Gamma-ray Bursts: Duration Isn’t Everything (Cosmology)

The discovery of supernova emission associated with the short gamma-ray burst Grb 200826A seems to contradict current scenarios on the origin of these extreme phenomena. But how much can we really rely on observed duration to locate the progenitor of a Grb? We host an editorial on the subject by Lorenzo Amati, research manager at INAF in Bologna, author of an article “News & Views” published today in the same issue of Nature Astronomy which reports the discovery

Despite enormous observational efforts, it took a good thirty years since their discovery in the 1960s to unravel the cosmological origin of Grb , flashes of X / gamma photons detected about once a day from random directions and so intense that they obscure any another high energy source in the sky. It then took another twenty years of effort by many space and ground-based telescopes, as well as intense theoretical work and highly sophisticated numerical simulations, to build and consolidate the current scenario for their ancestors: collapse of the core of peculiar very massive stars. for the longer ones and coalescence of a binary system formed by two neutron stars or a neutron star and a black hole for the shorter ones.

In this context, the discovery, reported in today’s issue of Nature Astronomy , by Tomas Ahumada , Bin-Bin Zhang and their collaborators, of the association of a short Grb with a stellar explosion is apparently unsettling . So do we need to reconsider our main paradigm on Grb? To understand this, let’s take a few steps back.

The growing evidence of the existence of two main classes of Grb – the short (from a few tens of milliseconds up to 1-2 seconds) and the long ones (typically, from a few seconds to a few minutes) -, based on the bi-modal distribution of durations of these events, was one of the first substantial advances in our understanding of these exceptional but elusive phenomena. A further very important step forward in this field of study occurred towards the end of the 1990s, when the first systematic localizations with the precision of a few arc minutes made it possible to discover that long flashes are followed by a weaker emission. , called afterglow, which decays according to roughly a power law and is observable from X-rays to radio waves. The localization up to a few arc seconds and the spectroscopy of the afterglow emission in the optical and in the near infrared by large terrestrial and Hst telescopes led to the confirmation that these events come from cosmological distances (at least up to redshift of about 9-10, or a few hundred million years from the “Big-Bang”), as well as the first identifications and characterizations of their host galaxies, up to the direct detection of a peculiar supernova (Sn) of type Ib / c associated with Grb 980425 .

This impressive wealth of discoveries provided strong support for the hypothesis that long Grbs are produced by the collapse of the nucleus of peculiar massive stars, a scenario already postulated in the 1970s, able to explain the long duration and the very high radiated energy ( up to at least 10 53 ergs, or about as much as a star like the Sun radiates in 10 billion years). This scenario has been further strengthened in the last twenty years by the detection, in nearby events, of optical-infrared emission with spectral characteristics similar to those of Sn 1998bw or, more generally, typical of type Ib / c supernovae, superimposed on the typical afterglow light curves. Other evidence supporting this origin for long Grbs includes their typical location in star-forming regions in their host galaxies, evidence of a metal  enriched circum-burst environment, and their redshift distribution , which roughly follows that of the star formation rate in the universe.

On the other hand, since the early 2000s, we have begun to reveal and characterize the afterglow emission of short Grbs too, learning that the redshift distributionof these events extends to much lower values ​​than that of the long ones and shows no relationship with the evolution of the star formation rate in the universe, that their typical energy released is about two orders of magnitude lower and that often they are found in the outermost regions of their host galaxies, with no association with star-forming regions. Together with their duration, these properties have increasingly supported the hypothesis that short Grbs originated from the merger of binary systems consisting of two neutron stars (Ns-Ns) or a neutron star and a black hole (Ns- Bh). This scenario was finally confirmed in a direct and spectacular way by the historical revelation of a short gamma-ray burst (Grb 170817A) associated with thefirst gravitational wave signal produced by the coalescence of a binary system of neutron stars (Gw 170817) ever detected (by Ligo / Virgo).

The observations and analyzes reported today by Ahumada et al . and Zhang et al. in Nature Astronomy they seem, however, to undermine this standard scenario on the progenitors of Grb. For the first time, in fact, an excess compared to the normal afterglow emission was revealed even for a short gamma-ray burst (Grb 200826A)in the optical and near infrared, excess showing the typical photometric and temporal evolution properties of supernovae associated with long Grb. At first glance, this result clearly questions our current understanding of the GRB phenomenon. However, a more global view of its properties, combined with several additional evidences that have emerged in recent years, show that this event, although peculiar and of great interest, may not be so “special”.

Lorenzo Amati, research manager at INAF OAS Bologna, author of the “News & Views” on gamma-ray bursts published today in Nature Astronomy © INAF

First of all, the duration of about 1 second, while making Grb 200826A by far the shortest gamma-ray bursts with evidence of supernova association, falls within the range where the duration distributions of short and long Grb overlap. again, potentially making this an extreme event in the short-lived queue of the distribution of long GRB durations. The position in the spectral hardness plane vs. duration further supports a classification of this event as “short” Grb, but the probability that Grb 200826A actually belongs to the “long” class is not negligible, as reported by Ahuamada et al . and Zhang et al. Furthermore, theoretical considerations and numerical simulations show that the duration of a Grb produced by the collapse of the core of a supermassive star, which depends both on the time during which the “central engine” (for example, an accreting black hole) is to work that from the time taken by the relativistic jet that then produces the lightning to exit the stellar envelope, can be even shorter than 0.5 seconds.

This is why, in reality, duration is increasingly considered only one of the indicators of the origin of a Grb. So much so that, more and more frequently, instead of “long” or “short” Grb, we speak of “Type I” events, those produced by the coalescence of binary systems Ns-Ns or Ns-Bh, and flashes of ” Type II ”, or the flashes produced by the core-collapse of super-massive stars. In addition to duration, spectral hardness, location in the host galaxy and properties of the host galaxy, the indicators used to discriminate these two classes of events include time-lag , i.e. the delay of the emission peak as a function of the energy band, and the relationship between the photon energy at which the peak of the Grb energy spectrum occurs (peak energy, or Ep, i) and the equivalent isotropic radiated energy ( Eiso ). The analysis of these indicators for Grb 200826A shows that, despite its very short duration, it actually belongs to the Type II class. Thus, the association of this event with a type Ib / c supernova is no longer surprising, but instead provides strong confirmation of the new paradigm’s efficiency to identify the progenitor of a Grb. In this respect, however, Grb 200826A is the opposite case of another famous and demanding event, Grb 060614 , which was a technically “long” Grb (lasting several tens of seconds) for which there was strong evidence of none association with a supernova. Even in that case it was thanks to indicators other than duration, for example time-lagand placement in the Ep plan , i vs. Eiso , that it was possible to overcome the apparent paradox and classify the event as a Type I gamma-ray burst.

The extended sensitivity and energy bandwidth of next-generation Grb detectors (e.g. those aboard the Franco-Chinese Svom mission , which will be launched next year), combined with improved follow-up capabilities provided by future observers ( e.g. example, Elt or Tmt in the optical / infrared, Ska in the radio, Athena in the X, Cta at very high energies), will allow us to definitively go beyond the classification of Grb based on duration and probably to identify a richer variety of progenitors which extremely magnetized neutron stars ( magnetar) and the possible connection with Fast Radio Burst ( Frb ) sources for short Grb, and different types of progenitor stars (possibly including those of population-III ) and explosive mechanisms ( core-collapse , pair instability) for those long. A better understanding of the subclasses and ancestors of Grb is also crucial for the growing relevance of Type II events for cosmology (study of the early universe and cosmological parameters) and Type I events for multi-messenger astrophysics ( as demonstrated by the extraordinary case of Gw 170817 / Grb 170817A).

Featured image: At the center, the bi-modal distribution of the Grb durations, flanked by the images of two typical host galaxies of the “short” and “long” Grb and the location of these events in them. The two panels at the top show the different placement of “short” and “long” Grb in the observed peak energy planes (Ep) vs. duration (T90) and intrinsic peak energy (Ep, i) –vs. radiated energy (Eiso). The two panels. The two panels below illustrate the two different scenarios for the origin of “short” and “long” (or, better, “Type I” and “Type II”, respectively). Credits: Nature Astronomy, Volume 5, Issue 7, by Springer Nature (republished with kind permission of the publisher) © INAF

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