The Northern Cross Enters the Frb Era (Astronomy)

For some years now, a small team has been involved in the technological reconversion of the Northern Cross, the historic low-frequency radio astronomy antenna of Medicina, near Bologna, to adapt this instrument to recently emerged research lines. Modernizations culminated in the observation of the first fast radio burst last March, an important step that opens up new scientific scenarios for the dean of Italian radio telescopes

A piece of this story begins on March 3, 2021 at the Radio Astronomy Station of Medicine of the National Institute of Astrophysics. We are in the lower Bologna area, 30 kilometers east of the Emilian capital, towards Ravenna and the Adriatic. In winter snow is not uncommon , in summer there is no shortage of mosquitoes , and legend has it that half a century ago, in this corner of the Po Valley, there was a restaurant that served fabulous tortelloni . It is here that, at 16:17:29 local time, a signal from the depths of the cosmos reaches six of the 64 metal cylinders that make up one of the two branches of Italy’s first radio telescope, the Northern Cross .

Despite the restrictions imposed by the third wave of the Covid-19 pandemic, a small group of radio astronomers is in Medicina. They work on different projects and continue to work even when the instrument, diligent and silent, picks up the very fast signal. Eight thousandths of a second. Ten, fifteen times faster than the proverbial blink of an eye. A timing that intuitively almost clashes with the not so lively dynamics of the Northern Cross which, on the other hand, is precisely what allowed the historic radio telescope to intercept this brief flash in low frequency radio waves.

«We can aim it only in declination», explains Germano Bianchi, Inaf researcher and head of the Northern Cross, “and can observe all objects that, during their apparent motion from east to west, pass on the local meridian”. Astronomers speak of a “transit” instrument. He stands there and watches the sky pass over him, like when you lie down on your stomach on a summer night, letting the earth’s rotation take its course, unrolling a continuous stream of astronomical springs on the celestial vault. Except that the Northern Cross, being a radio telescope, can scan the sky even during the day. Bianchi calls into question the blinders of a horse who, on the edge of a road, looks ahead and occasionally sees a car pass by: “the machines are the radio sources and the blinders are the field of view of the Cross”.

Technologies of yesterday (and today)

Transit radio telescopes like this, a technological avant-garde at the time of its construction, were very much in vogue until the 70s-80s of the last century because they were relatively simple to build, without expensive mechanical parts that often need maintenance. Inaugurated in 1964 , the Northern Cross is still one of the largest instruments of its kind in the world, with a collection area of ​​about 30,000 square meters – the equivalent of six football fields – which guarantees high sensitivity in observations.

Detail on the antennas of the Northern Cross in a photo taken in mid-March 2021; in the distance you can recognize the 32-meter dish. Credits: G. Bianchi

Then came the large orientable antennas, such as the 32-meter-diameter one that has flanked the Northern Cross in Medicine since 1983, or the Sardinia Radio Telescope (Srt), which with its powerful 64-meter dish has been the new for several years. flagship of Italian radio astronomy. The great transit instruments have faded into the background for many years, only to return surprisingly to the international astronomy scene only recently. The Canadian Hydrogen Intensity Mapping Experiment ( Chime ), operational since 2017 in British Columbia, the Canadian province overlooking the Pacific Ocean, is nothing more than a hyper-modern version of the Northern Cross.

One of the reasons for the great revival of these radio telescopes has a lot to do with the signal received on March 3 in Medicine, and can be summed up in three words: fast radio burst (Frb), or fast radio flashes. As the name implies, these are very short signals received in the radio band , almost exclusively from sources beyond our galaxy, discovered for the first time in 2007. To date, about a hundred sources of Frb are known, but their nature remains mysterious, with dozens and dozens of models proposed to try to explain the mechanism underlying these emissions that peep out unexpectedly in the radio firmament. A year ago, the discovery of the first Frb in the Milky Way , moreover in association with a magnetar, seems to indicate that these highly magnetized neutron stars may be hiding (at least) behind some of the flashes observed so far, but a general understanding of the phenomenon is still lacking. In the meantime, a lively research activity has developed around this enigma on all fronts, from radio band observations to the search for their high-energy counterparts, to theoretical modeling.

According to Gianni Bernardi, Inaf researcher in Bologna and coordinator of the research program of the Frb with the Northern Cross, there are many scientific cases today, “relevant above all for cosmology but in our case also for transients, where you do not necessarily have a favorite source to look at , but the event can happen anywhere ». If the direction in which you observe is not important, there is no need to move the telescope from one point of the sky to another, as is done with large orientable antennas to study specific radio sources in detail, and transit radio telescopes. In the case of Frb, which last a few milliseconds and can manifest without warning in any part of the sky, “you don’t have a great need to know where to point, you just need to stand there in the sky and wait for one to arrive.

Sometimes they come back: repeated flashes

Artistic impression of a Fast Radio Burst on its way to Earth. Credits: Jingchuan Yu, Beijing Planetarium

It takes time to frame a relatively new and decidedly inconstant phenomenon like the Frb. “In general, they are not periodicals,” underlines Maura Pilia , a researcher at INAF in Cagliari who is in charge of processing the data received from radio telescopes, in search of the elusive lightning. Yet, in the almost 14 years since the first survey, some lines among the many dots are beginning to be glimpsed. In particular, in 2016 the discovery that some Frb are repeated . “Out of a hundred who know each other now, only 20 repeat themselves, that is, they have been seen more than once.” Even when they recur, there doesn’t seem to be any regularity for these unpredictable lightning strikes in the sky of radio astronomy.

Or at least, this was the situation before January 2020, when the Chime collaboration first announces that it has found a fast radio burst that recurs regularly. It is called Frb 180916.J10158 + 56 , but experts are satisfied with the initials 180916, which identifies its first observation, in September 2018. Since then, as the authors explain in an article published in Nature last June, the Canadian radio telescope has observed the flash recur 38 timesover the course of a year and a half, on time every two weeks – 16.35 days to be exact – with an activity phase of about five days for each cycle. “It is not the periodicity that was expected, like that of pulsars for example,” comments Pilia. “The fact that it’s been days, tens of days, suggests that it’s a binary system.”

Regularity is not just a valuable indicator for trying to grasp the nature of these impenetrable springs. From a practical point of view, it means being able to optimize observations and data analysis. For the research program of the Frb with the Northern Cross, then recently started and still in an experimental form, the periodical reappearing of 180916 is a real well of information. As soon as the team learns of it, Pilia says, they immediately decide to start observing it.

In the foreground, some antennas of the north-south branch of the Northern Cross; in the distance, the antennas of the east-west branch. Credits: G. Bianchi

So for over a year, twice a month, for six days in a row, an hour every day, the Medicine observatory has turned its radio eyes towards the constellation of Cassiopeia, where the source of this Frb with a reassuring regularity, synchronizing the observations in the time interval in which it is very likely that this is active, waiting to capture a signal. “We are able to track the source during the transit within the field of view of the antenna”, explains Giovanni Naldi , an Inaf researcher who works in Medicine and who, together with his colleague Giuseppe Pupillo , takes care of the back-end of the radio telescope, including the planning of observations, data acquisition and preliminary processing using software ofpre-processing . “We have this automatic system that synthesizes an electronic beam that moves over time, effectively following the source that moves in the field of view. We are chasing this Frb for about an hour a day ».

Those who seek find …

The observing campaign started last year not only includes the Northern Cross but also Srt, which manages to capture three flashes from this source already between 22 and 24 February 2020. In addition to radio telescopes, optical band observers also participate, including including the Galileo National Telescope in La Palma and the Copernicus Telescope in Asiago, both of INAF, and satellites that scan the sky in high energy, including the Italian Agile , which is contributing to the wider monitoringnever made in X-rays for a source of Frb. “It was an important cover,” adds Pilia. “Among other things, in some situations, the Cross was the only Italian radio telescope capable of observing. We managed to make the observations, the analysis is still in progress ».

Yes, because the project of the group led by Bianchi and Bernardi was born without dedicated funds – a small allocation has arrived only recently – and is carried out, with passion and tenacity, by a small group engaged in various other activities. “The dedicated staff is very little, with FTE [ full-time equivalent , a measure that quantifies the time of the staff actually dedicated to a particular project – ed ] borrowed from other projects”, notes Bianchi. An observation program conducted so far between one project and another, but with a clear vision: redevelop an instrument to catch signals discovered for the first time more than forty years after its construction .

The first fast radio burst picked up by the Northern Cross. The top panel shows the profile of the received signal. In the second panel, the signal was corrected taking into account the dispersion in the interstellar medium; the third indicates the significance of the measure of dispersion, and the fourth shows the measure of dispersion, identified by the diagonal lines. Credits: G. Bernardi et al (2021)

Everything changes on March 12th. “For the first time I jumped on my chair”, recalls Pilia who, from Cagliari, processes the data with burst detection software and is therefore in charge of the group to say whether the Medicina antenna has actually seen a Frb or not . «Until now we had had some hopes of detection both with this source and with others, but they were at a level of significance that could be doubtful. Instead this time it was without a doubt our source, we got a nice burst ».

The unequivocal signature of the first Frb captured by the Northern Cross is the profile of the signal that stands out against the noise, much clearer than that of possible bursts recorded previously. Confirming the detection is also what astronomers call ‘dispersion measurement’, a delay caused by the interaction of the signal with the interstellar medium that pervades the Milky Way. When a signal arrives from far away, such as in the case of a Frb, it experiences a higher delay in the lower frequencies than in the higher ones. “We say that he is missing,” explains Pilia. And in this case, “we see that the peak is just around the measure of dispersion in which we expect it”.

A new life for the Northern Cross

The plan is to transform the Medicina radio telescope into an instrument dedicated to the search for these enigmatic and lightning-fast radio echoes, an ambitious goal for which numerous modernization interventions were necessary. Working at low frequencies, this radio telescope does not have a reflective surface formed by panels, as in satellite dishes, but is made up of a multitude of steel wires. “If a particularly windy day arrives in Medicina, the steel wires begin to vibrate,” recalls Bianchi. “As with a stringed instrument, the antenna starts to sound like a big harp: it makes a hiss that I find very pleasant to listen to.” A sound that could bring to the minds of cinema fans that “noise of the stars” that intrigued the protagonist of the film “The Red Desert” by Michelangelo Antonioni, played by Monica Vitti, in a scene shot right under the east-west branch of the iconic radio telescope, whose construction was then almost finished.

The antennas of the east-west branch of the Northern Cross and, below, some of the “cylinders” of the north-south branch. Credits: G. Bianchi

The branch of the Northern Cross immortalized in the Leone d’Oro film at the 1964 Venice Film Festival includes 25 structures that together form a single large cylindrical-parabolic antenna, 564 meters long. The technological upgrade in progress involves the other branch, the one oriented in a north-south direction, consisting of 64 individual antennas, also cylindrical-parabolic in shape, which follow each other for 625 meters at regular intervals, one every 10 meters . These “cylinders” collect the radio waves coming from the cosmos and reflect them, conveying the signals on the focal line, where they are then transformed into electrical impulses to be analyzed.

“Currently a cylinder has a single focal line,” explains Bianchi. “We divided it into 4 and so we expanded the field of view by 4 times.” This allows us to observe a slightly larger portion of the sky, in the hope of capturing some more bursts , but also satellites and space debris . «The first part of the upgradeof the first 8 cylinders began thanks to Stelio Montebugnoli ”, adds Naldi, recalling the previous manager of the Medicine radio astronomy station as“ the person who saw this great potential and had the foresight to give this new life back to the Northern Cross ”. Another piece of this story begins in those years, from 2006 onwards, when the radio telescope was proposed to the Ska consortium, which was then emerging , as the tool on which to test the algorithms that would then be applied for the Ska Observatory. , the largest facility in the world for radio astronomy, currently under construction between Australia and South Africa.

Above, aerial view of the Medicine Station, with the two perpendicular branches of the Northern Cross. Below, diagram of the “cylinders” that form the north-south branch. Source: N. Locatelli et al. Mnras, 2020

Since then, the transformation of the Northern Cross has proceeded by successive steps. After the first 8 cylinders it has gone to 16, this year it has reached 32, and by 2023 the update of the entire branch should be completed. The modernization of the structure also includes the installation of optical fiber for the transport of the signal, which replaced the old coaxial cables, and of new more powerful and efficient machines inside the control room. Technological innovations begin to bear fruit last year , when six cylinders of the north-south branch manage to capture radio pulses from pulsar B0329 + 54, a neutron star that rotates in less than a second. A quick signal, therefore, not too dissimilar to that of a Frb. This observation gives confidence to the small team, which in a year continues to accumulate data by chasing different sources of Frb, thanks also to the technical support of several colleagues working at the Medicine station and collaborators at the universities of Oxford and Malta.

Until that fateful March 12, when Pilia’s analysis software confirms the first detection . “I was actually in Srt, I was making other observations, in the meantime I was looking at the data from the Cross,” says the researcher. Meanwhile, in Bologna, Bernardi is attending a meeting remotely. Out of the corner of her eye she glimpses an email sent by her colleague from Cagliari, who she imagines contains a figure attached to it to be included in a document in preparation, precisely on the observation program of the Northern Cross. But he doesn’t have time to open it. Then a message on Whatsapp: “Didn’t you see the email?”

«When I opened the email I immediately called Maura and in the Skype call we added Germano, Giovanni and Giuseppe», recalls Bernardi. Magically, the sparkling wine was also ready. “I took this bottle that I had been keeping in the fridge for some time and we made a big toast.”

Left: Germano Bianchi learns from Maura Pilia about the first detection of a fast radio burst by the Northern Cross. Right: Bianchi, Pilia and Gianni Bernardi celebrate the result on Skype.

Data analysis continues. The telescope probably picked up some other bursts even before last March, but the traces are still hidden in the mass of data to be processed. A job that cannot overlap with the observational one, since the machine that acquires the data is the same one used by the team to analyze it. Frb are not uncommon: Chime, who looks at the whole sky every day – ten minutes per portion of the sky – has recorded a thousand in about two years of observations , many of which are burstsrepeated from the same sources. Of course, it is a somewhat different telescope from the Medicine one, which involves more staff and more substantial funding. Now, with the first observational feedback behind them, the researchers hope to be able to create a group dedicated to the study of Frb with the Northern Cross.

The biggest story

Sometimes, even here on Earth, certain stories set in motion well before they catapult into the lives of some of their protagonists. This is even more true in the cosmos, where light signals travel for millions, billions of years at 300,000 kilometers per second before reaching the telescopes and astronomers who, by interpreting those signals, try to decipher their origin. This is precisely the case of this story, which does not begin in March 2021 with the arrival of the signal in Medicine, nor in January 2020 with the announcement of the frequency of this Frb, nor in 2006 with the relaunch of the Northern Cross or even in 1964 with its inauguration. This story begins about 485 million years ago , when on Earth it was the dawn of the Ordovician geological period, was Paleozoic. No mammals had appeared yet, no birds or reptiles, and not even many of the plants we know today. Life on our planet was mainly aquatic, and the first vertebrate organisms were appearing in the underwater depths. At the same time, in the periphery of the spiral galaxy Sdss J015800.28 + 654253.0, an unidentified celestial body, possibly part of a binary system, emitted a rapid flash, one of many, in one of its regular cycles, punctuated every 16. , 35 rotations of a distant, insignificant rocky planet, the third orbiting a star much later to be called the Sun.

Image of the host galaxy of Frb 180916 (center) acquired with the Gemini-North telescope of 8 meters in Hawaii. The location of the fast radio flash in the spiral arm of the galaxy is marked with a green circle. Credits: Gemini Observatory / Nsf OiarLab / Aura

“This spring is very interesting, it is very close,” notes Pilia. Astronomically close, of course. Compared to other distant billions of light-years, the galaxy hosting the source of Frb 180916 is not terribly distant: “only” 485 million light-years. “It was quickly located and then it’s very active, so we can study it in great detail, at least hopefully.” The team continues to keep an eye on it with the Northern Cross, along with other sources of Frb, also in view of the possibility of observing these objects on the opposite side of the electromagnetic spectrum, in X and gamma rays. Among these sources there is also the only known in our galaxy, the one that has made so much talk of a possible correlation between fast radio bursts and magnetars over the last year.. And while the physical mechanisms behind this phenomenon remain unknown, the entry of a new telescope into such a compelling line of research can only be welcome.

“We understood that we had made a piece of history, that we had reached an important milestone “, comments Bernardi. Shortly after the group Skype call, the researcher sends the figure showing the detection to one of the collaborators of the project, Giancarlo Setti , professor emeritus of the University of Bologna, involved in the realization of the Northern Cross since the first developments . «He told me:“ Beautiful, to be framed ”. Afterwards he also told me: “In fact we knew from the beginning that it was possible”. I replied: “Let’s say that now that now that we have seen it, I am more convinced” ».

To know more:

Featured image: The Medicina station, with the two perpendicular branches of the Northern Cross in the center. On the left, the 32-meter satellite dish. Credits: G. Bianchi


Provided by INAF

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