Tag Archives: #magnetars

Galactic Gamma Ray Bursts Predicted Last Year Show Up Right On Schedule (Planetary Science)

Sherlock Holmes story gives clue to successful prediction of bursts from nearby magnetar

Magnetars are bizarre objects — massive, spinning neutron stars with magnetic fields among the most powerful known, capable of shooting off brief bursts of radio waves so bright they’re visible across the universe.

A team of astrophysicists has now found another peculiarity of magnetars: They can emit bursts of low energy gamma rays in a pattern never before seen in any other astronomical object.

It’s unclear why this should be, but magnetars themselves are poorly understood, with dozens of theories about how they produce radio and gamma ray bursts. The recognition of this unusual pattern of gamma ray activity could help theorists figure out the mechanisms involved.

“Magnetars, which are connected with fast radio bursts and soft gamma repeaters, have something periodic going on, on top of randomness,” said astrophysicist Bruce Grossan, an astrophysicist at the University of California, Berkeley’s Space Sciences Laboratory (SSL). “This is another mystery on top of the mystery of how the bursts are produced.”

The researchers — Grossan and theoretical physicist and cosmologist Eric Linder from UC Berkeley and postdoctoral fellow Mikhail Denissenya from Nazarbayev University in Kazakhstan — discovered the pattern in bursts from a soft gamma repeater, SGR1935+2154, that is a magnetar, a prolific source of soft or lower energy gamma ray bursts and the only known source of fast radio bursts within our Milky Way galaxy. They found that the object emits bursts randomly, but only within regular four-month windows of time, each active window separated by three months of inactivity.

On March 19, the team uploaded a preprint claiming “periodic windowed behavior” in soft gamma bursts from SGR1935+2154 and predicted that these bursts would start up again after June 1 — following a three month hiatus — and could occur throughout a four-month window ending Oct. 7.

On June 24, three weeks into the window of activity, the first new burst from SGR1935+2154 was observed after the predicted three month gap, and nearly a dozen more bursts have been observed since, including one on July 6, the day the paper was published online in the journal Physical Review D.

“These new bursts within this window means that our prediction is dead on,” said Grossan, who studies high energy astronomical transients. “Probably more important is that no bursts were detected between the windows since we first published our preprint.”

Linder likens the non-detection of bursts in three-month windows to a key clue — the “curious incident” that a guard dog did not bark in the nighttime — that allowed Sherlock Holmes to solve a murder in the short story “The Adventure of Silver Blaze”.

“Missing or occasional data is a nightmare for any scientist,” noted Denissenya, the first author of the paper and a member of the Energetic Cosmos Laboratory at Nazarbayev University that was founded several years ago by Grossan, Linder and UC Berkeley cosmologist and Nobel laureate George Smoot. “In our case, it was crucial to realize that missing bursts or no bursts at all carry information.”

The confirmation of their prediction startled and thrilled the researchers, who think this may be a novel example of a phenomenon — periodic windowed behavior — that could characterize emissions from other astronomical objects.

Mining data from 27-year-old satellite

Within the last year, researchers suggested that the emission of fast radio bursts — which typically last a few thousandths of a second — from distant galaxies might be clustered in a periodic windowed pattern. But the data were intermittent, and the statistical and computational tools to firmly establish such a claim with sparse data were not well developed.

Grossan convinced Linder to explore whether advanced techniques and tools could be used to demonstrate that periodically windowed — but random, as well, within an activity window — behavior was present in the soft gamma ray burst data of the SGR1935+2154 magnetar. The Konus instrument aboard the WIND spacecraft, launched in 1994, has recorded soft gamma ray bursts from that object — which also exhibits fast radio bursts — since 2014 and likely never missed a bright one.

Linder, a member of the Supernova Cosmology Project based at Lawrence Berkeley National Laboratory, had used advanced statistical techniques to study the clustering in space of galaxies in the universe, and he and Denissenya adapted these techniques to analyze the clustering of bursts in time. Their analysis, the first to use such techniques for repeated events, showed an unusual windowed periodicity distinct from the very precise repetition produced by bodies rotating or in orbit, which most astronomers think of when they think of periodic behavior.

“So far, we have observed bursts over 10 windowed periods since 2014, and the probability is 3 in 10,000 that while we think it is periodic windowed, it is actually random,” he said, meaning there’s a 99.97% chance they’re right. He noted that a Monte Carlo simulation indicated that the chance they’re seeing a pattern that isn’t really there is likely well under 1 in a billion.

The recent observation of five bursts within their predicted window, seen by WIND and other spacecraft monitoring gamma ray bursts, adds to their confidence. However, a single future burst observed outside the window would disprove the whole theory, or cause them to redo their analysis completely.

“The most intriguing and fun part for me was to make predictions that could be tested in the sky. We then ran simulations against real and random patterns and found it really did tell us about the bursts,” Denissenya said.

As for what causes this pattern, Grossan and Linder can only guess. Soft gamma ray bursts from magnetars are thought to involve starquakes, perhaps triggered by interactions between the neutron star’s crust and its intense magnetic field. Magnetars rotate once every few seconds, and if the rotation is accompanied by a precession — a wobble in the rotation — that might make the source of burst emission point to Earth only within a certain window. Another possibility, Grossan said, is that a dense, rotating cloud of obscuring material surrounds the magnetar but has a hole that only periodically allows bursts to come out and reach Earth.

“At this stage of our knowledge of these sources, we can’t really say which it is,” Grossan said. “This is a rich phenomenon that will likely be studied for some time.”

Linder agrees and points out that the advances were made by the cross-pollination of techniques from high energy astrophysics observations and theoretical cosmology.

“UC Berkeley is a great place where diverse scientists can come together,” he said. “They will continue to watch and learn and even ‘listen’ with their instruments for more dogs in the night.”

Featured image: Since 2014, a magnetar in our galaxy (SGR1935+2154) has been emitting bursts of soft gamma rays (black stars). UC Berkeley scientists concluded that they occurred only within certain windows of time (green stripes) but were somehow blocked during intervening windows (red). They used this pattern to predict renewed bursts starting after June 1, 2021 (stripes outlined in blue at right), and since June 24, more than a dozen have been detected (blue stars): right on schedule. © Mikhail Denissenya, Nazarbayev University, Kazakhstan

Reference: Mikhail Denissenya, Bruce Grossan, and Eric V. Linder, “Distinguishing time clustering of astrophysical bursts”, Phys. Rev. D 104, 023007 – Published 6 July 2021. DOI: https://doi.org/10.1103/PhysRevD.104.023007

Provided by UC Berkeley

Could Magnetised OB stars Produce All Magnetars? (Planetary Science)

The origin of magnetic fields in massive stars remains enigmatic. Several hypothesis have been proposed. In 2019, Schneider and colleagues analysed a different scenario where strong magnetic fields occur as a result of a merger. They performed 3-dimensional magnetohydrodynamic simulations for the merger of two massive main sequence stars. They noticed that this process produces strong magnetic fields compatible with 9 kG. These strongly magnetised merger products could be the magnetar progenitors. Stellar mergers occur in 22 per cent of all binaries, but it is impossible to quantify currently what fraction of mergers end up strongly magnetised. Therefore, some massive stars could be very different from the bulk population because of their past binary interactions or because of their significant initial magnetisation. Thus, in 2006, Ferrario & Wickramasinghe suggested that magnetars could originate from strongly magnetised massive B stars and radio pulsars are from weakly magnetised ones which they called “the fossil field hypothesis”.

“Stars of spectral types O and B produce neutron stars (NSs) after supernova explosions. Most of NSs are strongly magnetised including normal radio pulsars with 𝐵 ∝ 1012 G and magnetars with 𝐵 ∝ 1014 G. A fraction of 7-12 per cent of massive stars are also magnetised with 𝐵 ∝ 103 G and some are weakly magnetised with 𝐵 ∝ 1 G. It was suggested that magnetic fields of NSs could be the fossil remnants of magnetic fields of their progenitors. Our work is dedicated to study this hypothesis.”

— wrote authors of the study

Now, Makarenko and colleagues check this hypothesis by comparing the magnetic field distributions both for massive OB stars and neutron stars (NSs). To do so, they collect all reliable, modern measurements of magnetic fields at surfaces on massive stars of spectral types O, B, A. Then, they developed a maximum likelihood technique to estimate the parameters of the magnetic field distribution. They found that the log-normal distribution describes well measurements of magnetic fields of O and B stars. In the case of A stars, they found significant deviations from the log-normal distribution possibly related to evolution. In the case of B stars, the parameters of the log-normal distribution are as following: 𝜇𝐵 = 2.83±0.1 log10 (G) i.e. ≈ 700 and 𝜎𝐵 = 0.65±0.09 for strongly magnetised and 𝜇𝐵 = 0.14 ± 0.5 log10 (G), 𝜎 = 0.7 for weakly magnetised.

They also noticed that the difference between magnetic field of strongly magnetised B-stars and weakly magnetised B stars is 2.7 DEX and magnetars represent 10 per cent of all young NSs and run population synthesis.

“We found that it is impossible to simultaneously reproduce pulsars and magnetars populations if the difference in their magnetic fields is 2.7 DEX. Therefore, we conclude that the simple fossil origin of the magnetic field is not viable for NSs.”

— Makarenko, lead author of the study

In addition, in accordance to the fossil field hypothesis, they assumed that weakly-magnetised B stars produce normal radio pulsars with 𝜇𝐵 = 11.7 and 𝜎𝐵 = 0.7 and strongly magnetised produce magnetars with 𝜇𝐵 = 14.45 and 𝜎𝐵 = 0.7. To check if the resulting population looks anything like an observed population of radio pulsars and magnetars they ran the population synthesis. They found that simple conservation of magnetic field in the core cannot explain the observed value of period and period derivative for normal radio pulsars. The cloud of radio pulsars is shifted towards too small period derivative values. In trying to improve their model, they guess that their original model for magnetic field conservation might be too simplistic. Therefore, they assumed values of 𝜇𝐵 = 12.65, 𝜎𝐵 = 0.7 for 90 per cent of NSs and 𝜇𝐵 = 15.35, 𝜎𝐵 = 0.7 for magnetars to keep 2.7 DEX difference. This model strongly overproduces bright magnetars with fluxes 𝑆𝑋 in the range 10¯10– 10¯8 erg cm¯21. This result they found, does not depend on the value of the crust impurity parameter.

Therefore, they concluded that the fossil field hypothesis cannot simply explain NS magnetic field distribution. Finally, they suggested that in order to correct this hypothesis there is a need of a mechanism that decreases the difference of 2.7 DEX between two groups of stars to the difference of ≈ 1 DEX seen between magnetic fields of magnetars and normal radio pulsars. One such mechanism could be the field instability at the proto-NS stage.

“It is interesting to note that even if we miss most of the distribution for weakly magnetised massive stars (e.g. due to instrumental limitations) and estimate only the exponential tail, our conclusion still holds. In this case, the mean value of magnetic fields for weakly magnetised stars is located at even smaller values and the actual difference is more than 2.7 DEX.”

— Makarenko, lead author of the study

Featured image: Schematic view of magnetic field lines inside and outside of a massive star. The blue region is a radiative envelope, the grey region is a convective core and solid lines show the magnetic field lines. © Makarenko et al.

Reference: Ekaterina I. Makarenko, Andrei P. Igoshev, A.F. Kholtygin, “Testing the fossil field hypothesis: could strongly magnetised OB stars produce all known magnetars?”, Arxiv, pp. 1-23, 2021. https://arxiv.org/abs/2104.10579

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Astronomers Yield a New Measurement Of The Hubble Constant Using Fast Radio Bursts (Astronomy)

Fast radio bursts (FRBs) are very short and bright transients visible over extragalactic distances. The mechanism causing the burst is still unknown, but at least some of them are associated with magnetars. A subset of bursts have been found to repeat, either with a fixed period or in cyclical phases of irregular activity. It is therefore possible that the observed FRB events actually fall into a mix of different populations or progenitor mechanisms. The observed radio bursts are bright enough to be visible over extragalactic distances, which opens up exciting possibilities to use FRBs for studying cosmological scales.

The radio pulse from the burst undergoes dispersion while travelling through the ionized intergalactic medium, which is caused by free electrons along the line of sight, most of which are associated with the large-scale structure (LSS). The total dispersion measure therefore increases with the line of sight and provides a distance estimate to the source.

Now, Hagstotz, Reischke and Lilow presented a new measurement of the Hubble constant 𝐻0 based on the dispersion measure – redshift relation of fast radio bursts (FRBs). The method is similar to the determination of 𝐻0 from the luminosity – redshift relation of calibrated SN Ia. The total dispersion measure (DM) is dominated by the cosmological signal for redshifts 𝑧 > 0.3. With the current small sample of nine FRBs with known host galaxies, they constrained the Hubble constant to 𝐻0 = 62.3±9.1 km s¯1 Mpc¯1.

Figure 1. PDFs for the Hubble constant 𝐻0 from the nine individual FRBs with known redshift (thin colored lines) and the joint constraint 𝐻0 = 62.3 ± 9.1 km s¯1 Mpc¯1 of the sample (solid black). They also included the constraint from the six well-identified gold sample FRBs alone (dotted black), which results in 𝐻0 = 62.5 ± 10.1 km s¯1 Mpc¯1. The 𝐻0 values with 2𝜎 error bars from Planck CMB measurements ( blue) and from Cepheid-calibrated supernovae by the SH0ES collaboration are shown as shaded bands for reference. Hagstotz et al.

They also limit the analysis to the six events with the most reliable host identification (the gold sample), with an almost identical result of 𝐻0 = 62.5 ± 10.1 km s¯1 Mpc¯1, since most of the excluded FRBs are located at low redshifts.

“The current main limitations lie in the very small number of available events with sufficient localisation and in the uncertainty about the DM contribution from the host galaxy. Both of these can be solved by a larger sample of localised FRBs. In fact, dedicated searches with excellent angular resolution are expected to detect hundreds of bursts and their host galaxies over the next years.”

— wrote authors of the study

They demonstrated with a forecast that, with the data available in the near future, it is possible to set precision constraints on 𝐻0 fully independent from the CMB or other cosmological measurements, while simultaneously determining the stochastic host halo contribution. Since the cosmological and the stochastic contributions to the DM scale differently with redshift, a sample of a few hundred FRBs can reliably distinguish the two effects. This demonstrates the potential of FRBs for precision measurements of cosmological parameters.

Reference: Steffen Hagstotz, Robert Reischke, Robert Lilow, “A new measurement of the Hubble constant using Fast Radio Bursts”, Arxiv pp. 1-6, 2021. https://arxiv.org/abs/2104.04538v1

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Neutron Tunneling: A New Mechanism to Power Explosive Phenomena in Neutron Stars, Magnetars,And Neutron Star Mergers (Astronomy)

Bertulani and Lobato in their recent work clarifies aspects of ‘neutron tunneling’, the process that can occur in the outer regions of neutron stars when oscillations or cataclysmic events increase the ambient density. They use a time-dependent Hartree-Fock-Bogoliubov (TDHFB) formalism to determine the rate of neutron diffusion and find that large amounts of energy can be released rapidly due to fast tunneling times of loosely bound neutrons.

Neutron in nuclear orbits © Wallhere

Diffusion is a quantitative physical exchange of particles in environments with inhomogeneous particle spatial distribution. In nuclear physics it has been studied in central collisions between heavy ions due to a momentarily local imbalance of protons and neutrons. The goal in such studies is to understand the contribution of the symmetry energy term to the equation of state of neutron stars. Energy and isospin imbalance also occurs in the crust of neutron stars where dense neutron regions coexist with neutron poorer regions. In a non-equilibrated environment, neutrons will diffuse to lower the energy leading to a near homogenization of the neutron density. Neutron transfer can also occur between isospin unbalanced nuclear processes that could modify the cooling rates in transiently accreting neutron stars. Neutron transfer, or diffusion, in neutron star mergers will also influence the rate at which a locally homogeneous density can be achieved. Clumps and voids are certainly formed during the merging process and/or during a fallback mechanism in a core-collapse supernovae. Supernova fallback accretion has been intensively studied as possible site for r-process and as source of long-duration gamma-ray bursts in newly formed magnetars.

Bertulani and Robato explore the physics of diffusion in inhomogeneous neutron media due to tunneling, considering the flow of individual neutrons as well as neutron-pairs through the nuclear mean-field. For simplicity, they assume charge-neutral systems, i.e., pure neutron matter. Their goal is to identify general features and possible scaling laws for the diffusion rates that can be used to estimate cooling properties of neutron stars, and homogenization lifetimes in their crusts.

“We investigated the role of nuclear binding, the two-body interaction and pairing, on the neutron diffusion times and consider a one-dimensional quantum diffusion model and extend our analysis to study the impact of diffusion in three-dimensions. We find that these novel neutron transfer reactions can generate energy at the amount of 10^40-10⁴⁴ ergs under suitable conditions.”, write authors of the study.

They propose that the fast tunneling times of loosely bound neutrons can trigger short gamma/X-ray bursts/flaring activities in magnetars and fast radio burst (FRBs) through the liberation of photons in the crust or in the star magnetic field. Beta decay particles in the strong magnetic field move perpendicular to it in quantized Landau levels and the electron-cyclotron energy will be equal to the electron rest-mass energy. In this scenario these particles will also act as a seed for the high energy electromagnetic radiation. The origin of these electromagnetic activities as well as the sources of the FRBs are unknown.

They also found that the binding energies per neutron are much larger than for a regular nuclear system, however the physics associated with the diffusion rate of the neutrons can be well understood with this model.

Table 1. Single particle energy, Es.p., kinetic energy, Ekin, interaction energy, Eint, pairing energy, Epair, total energy, Etotal and neutron separation energy, Sn, for a system of N = 20, 40, 80 and 160 neutrons confined in a potential with parameters described in text. All energies are in units of MeV
Figure 1. Neutron diffusion rates as a function of time for the neutron clumps separated by 20 fm with neutron numbers listed in Table 1 and a periodic row of Woods-Saxon potentials.

While, the transfer rates reported by researchers in their study are large compared to typical ones in nuclear reactions because of the large neutron numbers and small separation energies they have adopted for the valence neutrons. Because of the Coulomb barriers and symmetry energies in normal nuclei, and for large separation between them, the transfer rates are much smaller. On the other hand, as the neutron number increases in the envelope and crust of neutron stars, the reaction rates are expected to increase accordingly when inhomogeneous conditions develop. They have considered neutron clumps with 500 and 1000 neutrons with widths and depths of the confining potentials U(x) adjusted to accommodate all neutrons within the potential wells while keeping the valence neutrons at about 1 MeV binding. They found that the results are in agreement with the increase of the diffusion rate with the neutron number, but it is manifestly stronger for large clumps. It is a probable scenario in a cataclysmic event, i.e., large neutron clumps separated by large distances. If the distances become smaller due to compression waves, the neutron diffusion process can release enormous amounts of energy.

Their time-dependent microscopic calculations showed that the subject of density homogenization and isospin diffusion in nuclear reactions and in stellar environments deserves more extensive studies. While, perturbative estimates are likely to yield poor results because of the microscopic properties of strongly interacting systems such as the different contributions of the interactions to the total energy and the related energy rearrangement due to tunneling.

Researchers concluded that perturbations within cataclysmic environments such as supernovae, neutron star mergers, or the creation of inhomogeneous regions in the crust of neutron stars due to compression waves from a star quake, can all lead to gamma or radio bursts when proper conditions for neutron diffusion is attained. Nuclear physics dictates that such conditions depend on the existence of inhomogeneous neutron matter clumps separated by relatively small distances.

Reference: Carlos A. Bertulani, Ronaldo V. Lobato, “Neutron tunneling: A new mechanism to power explosive phenomena in neutron stars, magnetars, and neutron star mergers”, pp. 1-9, 2020. https://arxiv.org/abs/2011.14953v1

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NASA’s Hubble sees Unexplained Brightness from Colossal Explosion (Astronomy)

Long ago and far across the universe, an enormous burst of gamma rays unleashed more energy in a half-second than the Sun will produce over its entire 10-billion-year lifetime. In May of 2020, light from the flash finally reached Earth and was first detected by NASA’s Neil Gehrels Swift Observatory. Scientists quickly enlisted other telescopes — including NASA’s Hubble Space Telescope, the Very Large Array radio observatory, the W. M. Keck Observatory, and the Las Cumbres Observatory Global Telescope network — to study the explosion’s aftermath and the host galaxy. It was Hubble that provided the surprise.

This image shows the glow from a kilonova caused by the merger of two neutron stars. The kilonova, whose peak brightness reaches up to 10,000 times that of a classical nova, appears as a bright spot (indicated by the arrow) to the upper left of the host galaxy. The merger of the neutron stars is believed to have produced a magnetar, which has an extremely powerful magnetic field. The energy from that magnetar brightened the material ejected from the explosion. Credits: NASA, ESA, W. Fong (Northwestern University), and T. Laskar (University of Bath, UK)

Based on X-ray and radio observations from the other observatories, astronomers were baffled by what they saw with Hubble: the near-infrared emission was 10 times brighter than predicted. These results challenge conventional theories of what happens in the aftermath of a short gamma-ray burst. One possibility is that the observations might point to the birth of a massive, highly magnetized neutron star called a magnetar.

“These observations do not fit traditional explanations for short gamma-ray bursts,” said study leader Wen-fai Fong of Northwestern University in Evanston, Illinois. “Given what we know about the radio and X-rays from this blast, it just doesn’t match up. The near-infrared emission that we’re finding with Hubble is way too bright. In terms of trying to fit the puzzle pieces of this gamma-ray burst together, one puzzle piece is not fitting correctly.”

Without Hubble, the gamma-ray burst would have appeared like many others, and Fong and her team would not have known about the bizarre infrared behavior. “It’s amazing to me that after 10 years of studying the same type of phenomenon, we can discover unprecedented behavior like this,” said Fong. “It just reveals the diversity of explosions that the universe is capable of producing, which is very exciting.”

Light Fantastic

The intense flashes of gamma rays from these bursts appear to come from jets of material that are moving extremely close to the speed of light. The jets do not contain a lot of mass — maybe a millionth of the mass of the Sun — but because they’re moving so fast, they release a tremendous amount of energy across all wavelengths of light. This particular gamma-ray burst was one of the rare instances in which scientists were able to detect light across the entire electromagnetic spectrum.

“As the data were coming in, we were forming a picture of the mechanism that was producing the light we were seeing,” said the study’s co-investigator, Tanmoy Laskar of the University of Bath in the United Kingdom. “As we got the Hubble observations, we had to completely change our thought process, because the information that Hubble added made us realize that we had to discard our conventional thinking, and that there was a new phenomenon going on. Then we had to figure out what that meant for the physics behind these extremely energetic explosions.”

This illustration shows the sequence for forming a magnetar-powered kilonova, whose peak brightness reaches up to 10,000 times that of a classical nova. 1) Two orbiting neutron stars spiral closer and closer together. 2) They collide and merge, triggering an explosion that unleashes more energy in a half-second than the Sun will produce over its entire 10-billion-year lifetime. 3) The merger forms an even more massive neutron star called a magnetar, which has an extraordinarily powerful magnetic field. 4) The magnetar deposits energy into the ejected material, causing it to glow unexpectedly bright at infrared wavelengths. Credits: NASA, ESA, and D. Player (STScI)

Gamma-ray bursts — the most energetic, explosive events known — live fast and die hard. They are split into two classes based on the duration of their gamma rays.

If the gamma-ray emission is greater than two seconds, it’s called a long gamma-ray burst. This event is known to result directly from the core collapse of a massive star. Scientists expect a supernova to accompany this longer type of burst.

If the gamma-ray emission lasts less than two seconds, it’s considered a short burst. This is thought to be caused by the merger of two neutron stars, extremely dense objects about the mass of the Sun compressed into the volume of a city. A neutron star is so dense that on Earth, one teaspoonful would weigh a billion tons! A merger of two neutron stars is generally thought to produce a black hole.

Neutron star mergers are very rare but are extremely important because scientists think that they are one of the main sources of heavy elements in the universe, such as gold and uranium.

Accompanying a short gamma-ray burst, scientists expect to see a “kilonova” whose peak brightness typically reaches 1,000 times that of a classical nova. Kilonovae are an optical and infrared glow from the radioactive decay of heavy elements and are unique to the merger of two neutron stars, or the merger of a neutron star with a small black hole.

Magnetic Monster?

Fong and her team have discussed several possibilities to explain the unusual brightness that Hubble saw. While most short gamma-ray bursts probably result in a black hole, the two neutron stars that merged in this case may have combined to form a magnetar, a supermassive neutron star with a very powerful magnetic field.

“You basically have these magnetic field lines that are anchored to the star that are whipping around at about a thousand times a second, and this produces a magnetized wind,” explained Laskar. “These spinning field lines extract the rotational energy of the neutron star formed in the merger, and deposit that energy into the ejecta from the blast, causing the material to glow even brighter.”

Video: These two images taken on May 26 and July 16, 2020, show the fading light of a kilonova located in a distant galaxy. The kilonova appears as a spot to the upper left of the host galaxy. The glow is prominent in the May 26 image but fades in the July 16 image. The kilonova’s peak brightness reaches up to 10,000 times that of a classical nova. A merger of two neutron stars — the source of the kilonova — is believed to have produced a magnetar, which has an extremely powerful magnetic field. The energy from that magnetar brightened the material ejected from the explosion, causing it to become unusually bright at infrared wavelengths of light.
Credits: NASA, ESA, W. Fong (Northwestern University), T. Laskar (University of Bath, UK), and A. Pagan (STScI)

If the extra brightness came from a magnetar that deposited energy into the kilonova material, then within a few years, the team expects the ejecta from the burst to produce light that shows up at radio wavelengths. Follow-up radio observations may ultimately prove that this was a magnetar, and this may explain the origin of such objects.

“With its amazing sensitivity at near-infrared wavelengths, Hubble really sealed the deal with this burst,” explained Fong. “Amazingly, Hubble was able to take an image only three days after the burst. Through a series of later images, Hubble showed that a source faded in the aftermath of the explosion. This is as opposed to being a static source that remains unchanged. With these observations, we knew we had not only nabbed the source, but we had also discovered something extremely bright and very unusual. Hubble’s angular resolution was also key in pinpointing the position of the burst and precisely measuring the light coming from the merger.”

NASA’s upcoming James Webb Space Telescope is particularly well-suited for this type of observation. “Webb will completely revolutionize the study of similar events,” said Edo Berger of Harvard University in Cambridge, Massachusetts, and principal investigator of the Hubble program. “With its incredible infrared sensitivity, it will not only detect such emission at even larger distances, but it will also provide detailed spectroscopic information that will resolve the nature of the infrared emission.”

References: https://iopscience.iop.org/journal/0004-637X

Provided by NASA Goddard

“Slow” Radio Bursts Are Produced by Galactic Magnetars (Astronomy)

Fast radio bursts (FRBs) are super intense, millisecond-long bursts of radio waves produced by unidentified sources in the distant cosmos. The detection of a 1.5 MJy ms fast radio burst (FRB) in the Milky Way galaxy, i.e. FRB 200428 in association with a bright X-ray burst from the magnetar SGR J1935+2154, established the magnetar origin of at least some, probably all FRBs. On the other hand, deep monitoring of SGR J1935+2154 by FAST suggested that the FRB-SGR-burst associations are rather rare. During an active phase of SGR J1935+2154 when 29 other X-ray bursts were emitted from the source, no single FRB-like event was detected. Whereas whether the FRB-associated X-ray burst is physically special is still subject to debate, one plausible possibility is that the FRB emission is much more narrowly beamed than the SGR burst emission.

(Credits, Discover: William Zuback, Roen Kelly, Alison Mackey)

Within this picture, an FRB can be detected only when the narrow beam points towards Earth. Outside the FRB “jet”, due to the rapid drop of the Doppler factor, one would expect that the flux drops rapidly, spectrum becomes softer, and duration becomes longer. These off-beam events are not likely detectable from cosmological FRB sources. However, in view of the huge specific fluence of FRB 200428, it is entirely possible that some off-axis, longer and softer bursts from Galactic magnetars such as SGR J1935+2154 can be detected above the available telescopes sensitivity threshold. Bing Zhang and colleagues define these events as “slow” radio bursts (SRBs) and studied their properties in their recent research paper.

They have discussed a type of radio burst from Galactic magnetars that could be FRBs viewed off-beam. These bursts, dubbed SRBs, could have much longer durations and lower specific fluences than FRBs because of their smaller Doppler factors than on-beam FRBs.

They derived a “closure relation”, (which I shown above), among the ratios of burst specific fluence, width, and observing frequency between SRBs and FRBs, which could be used to judge whether a radio burst is an SRB. They showed that the 2.2-s long, 111 MHz radio burst detected from SGR J1935+2154 by the BSA LPI radio telescope could be interpreted as an SRB if the spectral slope is positive.

They estimated the relative event rates of Galactic SRBs with respect to Galactic FRBs and found that the rate of Galactic SRBs could be much higher than that of Galactic FRBs if all SGR-bursts are associated with narrow-beam FRBs. According to authors, “A systematic search for SRBs from SGR J1935+2154 and other Galactic magnetars can place important constraints on this hypothesis. Non-detection would rule out this suggestion. Detections, on the other hand, would confirm the beaming nature of FRBs and allow direct constraints on physical parameters of the FRB emitters.”

They concluded that SRBs may not be only produced by Galactic magnetars. If other sources in the Milky Way can make Galactic FRBs and be detected by future wide-field radio telescope arrays, SRBs may be also produced from those objects based on the same reasoning discussed in their paper.

References: Bing Zhang, ““Slow” Radio Bursts from Galactic Magnetars?”, ArXiv, pp. 1-4, 2020. https://arxiv.org/abs/2011.09921

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Cosmic Flashes Come In All Different Sizes (Planetary Science)

By studying the site of a spectacular stellar explosion seen in April 2020, a Chalmers-led team of scientists have used four European radio telescopes to confirm that astronomy’s most exciting puzzle is about to be solved. Fast radio bursts, unpredictable millisecond-long radio signals seen at huge distances across the universe, are generated by extreme stars called magnetars – and are astonishingly diverse in brightness.

On May 24, four European telescopes took part in the global effort to understand mysterious cosmic flashes. The telescopes captured flashes of radio waves from an extreme, magnetised star in our galaxy. All are shown in this illustration. ©Danielle Futselaar/artsource.nl

For over a decade, the phenomenon known as fast radio bursts has excited and mystified astronomers. These extraordinarily bright but extremely brief flashes of radio waves – lasting only milliseconds – reach Earth from galaxies billions of light years away.

In April 2020, one of the bursts was for the first time detected from within our galaxy, the Milky Way, by radio telescopes CHIME and STARE2. The unexpected flare was traced to a previously-known source only 25 000 light years from Earth in the constellation of Vulpecula, the Fox, and scientists all over the world coordinated their efforts to follow up the discovery.

In May, a team of scientists led by Franz Kirsten (Chalmers) pointed four of Europe’s best radio telescopes towards the source, known as SGR 1935+2154. Their results are published today in a paper in the journal Nature Astronomy. http://www.nature.com/articles/s41550-020-01246-3

“We didn’t know what to expect. Our radio telescopes had only rarely been able to see fast radio bursts, and this source seemed to be doing something completely new. We were hoping to be surprised!”, said Mark Snelders, team member from the Anton Pannekoek Institute for Astronomy, University of Amsterdam.

The radio telescopes, one dish each in the Netherlands and Poland and two at Onsala Space Observatory in Sweden, monitored the source every night for more than four weeks after the discovery of the first flash, a total of 522 hours of observation.

On the evening of May 24, the team got the surprise they were looking for. At 23:19 local time, the Westerbork telescope in the Netherlands, the only one of the group on duty, caught a dramatic and unexpected signal: two short bursts, each one millisecond long but 1.4 seconds apart.

Kenzie Nimmo, astronomer at Anton Pannekoek Institute for Astronomy and ASTRON, is a member of the team.

“We clearly saw two bursts, extremely close in time. Like the flash seen from the same source on April 28, this looked just like the fast radio bursts we’d been seeing from the distant universe, only dimmer. The two bursts we detected on May 24 were even fainter than that”, she said.

Onsala Space Observatory in western Sweden. ©Magnus Falck/Chalmers University of Technology

This was new, strong evidence connecting fast radio bursts with magnetars, the scientists thought. Like more distant sources of fast radio bursts, SGR 1935+2154 seemed to be producing bursts at random intervals, and over a huge brightness range.

“The brightest flashes from this magnetar are at least ten million times as bright as the faintest ones. We asked ourselves, could that also be true for fast radio burst sources outside our galaxy? If so, then the universe’s magnetars are creating beams of radio waves that could be criss-crossing the cosmos all the time – and many of these could be within the reach of modest-sized telescopes like ours”, said team member Jason Hessels (Anton Pannekoek Institute for Astronomy and ASTRON, Netherlands).

Neutron stars are the tiny, extremely dense remnants left behind when a short-lived star of more than eight times the mass of the Sun explodes as a supernova. For 50 years, astronomers have studied pulsars, neutron stars which with clock-like regularity send out pulses of radio waves and other radiation. All pulsars are believed to have strong magnetic fields, but the magnetars are the strongest known magnets in the universe, each with a magnetic field hundreds of trillions of times stronger than the Sun’s.

In the future, the team aims to keep the radio telescopes monitoring SGR 1935+2154 and other nearby magnetars, in the hope of pinning down how these extreme stars actually make their brief blasts of radiation.

Scientists have presented many ideas for how fast radio bursts are generated. Franz Kirsten, astronomer at Onsala Space Observatory, Chalmers, who led the project, expects the rapid pace in understanding the physics behind fast radio bursts to continue.

“The fireworks from this amazing, nearby magnetar have given us exciting clues about how fast radio bursts might be generated. The bursts we detected on May 24 could indicate a dramatic disturbance in the star’s magnetosphere, close to its surface. Other possible explanations, like shock waves further out from the magnetar, seem less likely, but I’d be delighted to be proved wrong. Whatever the answers, we can expect new measurements and new surprises in the months and years to come”, he said.

References: Kirsten, F., Snelders, M.P., Jenkins, M. et al. Detection of two bright radio bursts from magnetar SGR 1935 + 2154. Nat Astron (2020). https://www.nature.com/articles/s41550-020-01246-3 https://doi.org/10.1038/s41550-020-01246-3

Provided by Chalmers University Of Technology

What Is The Role Of Quark Matter Surface Tension in Magnetars? (Astronomy)

Grunfeld and Lugones analyzed the behavior of the surface tension of three-flavor quark matter in the outer and inner core of cold deleptonized magnetars, proto magnetars born in core collapse supernovae, and hot magnetars produced in binary neutron stars mergers.

Magnetars are apparently isolated compact objects characterized by a variable X-ray activity, whose main properties can be explained by the presence of extremely strong magnetic fields.

Computer artwork showing the magnetic field (lines) around a magnetar. A magnetar is a type of neutron star with an incredibly strong magnetic field (a million billion times stronger than that of the Earth), which is formed when certain stars undergo supernova explosions. © Getty images

The internal composition of magnetars is not yet fully understood since baryon number densities inside them may be as large as several times the nuclear saturation density, no ≈ 0.16 fm-³ and first principle calculations are not available in this density regime. In spite of this, it is widely accepted that deconfined quark matter would be present in neutron star cores. In this context, it is important to know the value of the surface tension of quark matter because it is essential to understand whether the hadron-quark interface is simply a sharp discontinuity or a mixed phase where quarks and hadrons form geometrical structures that coexist over a wide density region of the star. If the energy cost of surface effects does not exceed the gain in bulk energy, the scenario involving a mixed phase turns out to be favorable. Surface tension also plays a crucial role in quark matter nucleation during the formation of compact stellar objects, because it influences the nucleation rate and the associated critical size of the nucleated drops. It also determines the internal structure of strange stars which may fragment into a charge-separated mixture, involving positively-charged strangelets immersed in a negatively charged sea of electrons.

Now, in this work, Grunfled and Lugones studied the surface tension of three-flavor quark matter in cold magnetars as well as in hot magnetars with trapped neutrinos. They used MIT bag model to describe quark matter is a locally charge neutral mixture of u, d, s quarks, electrons and neutrinos, all in chemical equilibrium under weak interactions and immersed in a strong magnetic field. While, finite size effects were included by means of the MRE formalism.

They found that the presence of a strong magnetic field quantises the transverse motion of charged particles into Landau levels resulting in a surface tension that has a different value in the parallel and the transverse directions with respect to the magnetic field. As input parameters they have picked two values of the magnetic field, eBlow = 5 × 10−³ GeV and eBhigh = 5 × 10−² GeV² and two different sizes of the quark matter droplets, V / S = 2 fm and 10 fm. For the baryon number density they considered nB = n0 which is a typical value for the outer core and nB = 4n0 which is representative of the inner core of a magnetar.

They focused their analysis on three different astrophysical scenarios, according to their temperature and the amount of trapped neutrinos.

Cold magnetars (CM). The thermodynamic state is characterized by a very low temperature and neutrino transparency. They shall consider here 1 MeV < T < 10 MeV and μve= 0.
Proto magnetars (PM). Matter is non-degenerate and there is a considerable amount of trapped neutrinos. As a representative case they adopt 10 MeV < T < 50 MeV and μve = 100 MeV.
Post merger magnetar (PMM). According to numerical simulations, a compact object produced in a neutron star merger may attain temperatures of several tens of MeV and contains a large amount of trapped neutrinos. They consider here 50 MeV < T < 100 MeV and the extreme value μve= 200 MeV.

Their results showed that, for the densities considered there, the surface tension spans values between 0.2 MeV∕fm² and 15 MeV∕fm². They also find that, for magnetic fields smaller than eBlow, there is no difference between the transverse and the longitudinal values of the surface tension, meaning that quark drops must have a spherical shape. However, for larger magnetic fields the longitudinal contribution is larger than the transverse one and elongated shapes in the direction of the magnetic field are more prone to occur. This effect is more pronounced at low temperatures.

Surface tension has a key role in several astrophysical contexts. It plays a crucial role in quark matter nucleation during the formation of compact stellar objects, because it determines the nucleation rate and the associated critical size of the nucleated drops. Small values of the surface tension, as the ones obtained in this study, tend to favor faster nucleation timescales than other models such as NJL.

Finite size effects are also determinant in the formation of mixed phases at the core of hybrid stars which may arise only if the surface tension is smaller than a critical value σcrit ≈ 60 MeV∕fm². Below this value, the structure of the mixed phase becomes mechanically unstable and local charge neutrality is recovered. Their results are significantly smaller than σcrit indicating that a mixed phase would be favoured. Although their calculations assumed that matter is locally electrically neutral, they don’t expect that a more realistic calculation assuming global charge neutrality will change the order of Magnitude of surface tension σ, and therefore the above conclusion would still be valid. However, since large magnetic fields lead to different values for parallel ∥ and transverse ⟂, the pasta configurations that may appear in the mixed phase (droplets, rods, slabs, tubes, and bubbles) may be different in non-magnetized hybrid stars and in hybrid magnetars.

FIGURE 1. Parallel surface tension (upper panels) and transverse surface tension (lower panels) for nB= n0. Panels (a) and (d) correspond to CMs, panels (b) and (e) to PMs and panels (c) and (f) to PMMs.
FIGURE 2. Same as in Fig. 1 but for nB = 4n0.

Finally, they also found that the surface tension may affect decisively the internal structure of self-bound strange stars which may fragment into a charge-separated mixture, involving positively-charged strangelets immersed in a negatively charged sea of electrons, presumably forming crystalline solid matter. This would happen below another critical surface tension σ′crit whose value depends strongly on the equation of state. Present estimations of σ′crit give values in the range 0.5 − 18 MeV∕fm². Assuming global charge neutrality and non-magnetized matter, they have shown that this phase is favored in a scenario of high enough σ′crit (above ∼ 10 MeV∕fm²). However, they have shown that for strongly magnetized matter, σ^∥ grows and σ^⟂ diminishes with respect to the non-magnetized case. This leads to a much more involved situation because a single critical surface tension is not enough to determine whether a charge-separated mixture is favoured.

They concluded that more detailed analysis would be needed to determine the true ground state of self-bound ultra-magnetized matter.

References: A.G. Grunfeld, G. Lugones, “The role of quark matter surface tension in Magnetars”, ArXiv, 2020. https://arxiv.org/abs/2011.06131

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Modelling Extreme Magnetic Fields And Temperature Variation On Distant Stars (Astronomy)

New research is helping to explain one of the big questions that has perplexed astrophysicists for the past 30 years – what causes the changing brightness of distant stars called magnetars.

Magnetars were formed from stellar explosions or supernovae and they have extremely strong magnetic fields, estimated to be around 100 million, million times greater than the magnetic field found on earth.

The maps show the heat distribution. The bue regions are cooler – and the yellow regions are hotter. It describes data taken from the following magentars: 4U 0142+61, 1E 1547.0-5408, XTE J1810-197, SGR 1900 + 14. ©University of Leeds

The magnetic field generates intense heat and x-rays. It is so strong it also affects the physical properties of matter, most notably the way that heat is conducted through the crust of the star and across its surface, creating the variations in brightness across the star which has puzzled astrophysicists and astronomers.

A team of scientists – led by Dr Andrei Igoshev at the University of Leeds – has developed a mathematical model that simulates the way the magnetic field disrupts the conventional understanding of heat being distributed uniformly and creates hotter and cooler regions where there may be a difference in temperature of one million degrees Celsius.

This is a GIF simulation of simulation 1
©University of Leeds

Those hotter and cooler regions emit x-rays of differing intensity – and it is that variation in x-ray intensity that is observed as changing brightness by space-borne telescopes.

The findings are published today (12 October) in the journal Nature Astronomy. The research was funded by the Science and Technology Facilities Council (STFC).

Dr Igoshev, from the School of Mathematics at Leeds, said: “We see this constant pattern of hot and cold regions. Our model – based on the physics of magnetic fields and the physics of heat – predicts the size, location and temperature of these regions – and in doing so, helps explain the data captured by satellite telescopes over several decades and which has left astronomers scratching their heads as to why the brightness of magnetars seemed to vary.

The maps show the heat distribution. The blue regions are cooler – and the yellow regions are hotter. It describes data taken from the following magnetars: SGR 0418+5729, PSR J1119-6127, CXOU J164710.0-455216, CXOU J171405.7-381031, Swift J1822.3-1606, 1E 1841-045. ©University of Leeds

“Our research involved formulating mathematical equations that describe how the physics of magnetic fields and heat distribution would behave under the extreme conditions that exist on these stars.

“To formulate those equations took time but was straightforward. The big challenge was writing the computer code to solve the equations – that took more than three years.”

Once the code was written, it then took a super-computer to solve the equations, allowing the scientists to develop their predictive model.

The team used the STFC-funded DiRAC supercomputing facilities at the University of Leicester.

Dr Igoshev said once the model had been developed, its predictions were tested against the data collected by space-borne observatories. The model was correct in ten out of 19 cases.

The magnetars studied as part of the investigation are in the Milky Way and typically 15 thousand light years away.

The other members of the research team were Professor Rainer Hollerbach, also from Leeds, Dr Toby Wood, from the University of Newcastle, and Dr Konstantinos N Gourgouliatos, from the University of Patras in Greece.

References: Igoshev, A.P., Hollerbach, R., Wood, T. et al. Strong toroidal magnetic fields required by quiescent X-ray emission of magnetars. Nat Astron (2020). https://doi.org/10.1038/s41550-020-01220-z link: https://www.nature.com/articles/s41550-020-01220-z

Provided by University Of Leeds