A three-dimensional step towards sorting out the GRB zoo

by Lori White

Maria Dainotti. (Courtesy M. Dainotti.)

 

 

 

A powerful tool for characterizing and classifying gamma-ray bursts (GRBs) has recently been presented by an international team of researchers led by KIPAC member Dr. Maria Dainotti (Marie Curie outgoing Fellow at INAF, Italy and Stanford University and assistant professor at Jagiellonian University, Poland). 

The work, which has been published in the Astrophysical Journal (arXiv link), is a statistical analysis of the properties of the still-mysterious GRBs, aimed at determining the physical origin of these systems and ultimately allowing their use as tracers of the expansion history of the Universe. The tool builds on work presented in previous KIPAC blogpost Swift GRBs: A 3D step toward standard candles

GRBs are the most powerful high-energy events known in the Universe, lasting anywhere between a few seconds to a few hours. During their short-lived phase of very high energy gamma ray emission (called the prompt phase), they emit the same amount of energy the Sun releases over its entire lifetime. Thus, they are detected out to enormous distances, some of them as far back until just shortly after the Big Bang, when the Universe was just about 4% of its current age. Despite having been observed for decades, there is still little definitively known about GRBs regarding the physical mechanisms which produce them. There is, in fact, no shortage of proposed origins: for example, the explosions of extremely massive stars, merger of two neutron stars, or the spin down of magnetized massive stars.

GRB origins make a satisfying puzzle for study, but the objects themselves could serve a larger purpose. Since they can be detected out to much earlier epochs than supernovae, determining their detailed physical characteristics—in particular the intrinsic luminosity of each distinct observed GRB—could give researchers the means to use GRBs to trace the expansion history of the Universe. They could provide the key to pinning down significantly more remote cosmological times than currently possible, allowing more precise testing of theories on the origin of the Universe.

Dainotti has shown that, by considering the details of the less energetic but much longer lasting X-ray afterglow, called the plateau phase, a sub-class of long GRBs can be defined such that a very tight correlation appears between the duration of the plateau phase, its luminosity, and the luminosity of the prompt phase. (See figure below.)

Fundamental plane of GRB characteristics. (Credit: M.G. Dainotti.)
The picture shows 183 GRBs in the L_a (luminosity at the end of the X-ray plateau), T_a (the rest frame duration of the plateau) and L_peak, (the peak luminosity in the prompt emission) space, with a plane, including GRBs with associated supernovae (cones),  X-ray flashes (spheres), Short with Extended Emission (cuboids), long GRBs (circles), and ultra-long GRBs (polyhedrons). Darker colors indicate data points above the plane, while lighter colors data points below the plane. (Credit: M.G. Dainotti.)

 

This three-parameter correlation pinpoints a plane graph in which the three axes of the graph, length, width, and height, represent these quantities. Building on previous work (Dainotti et al. 2013a and 2015b) done under the supervision of KIPAC professor Vahe’ Petrosian (which is related to even earlier work on selection bias; see Efron & Petrosian 1992), it has also been directly established that the parameters used in this study and their correlations (Lpeak-La and  La-Ta) are intrinsic to the physical systems studied, and not the result of selection or observational effects. Thus, it naturally follows that the 3D correlation is intrinsic to GRBs. It is not the result of selection bias.

In the current paper, the researchers have shown that by taking only GRBs presenting a close to constant X-ray plateau phase and subdividing the sample into categories, a fundamental plane appears. The plane identified by gold GRBs, the ones with very well-defined and not-so-steep X-ray plateaus, shows a very small scatter, suggesting its use for cosmological studies where it is essential to know the precise brightness of the cosmological tracers used, as well as identifying the average distance to this plane as a crucial parameter leading to the identification of distinct underlying physical processes behind the various classes treated. 

Distances from the fundamental plane of GRBs under study. (Courtesy M.G. Dianotti.)

A plot showing distances from the fundamental plane of different categories of GRBs. (Credit: M. Dainotti.)
A plot showing Gaussian fits of the distributions of the distances from the fundamental plane for GRBs of each category, including GRB-SNe (orange), XRF (blue), SEE (red), gold sample GRBs (purple), long GRBs (black). The perpendicular line shows the central value for of the gold sample for reference compared to the other categories. (Credit: M.G. Dainotti.)

 

“We see evidence for a different physical origin for short GRBs presenting extended emission from the other various classes," Dainotti says. Thus, the distance to the fundamental plane defined by the gold GRBs becomes a crucial tool to discern among GRB categories and lead to a more profound understanding of their nature.”  The figure above shows a statistical difference between the gold sample (black solid line) plane and the plane identified by short-with-extended-emission GRBs (red solid line). This last is of relevance to the nascent field of gravitational wave astronomy, where a distinct signal might hence be expected in connection to events having a short or long GRB association.

A detailed study of the evolution of the short GRBs which here constitute a different fundamental plane will follow soon (Petrosian & Dainotti in preparation).

This approach has been allowed by the growing sample of well-studied GRBs from both the Swift and the Fermi satellites, which now permits subdividing the available sample into increasingly detailed and ever more finely defined classes. The fact that the fundamental plane is also observed by the Gamma-ray Burst Monitor, the second of two primary instruments on Fermi, clearly shows the independence of this plane from the particular energy range chosen.

The key point is that the gold fundamental plane still remains with the smallest intrinsic scatter and thus it constitutes a step forward in the determination of a standard candle and in the use of this plane as a cosmological tool.

This is possible based on a previous study that showed that adding the 2D Dainotti relation, L_a-T_a, to other five GRB relations reduces the resulting confidence intervals on the inferred distance moduli by 14%. The sample of this 2D relation was composed of only 28 GRBs versus the 45 GRBs presented here. The intrinsic scatter of the 2D relation was 0.33 versus 0.30 of the current 3D relation. Thus, this increase of 61% in the sample size and 10% decrease in the scatter will allow a reduction in the inferred cosmological parameters if the 2D Dainotti relation is replaced with the current 3D gold fundamental plane, together with the other five GRB relations. 

The situation resembles walking through an impenetrable jungle while listening to the calls of unknown animals in the distance, Dainotti explained; a certain distinction between long and short calls will probably be evident, but then, within each of those broad classes, a wild assortment of sounds and animals will still remain. If, however, one begins a more subtle classification based on other finer features, class-specific groupings will appear within which much tighter and more meaningful trends will start to become evident.

Dr. Dainotti, together with the scientists in the current publication (Dr. Xavier Hernandez from the Institute of Astronomy, UNAM, Mexico, Dr. Sergey Postnikov, from Indiana University, Bloomington, Dr. Shigehiro Nagataki from RIKEN, Japan, Professor Paul Obrien and Professor Richard Willingale from Leicester University, UK, and Stephanie Striegel from San Jose State University, USA) are taking steps towards the identification of the diverse mixture of species which comprise the gamma-ray zoo, a very challenging program which will ultimately allow astrophysicists to determine the detailed physical mechanisms responsible for the many varieties of gamma ray bursts, and finally fulfill the promise of allowing their use as cosmological probes.

Notes:

  1. The paper "A Study of the Gamma-Ray Burst Fundamental Plane" by Dainotti, M. G., Hernandez,  X., Postnikov S., Nagataki S., Obrien, P., Willingale, R. Striegel, S. has been accepted for publication in the Astrophysical Journal and will appear in the October issue of the Astrophysical Journal, Volume 848, Issue 2, Article 88. The preprint can be downloaded from: http://adsabs.harvard.edu/abs/2017arXiv170404908D.
     
  2. Article from Oct 13, 2017 about this work in Le Scienze, the Italian version of Scientific American.
     
  3. M.G. D. acknowledges the Marie Curie Program: research leading to these results has received funding from the European Union Seventh Framework Program (FP7-2007/2013) under grant agreement no. 626267.

     (Based on a Oct 13, 2017 Press Release by INAF)