Distribution Of Gamma-ray Bursts Research Articles

Investigation of redshift- and duration-dependent clustering of gamma-ray bursts

Gamma-ray bursts (GRBs) are detectable out to very large distances and as such are potentially powerful cosmological probes. Historically, the angular distribution of GRBs provided important information about their origin and physical properties. As a general population, GRBs are distributed isotropically across the sky. However, there are published reports that once binned by duration or redshift, GRBs display significant clustering. We have studied the redshift- and duration-dependent clustering of GRBs using proximity measures and kernel density estimation. Utilizing bursts detected by BATSE, Fermi/GBM and Swift/BAT, we found marginal evidence for clustering in very short duration GRBs lasting less than 100 ms. Our analysis provides little evidence for significant redshift-dependent clustering of GRBs.

THE TWO-POINT CORRELATION FUNCTION OF GAMMA-RAY BURSTS

In this paper, we examine the spacial distribution of gamma-ray bursts (GRBs) using a sample of 373 objects. We subdivide the GRB data into two redshift intervals over the redshift range $0<z< 6.7$. We measure the two-point correlation function (2PCF), $\xi(r)$ of the GRBs. In determining the separation distance of the GRB pairs, we consider two representative cosmological models: a cold dark matter universe plus a cosmological constant $\Lambda$, with $(\Omega_{{\rm m}}, \Omega_{{\rm \Lambda}})=(0.28,0.72)$ and an Einstein-de Sitter (EdS) universe, with $(\Omega_{{\rm m}}, \Omega_{{\rm \Lambda}})=(1,0)$. We find a $z$-decreasing correlation of the GRB distribution, which is in agreement with the predictions of the current structure formation theory. We fit a power-law model $\xi(r)=(r/r_0)^{-\gamma}$ to the measured $\xi(r)$ and obtain an amplitude and slope of $r_0= 1235.2 \pm 342.6~h^{-1}$ Mpc and $\gamma = 0.80\pm 0.19 $ ($1\sigma$ confidence level) over the scales $r=200$ to $10^4~h^{-1}$ Mpc. Our result provide a supplement to the measurement of matter correlation on large scales, while the matter distribution below $200~h^{-1}$ Mpc is usually described by the correlation function of galaxies.

Redshift distribution of gamma-ray bursts: Evidence for the long GRBs sources population nonuniformity

The results of redshifts distribution investigation for 275 γ-ray bursts (GRBs) with defined duration from the Swift experiment catalogue are discussed. Detailed investigation of the redshift distribution shows that the former has at least two maxima at the 95% significance level in contradiction with the similar distribution for uniform samples in our Metagalaxy. This pattern (demonstrating the nonuniformity of the long GRB sources population) persists even if short GRBs with durations of t90 < 2 s without redshift correction, which corresponds to the classical definition of short GRB events, are excluded from consideration and if the analysis uses any sample of a smaller size for previous years. Preliminary investigation of the GRBs distribution on redshift and duration also reveals another subgroup that can be interpreted as intermediate GRB events. This subgroup persists if the analysis is confined to the distribution of those GRBs whose redshifts z are determined using one method only, e.g., from analyzing afterglow absorption lines.

The star formation history inferred from long gamma-ray bursts with high pseudo-redshifts

By employing a simple semi-analytical star formation model where the formation rates of Population (Pop) I/II and III stars can be calculated, respectively, we account for the number distribution of gamma-ray bursts (GRBs) with high pseudo-redshifts that was derived from an empirical luminosity-indictor relationship. It is suggested that a considerable number of Pop III GRBs could exist in the present sample of Swift GRBs. By further combining the implication for the star formation history from the optical depth of the CMB photons, it is also suggested that only a very small fraction (∼0.6%) of Pop III GRBs could have triggered the Swift BAT. These results could provide an useful basis for estimating future detectability of Pop III stars and their produced transient phenomena.

Statistics of the Planck CMB Signal in direction of gamma-ray bursts from the BATSE and BeppoSAX catalogs

Distribution of gamma-ray bursts (GRBs) from catalogs of the BATSE and BeppoSAX space observatories relative to the cosmic microwave background (CMB) data by Planck space mission is studied. Three methods were applied for data analysis: 1) a histogram of CMB signal values in GRB directions, 2) mosaic correlation maps calculated for GRB locations and CMB distribution, 3) calculation of an average response in the area of "an average population GRB" on the CMB map. A correlation between GRB locations and CMB fluctuations was discovered which can be interpreted as systematic effects in the process of observations. Besides, in averaged areas of CMB maps a difference between distributions of average fluctuations for short and long GRBs was detected, which can be caused by different natures of these events.

SIMILAR RADIATION MECHANISM IN GAMMA-RAY BURSTS AND BLAZARS: EVIDENCE FROM TWO LUMINOSITY CORRELATIONS

Active galactic nuclei (AGNs) and gamma-ray bursts (GRBs) are powerful astrophysical events with relativistic jets. In this Letter the broadband spectral properties are compared between GRBs and the well-observed blazars. The distribution of GRBs are consistent with the well-known blazar sequence including the $\nu L_\nu(5\rm GHz)-\alpha_{\rm RX}$ and $\nu L_\nu(5\rm GHz)-\nu_{\rm peak}$ correlations, where $\alpha_{\rm RX}$ is defined as the broadband spectral slope in radio-to-X-ray bands, and $\nu_{\rm peak}$ is defined as the spectral peak frequency. Moreover, GRBs occupy the low radio luminosity end of these sequences. These two correlations suggest that GRBs could have a similar radiation process with blazars both in the prompt emission and afterglow phases, i.e., synchrotron radiation.

Possible structure in the GRB sky distribution at redshift two

Context. Research over the past three decades has revolutionized cosmology while supporting the standard cosmological model. However, the cosmological principle of Universal homogeneity and isotropy has always been in question, since structures as large as the survey size have always been found each time the survey size has increased. Until 2013, the largest known structure in our Universe was the Sloan Great Wall, which is more than 400 Mpc long located approximately one billion light years away. Aims. Gamma-ray bursts are the most energetic explosions in the Universe. As they are associated with the stellar endpoints of massive stars and are found in and near distant galaxies, they are viable indicators of the dense part of the Universe containing normal matter. The spatial distribution of gamma-ray bursts can thus help expose the large scale structure of the Universe. Methods. As of July 2012, 283 GRB redshifts have been measured. Subdividing this sample into nine radial parts, each containing 31 GRBs, indicates that the GRB sample having 1.6 < z < 2.1 differs significantly from the others in that 14 of the 31 GRBs are concentrated in roughly 1/8 of the sky. A two-dimensional Kolmogorov-Smirnov test, a nearest-neighbour test, and a Bootstrap Point-Radius Method explore the significance of this clustering. Results. All tests used indicate that there is a statistically significant clustering of the GRB sample at 1.6 < z < 2.1. Furthermore, this angular excess cannot be entirely attributed to known selection biases, making its existence due to chance unlikely. Conclusions. This huge structure lies ten times farther away than the Sloan Great Wall, at a distance of approximately ten billion light years. The size of the structure defined by these GRBs is about 2000-3000 Mpc, or more than six times the size of the Sloan Great Wall.

On the observed duration distribution of gamma-ray bursts from collapsars

The duration of the prompt emission of long gamma-ray bursts is generally considered to be fairly similar to the duration of the activity of the engine in the center of the progenitor star. Here, we investigate the relation between the duration of the engine activity and that of the observed light curve, using inputs from both numerical simulations and observations. We find that the observed burst duration is a good proxy for the engine duration after the time necessary for the jet to break out the star's surface is subtracted. However, the observed duration is a function of the viewing angle and can be significantly shorter than the duration of the engine activity. We also show that the observed, redshift-corrected burst duration evolves only moderately with redshift for both observations and synthetic light curves. We conclude that the broad distribution of the observed duration of long BATSE gamma-ray bursts is mostly accounted for by an engine lasting ~20 s, the dispersion being due to viewing and redshift effects. Our results do not rule out the existence of engines with very long duration. However, we find that they are constrained to be a small minority of the BATSE detected bursts.

A redshift–observation time relation for gamma-ray bursts: evidence of a distinct subluminous population

We show that the redshift and peak flux distributions of gamma-ray bursts (GRBs) have an observation time dependence that can be used to discriminate between different burst populations. We demonstrate how observation time relations can be derived from the standard integral distributions and that they can differentiate between GRB populations detected by both the Burst and Transient Source Experiment (BATSE) and Swift satellites. Using Swift data, we show that a redshift–observation time relation (log Z–log T) is consistent with both a peak flux–observation time relation (log P–log T) and a standard log N–log P brightness distribution. As the method depends only on rarer small-z events, it is invariant to high-z selection effects. We use the log Z–log T relation to show that subluminous GRBs are a distinct population occurring at a higher rate of the order of 150+ 180− 90 Gpc− 3 yr− 1. Our analysis suggests that GRB 060505 – a relatively nearby GRB observed without any associated supernova – is consistent with a subluminous population of bursts. Finally, we show that our relations can be used as a consistency test for some of the proposed GRB spectral energy correlations.

Neutrinos from GRBs in the Fermi Era

We investigate the consequences of the redshift distribution of gamma-ray bursts on diffuse gamma-ray burst neutrino fluxes using a Monte Carlo simulation. We determine the systematic error introduced by the extrapolation from small samples to the quasi-diffuse flux. In addition, we reanalyze the computation of neutrino spectra from photon spectra measured for example by the Fermi Gamma-Ray Burst Monitor.

Angular Distribution of GRBs

We studied the complete randomness of the angular distribution of BATSE gamma-ray bursts (GRBs). Based on their durations and peak fluxes, we divided the BATSE sample into 5 subsamples (short1, short2, intermediate, long1, long2) and studied the angular distributions separately. We used three methods to search for non-randomness in the subsamples: Voronoi tesselation, minimal spanning tree, and multifractal spectra. To study any non-randomness in the subsamples we defined 13 test-variables (9 from Voronoi tesselation, 3 from the minimal spanning tree and one from the multifractal spectrum). We made Monte Carlo simulations taking into account the BATSE’s sky-exposure function. We tested therandomness by introducing squared Euclidean distances in the parameter space of the test-variables. We recognized that the short1, short2 groups deviate significantly (99.90%, 99.98%) from the fully random case in the distribution of the squared Euclidean distances but this is not true for the long samples. In the intermediate group, the squared Euclidean distances also give significant deviation (98.51%).

Rest-frame properties of 32 gamma-ray bursts observed by theFermiGamma-ray Burst Monitor

Aims: In this paper we study the main spectral and temporal properties of gamma-ray bursts (GRBs) observed by Fermi/GBM. We investigate these key properties of GRBs in the rest-frame of the progenitor and test for possible intra-parameter correlations to better understand the intrinsic nature of these events. Methods: Our sample comprises 32 GRBs with measured redshift that were observed by GBM until August 2010. 28 of them belong to the long-duration population and 4 events were classified as short/hard bursts. For all of these events we derive, where possible, the intrinsic peak energy in the $\nu F_{\nu}$ spectrum (\eprest), the duration in the rest-frame, defined as the time in which 90% of the burst fluence was observed (\tninetyrest) and the isotropic equivalent bolometric energy (\eiso). Results: The distribution of \eprest has mean and median values of 1.1 MeV and 750 keV, respectively. A log-normal fit to the sample of long bursts peaks at ~800 keV. No high-\ep population is found but the distribution is biased against low \ep values. We find the lowest possible \ep that GBM can recover to be ~ 15 keV. The \tninetyrest distribution of long GRBs peaks at ~10 s. The distribution of \eiso has mean and median values of $8.9\times 10^{52}$ erg and $8.2 \times 10^{52}$ erg, respectively. We confirm the tight correlation between \eprest and \eiso (Amati relation) and the one between \eprest and the 1-s peak luminosity ($L_p$) (Yonetoku relation). Additionally, we observe a parameter reconstruction effect, i.e. the low-energy power law index $\alpha$ gets softer when \ep is located at the lower end of the detector energy range. Moreover, we do not find any significant cosmic evolution of neither \eprest nor \tninetyrest.

Cosmological effects on the observed flux and fluence distributions of gamma-ray bursts

AbstractSeveral claims have been put forward that an essential fraction of long-duration BATSE gamma-ray bursts should lie at redshifts larger than 5. This point-of-view follows from the natural assumption that fainter objects should, on average, lie at larger redshifts. However, redshifts larger than 5 are rare for bursts observed by Swift. The purpose of this article is to show that the most distant bursts in general need not be the faintest ones. We derive the cosmological relationships between the observed and emitted quantities, and arrive at a prediction that is tested on the ensembles of BATSE, Swift and Fermi bursts. This analysis is independent on the assumed cosmology, on the observational biases, as well as on any gamma-ray burst model. We arrive to the conclusion that apparently fainter bursts need not, in general, lie at large redshifts. Such a behaviour is possible, when the luminosities (or emitted energies) in a sample of bursts increase more than the dimming of the observed values with redshift. In such a case dP(z)/dz &gt; 0 can hold, where P(z) is either the peak-flux or the fluence. This also means that the hundreds of faint, long-duration BATSE bursts need not lie at high redshifts, and that the observed redshift distribution of long Swift bursts might actually represent the actual distribution.

Cosmological effects on the observed flux and fluence distributions of gamma-ray bursts: Are the most distant bursts in general the faintest ones?

Several claims have been put forward that an essential fraction of long-duration BATSE gamma-ray bursts should lie at redshifts larger than 5. This point-of-view follows from the natural assumption that fainter objects should, on average, lie at larger redshifts. However, redshifts larger than 5 are rare for bursts observed by Swift, seemingly contradicting the BATSE estimates. The purpose of this article is to clarify this contradiction. We derive the cosmological relationships between the observed and emitted quantities, and we arrive at a prediction that can be tested on the ensembles of bursts with determined redshifts. This analysis is independent on the assumed cosmology, on the observational biases, as well as on any gamma-ray burst model. Four different samples are studied: 8 BATSE bursts with redshifts, 13 bursts with derived pseudo-redshifts, 134 Swift bursts with redshifts, and 6 Fermi bursts with redshifts. The controversy can be explained by the fact that apparently fainter bursts need not, in general, lie at large redshifts. Such a behaviour is possible, when the luminosities (or emitted energies) in a sample of bursts increase more than the dimming of the observed values with redshift. In such a case dP(z)/dz > 0 can hold, where P(z) is either the peak-flux or the fluence. All four different samples of the long bursts suggest that this is really the case. This also means that the hundreds of faint, long-duration BATSE bursts need not lie at high redshifts, and that the observed redshift distribution of long Swift bursts might actually represent the actual distribution.

AN EVOLVING STELLAR INITIAL MASS FUNCTION AND THE GAMMA-RAY BURST REDSHIFT DISTRIBUTION

Recent studies suggest that Swift gamma-ray bursts (GRBs) may not trace an ordinary star formation history. Here we show that the GRB rate turns out to be consistent with the star formation history with an evolving stellar initial mass function (IMF). We first show that the latest Swift sample of GRBs reveals an increasing evolution in the GRB rate relative to the ordinary star formation rate at high redshifts. We then assume only massive stars with masses greater than the critical value to produce GRBs, and use an evolving stellar IMF suggested by Dav\'{e} (2010) to fit the latest GRB redshift distribution. This evolving IMF would increase the relative number of massive stars, which could lead to more GRB explosions at high redshifts. We find that the evolving IMF can well reproduce the observed redshift distribution of Swift GRBs.

Can past gamma-ray bursts explain both INTEGRAL and ATIC/PAMELA/Fermi anomalies simultaneously?

Gamma-ray bursts (GRBs) have been invoked to explain both the 511 keV emission from the Galactic bulge and the high-energy positron excess inferred from the ATIC, PAMELA, and Fermi data. While independent explanations can be responsible for these phenomena, we explore the possibility of their common GRB-related origin by modeling the GRB distribution and estimating the rates. For an expected Milky Way long GRB rate, neither of the two signals is generic; the local excess requires a 2% coincidence while the signal from the Galactic center requires a 20% coincidence with respect to the timing of the latest GRB. The simultaneous explanation requires a 0.4% coincidence. Considering the large number of statistical ``trials'' created by multiple searches for new physics, the coincidences of a few percent cannot be dismissed as unlikely. Alternatively, both phenomena can be explained by GRB if the Galactic rate is higher than expected. We also show that a similar result is difficult to obtain assuming a simplified short GRB distribution.

Is sky distribution of gamma-ray bursts random?

We investigate the full randomness of the angular distribution of gamma-ray bursts (GRBs) detected by the BATSE (the Burst and Transient Source Experiment). We divided the BATSE sample into 5 subsamples (short1, short2, intermediate, long1, long2 based on their durations and peak fluxes and studied the angular distributions separately. We used three methods, the Voronoi tesselation, minimal spanning tree and multifractal spectra to search for non-randomness in the subsamples. To investigate the eventual non-randomness in the subsamples we defined 13 test-variables (9 from the Voronoi tesselation, 3 from the minimal spanning tree and one from the multifractal spectrum). We performed Monte Carlo simulations taking into account the BATSE sky-exposure function. We tested the randomness by introducing squared Euclidean distances in the parameter space of the test-variables. We concluded that the short1, short2 groups deviate significantly (99.90%, 99.98%) from the full randomness in the distribution of the squared Euclidean distances, however, it is not the case with the long subsamples. In the intermediate group, the squared Euclidean distances also yield a significant deviation (98.51%).

Duration distributions for different softness groups of gamma-ray bursts

Gamma-ray bursts (GRBs) are divided into two classes according to their durations. We investigate if the softness of bursts plays a role in the conventional classification of the objects. We employ the BATSE (Burst and Transient Source Experiment) catalog and analyze the duration distributions of different groups of GRBs associated with distinct softness. Our analysis reveals that the conventional classification of GRBs with the duration of bursts is influenced by the softness of the objects. There exists a bimodality in the duration distribution of GRBs for each group of bursts and the time position of the dip in the bimodality histogram shifts with the softness parameter. Our findings suggest that the conventional classification scheme should be modified by separating the two well-known populations in different softness groups, which would be more reasonable than doing so with a single sample. According to the relation between the dip position and the softness parameter, we get an empirical function that can roughly set apart the short-hard and long-soft bursts: SP = (0.100 ± 0.028)T 90 − (0.85 ± 0.18) , where SP is the softness parameter adopted in this paper.

Modelling gamma-ray burst observations by Fermi and MAGIC including attenuation due to diffuse background light

Gamma rays from extragalactic sources are attenuated by pair-production interactions with diffuse photons of the extragalactic background light (EBL). Gamma-ray bursts (GRBs) are a source of high-redshift photons above 10 GeV, and could be therefore useful as a probe of the evolving UV background radiation. In this paper, we develop a simple phenomenological model for the number and redshift distribution of gamma-ray bursts that can be seen at GeV energies with the Fermi satellite and MAGIC atmospheric Cherenkov telescope. We estimate the observed number of gamma rays per year, and show how this result is modified by considering interactions with different realizations of the evolving EBL. We also discuss the bright Fermi GRB 080916C in the context of this model. We find that the LAT on Fermi can be expected to see a small number of photons above 10 GeV each year from distant GRBs. Annual results for ground-based instruments like MAGIC are highly variable due to the low duty cycle and sky coverage of the telescope. However, successfully viewing a bright or intermediate GRB from the ground could provide hundreds of photons from high redshift, which would almost certainly be extremely useful in constraining both GRB physics and the high-redshift EBL.

LOW-RESOLUTION SPECTROSCOPY OF GAMMA-RAY BURST OPTICAL AFTERGLOWS: BIASES IN THE SWIFT SAMPLE AND CHARACTERIZATION OF THE ABSORBERS

We present a sample of 77 optical afterglows (OAs) of Swift detected gamma-ray bursts (GRBs) for which spectroscopic follow-up observations have been secured. Our first objective is to measure the redshifts of the bursts. For the majority (90%) of the afterglows, the redshifts have been determined from the spectra. We provide line lists and equivalent widths (EWs) for all detected lines redward of Lyα covered by the spectra. In addition to the GRB absorption systems, these lists include line strengths for a total of 33 intervening absorption systems. We discuss to what extent the current sample of Swift bursts with OA spectroscopy is a biased subsample of all Swift detected GRBs. For that purpose we define an X-ray-selected statistical sample of Swift bursts with optimal conditions for ground-based follow-up from the period 2005 March to 2008 September; 146 bursts fulfill our sample criteria. We derive the redshift distribution for the statistical (X-ray selected) sample and conclude that less than 18% of Swift bursts can be at z > 7. We compare the high-energy properties (e.g., γ-ray (15-350 keV) fluence and duration, X-ray flux, and excess absorption) for three subsamples of bursts in the statistical sample: (1) bursts with redshifts measured from OA spectroscopy; (2) bursts with detected optical and/or near-IR afterglow, but no afterglow-based redshift; and (3) bursts with no detection of the OA. The bursts in group (1) have slightly higher γ-ray fluences and higher X-ray fluxes and significantly less excess X-ray absorption than bursts in the other two groups. In addition, the fractions of dark bursts, defined as bursts with an optical to X-ray slope βOX 39% in group (3). For the full sample, the dark burst fraction is constrained to be in the range 25%-42%. From this we conclude that the sample of GRBs with OA spectroscopy is not representative for all Swift bursts, most likely due to a bias against the most dusty sight lines. This should be taken into account when determining, e.g., the redshift or metallicity distribution of GRBs and when using GRBs as a probe of star formation. Finally, we characterize GRB absorption systems as a class and compare them to QSO absorption systems, in particular the damped Lyα absorbers (DLAs). On average GRB absorbers are characterized by significantly stronger EWs for H I as well as for both low and high ionization metal lines than what is seen in intervening QSO absorbers. However, the distribution of line strengths is very broad and several GRB absorbers have lines with EWs well within the range spanned by QSO-DLAs. Based on the 33 z > 2 bursts in the sample, we place a 95% confidence upper limit of 7.5% on the mean escape fraction of ionizing photons from star-forming galaxies. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, under programs 275.D-5022 (PI: Chincarini), 075.D-0270 (PI: Fynbo), 077.D-0661 (PI: Vreeswijk), 077.D-0805 (PI: Tagliaferri), 177.A-0591 (PI: Hjorth), 078.D-0416 (PI: Vreeswijk), 079.D-0429 (PI: Vreeswijk), 080.D-0526 (PI: Vreeswijk), 081.A-0135 (PI: Greiner), 281.D-5002 (PI: Della Valle), and 081.A-0856 (PI: Vreeswijk). Also based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Some of the data obtained herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck foundation.