We report our observation of the short gamma-ray burst (GRB) GRB 170817A, associated to the binary neutron star merger gravitational wave (GW) event GW 170817, performed in the X-ray band with ...XMM-Newton 135 d after the event (on 29 December, 2017). We find evidence for a flattening of the X-ray light curve with respect to the previously observed brightening. This is also supported by a nearly simultaneous optical Hubble Space Telescope observation and successive X-ray Chandra and low-frequency radio observations recently reported in the literature. Since the optical-to-X-ray spectral slope did not change with respect to previous observations, we exclude that the change in the temporal evolution of the light curve is due to the passage of the cooling frequency: its origin must be geometric or dynamical. We interpret all the existing afterglow data with two models: i) a structured jet and ii) a jet-less isotropic fireball with some stratification in its radial velocity structure. Both models fit the data and predict that the radio flux must decrease simultaneously with the optical and X-ray emission, making it difficult to distinguish between them at the present stage. Polarimetric measurements and the rate of short GRB-GW associations in future LIGO/Virgo runs will be key to disentangle these two geometrically different scenarios.
The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of γ-rays, a gravitational-wave signal, and a transient optical-near-infrared source ...powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process). Such transients, named 'macronovae' or 'kilonovae', are believed to be centres of production of rare elements such as gold and platinum. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short γ-ray burst at redshift z = 0.356, although findings indicating bluer events have been reported. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational-wave source GW170817 and γ-ray burst GRB 170817A associated with a galaxy at a distance of 40 megaparsecs from Earth. Using a series of spectra from ground-based observatories covering the wavelength range from the ultraviolet to the near-infrared, we find that the kilonova is characterized by rapidly expanding ejecta with spectral features similar to those predicted by current models. The ejecta is optically thick early on, with a velocity of about 0.2 times light speed, and reaches a radius of about 50 astronomical units in only 1.5 days. As the ejecta expands, broad absorption-like lines appear on the spectral continuum, indicating atomic species produced by nucleosynthesis that occurs in the post-merger fast-moving dynamical ejecta and in two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03 to 0.05 solar masses of material, including high-opacity lanthanides.
We present a carefully selected sample of short gamma-ray bursts (SGRBs) observed by the Swift satellite up to 2013 June. Inspired by the criteria we used to build a similar sample of bright long ...GRBs (the BAT6 sample), we selected SGRBs with favourable observing conditions for the redshift determination on ground, ending up with a sample of 36 events, almost half of which with a redshift measure. The redshift completeness increases up to about 70 per cent (with an average redshift value of z = 0.85) by restricting to those events that are bright in the 15–150 keV Swift Burst Alert Telescope energy band. Such flux-limited sample minimizes any redshift-related selection effects, and can provide a robust base for the study of the energetics, redshift distribution and environment of the Swift bright population of SGRBs. For all the events of the sample, we derived the prompt and afterglow emission in both the observer and (when possible) rest frame and tested the consistency with the correlations valid for long GRBs. The redshift and intrinsic X-ray absorbing column density distributions we obtain are consistent with the scenario of SGRBs originated by the coalescence of compact objects in primordial binaries, with a possible minor contribution (∼10–25 per cent) of binaries formed by dynamical capture (or experiencing large natal kicks). This sample is expected to significantly increase with further years of Swift activity.
In this paper, we compute rest-frame extinctions for the afterglows of a sample of Swift gamma-ray bursts (GRBs) complete in redshift. The selection criteria of the sample are based on observational ...high-energy parameters of the prompt emission and therefore our sample should not be biased against dusty sight-lines. It is therefore expected that our inferences hold for the general population of GRBs. Our main result is that the optical/near-infrared extinction of GRB afterglows in our sample does not follow a single distribution. 87 per cent of the events are absorbed by less than 2 mag, and 50 per cent suffer from less than 0.3-0.4 mag extinction. The remaining 13 per cent of the afterglows are highly absorbed. The true percentage of GRB afterglows showing high absorption could be even higher since a fair fraction of the events without reliable redshift measurement are probably part of this class. These events may be due to highly dusty molecular clouds/star-forming regions associated with the GRB progenitor or along the afterglow line of sight, and/or due to massive dusty host galaxies. No clear evolution in the dust extinction properties is evident within the redshift range of our sample, although the largest extinctions are at z ∼ 1.5-2, close to the expected peak of the star formation rate. Those events classified as dark are characterized, on average, by a higher extinction than typical events in the sample. A correlation between optical/near-infrared extinction and hydrogen-equivalent column density based on X-ray studies is shown, although the observed N
H appears to be well in excess compared to those observed in the Local Group. Dust extinction does not seem to correlate with GRB energetics or luminosity.
We present a carefully selected sub-sample of Swift long gamma-ray bursts (GRBs) that is complete in redshift. The sample is constructed by considering only bursts with favorable observing conditions ...for ground-based follow-up searches, which are bright in the 15-150 keV Swift/BAT band, i.e., with 1-s peak photon fluxes in excess to 2.6 photons s super(-1) cm super(-2). The sample is composed of 58 bursts, 52 of them with redshift for a completeness level of 90%, while another two have a redshift constraint, reaching a completeness level of 95%. For only three bursts we have no constraint on the redshift. The high level of redshift completeness allows us for the first time to constrain the GRB luminosity function and its evolution with cosmic times in an unbiased way. We find that strong evolution in luminosity ( delta sub()l = 2.3 + or - 0.6) or in density ( delta sub()d = 1.7 + or - 0.5) is required in order to account for the observations. The derived redshift distributions in the two scenarios are consistent with each other, in spite of their different intrinsic redshift distributions. This calls for other indicators to distinguish among different evolution models. Complete samples are at the base of any population studies. In future works we will use this unique sample of Swift bright GRBs to study the properties of the population of long GRBs.
Abstract
The low distance, z = 0.0591, of GRB 100316D and its association with SN 2010bh represent two important motivations for studying this host galaxy and the GRB's immediate environment with the ...integral field spectrographs like Very Large Telescope/Multi-Unit Spectroscopic Explorer. Its large field of view allows us to create 2D maps of gas metallicity, ionization level and the star formation rate (SFR) distribution maps, as well as to investigate the presence of possible host companions. The host is a late-type dwarf irregular galaxy with multiple star-forming regions and an extended central region with signatures of on-going shock interactions. The gamma-ray burst (GRB) site is characterized by the lowest metallicity, the highest SFR and the youngest (∼20–30 Myr) stellar population in the galaxy, which suggest a GRB progenitor stellar population with masses up to 20–40 M⊙. We note that the GRB site has an offset of ∼660 pc from the most luminous SF region in the host. The observed SF activity in this galaxy may have been triggered by a relatively recent gravitational encounter between the host and a small undetected (LH α ≤ 1036 erg s−1) companion.
Aims. We investigate the existence of a metallicity threshold for the production of long gamma-ray bursts (LGRBs). Methods. We used the host galaxies of the Swift/BAT6 sample of LGRBs. We considered ...the stellar mass, star formation rate (SFR), and metallicity determined from the host galaxy photometry and spectroscopy up to z = 2 and used them to compare the distribution of host galaxies to that of field galaxies in the mass-metallicity and fundamental metallicity relation plane. Results. We find that although LGRBs also form in galaxies with relatively large stellar masses, the large majority of host galaxies have metallicities below log (O/H) ~ 8.6. The extension to z = 2 results in a good sampling of stellar masses also above Log(M∗/M⊙) ~ 9.5 and provides evidence that LGRB host galaxies do not follow the fundamental metallicity relation. As shown by the comparison with dedicated numerical simulations of LGRB host galaxy population, these results are naturally explained by the existence of a mild (~0.7 Z⊙) threshold for the LGRB formation. The present statistics does not allow us to discriminate between different shapes of the metallicity cutoff, but the relatively high metallicity threshold found in this work is somewhat in disagreement to most of the standard single-star models for LGRB progenitors.
We derive, adopting a direct method, the luminosity function and the formation rate of long Gamma Ray Bursts through a complete, flux-limited, sample of Swift bursts which has a high level of ...completeness in redshift z (~82%). We parametrise the redshift evolution of the GRB luminosity as L = L0(1 + z)k and we derive k = 2.5, consistently with recent estimates. The de-evolved luminosity function φ(L0) of GRBs can be represented by a broken power law with slopes a = −1.32 ± 0.21 and b = −1.84 ± 0.24 below and above, respectively, a break luminosity L0,b = 1051.45±0.15 erg/s. Under the hypothesis of luminosity evolution we find that the GRB formation rate increases with redshift up to z ~ 2, where it peaks, and then decreases in agreement with the shape of the cosmic star formation rate. We test the direct method through numerical simulations and we show that if it is applied to incomplete (both in redshift and/or flux) GRB samples it can misleadingly result in an excess of the GRB formation rate at low redshifts.
Context.
Long gamma-ray bursts (GRBs) offer a promising tool for tracing the cosmic history of star formation, especially at high redshift, where conventional methods are known to suffer from ...intrinsic biases. Previous studies of GRB host galaxies at low redshift showed that high surface density of stellar mass and high surface density of star formation rate (SFR) can potentially enhance the GRB production. Evaluating the effect of such stellar densities at high redshift is therefore crucial to fully control the ability of long GRBs for probing the activity of star formation in the distant Universe.
Aims.
We assess how the size, stellar mass, and star formation rate surface densities of distant galaxies affect the probability of their hosting a long GRB, using a sample of GRB hosts at
z
> 1 and a control sample of star-forming sources from the field.
Methods.
We gathered a sample of 45 GRB host galaxies at 1 <
z
< 3.1 observed with the
Hubble
Space Telescope WFC3 camera in the near-infrared. Our subsample at 1 <
z
< 2 has cumulative distributions of redshift and stellar mass consistent with the host galaxies of known unbiased GRB samples, while our GRB host selection at 2 <
z
< 3.1 has lower statistics and is probably biased toward the high end of the stellar mass function. Using the GALFIT parametric approach, we modeled the GRB host light profile with a Sérsic component and derived the half-light radius for 35 GRB hosts, which we used to estimate the star formation rate and stellar mass surface densities of each object. We compared the distribution of these physical quantities to the SFR-weighted properties of a complete sample of star-forming galaxies from the 3D-HST deep survey at a comparable redshift and stellar mass.
Results.
We show that similarly to
z
< 1, GRB hosts are smaller in size and they have higher stellar mass and star formation rate surface densities than field galaxies at 1 <
z
< 2. Interestingly, this result is robust even when separately considering the hosts of GRBs with optically bright afterglows and the hosts of dark GRBs, as the two subsamples share similar size distributions. At
z
> 2, however, GRB hosts appear to have sizes and stellar mass surface densities more consistent with those characterizing the field galaxies. This may reveal an evolution with redshift of the bias between GRB hosts and the overall population of star-forming sources, although we cannot exclude that our result at
z
> 2 is also affected by the prevalence of dark GRBs in our selection.
Conclusions.
In addition to a possible trend toward a low-metallicity environment, other environmental properties such as stellar density appear to play a role in the formation of long GRBs, at least up to
z
∼ 2. This might suggest that GRBs require special environments to enhance their production.
We derive the luminosity function φ(L) and redshift distribution Ψ(z) of short gamma-ray bursts (SGRBs) using all the available observer-frame constraints (i.e. peak flux, fluence, peak energy and ...duration distributions) of the large population of Fermi SGRBs and the rest-frame properties of a complete sample of SGRBs detected by Swift. We show that a steep φ(L) ∝ L− α with α ≥ 2.0 is excluded if the full set of constraints is considered. We implement a Markov chain Monte Carlo method to derive the φ(L) and Ψ(z) functions assuming intrinsic Ep−Liso and Ep−Eiso correlations to hold or, alternatively, that the distributions of intrinsic peak energy, luminosity, and duration are independent. To make our results independent from assumptions on the progenitor (NS−NS binary mergers or other channels) and from uncertainties on the star formation history, we assume a parametric form for the redshift distribution of the population of SGRBs. We find that a relatively flat luminosity function with slope ~0.5 below a characteristic break luminosity ~3 × 1052 erg s-1 and a redshift distribution of SGRBs peaking at z ~ 1.5−2 satisfy all our constraints. These results also hold if no Ep−Liso and Ep−Eiso correlations are assumed and they do not depend on the choice of the minimum luminosity of the SGRB population. We estimate, within ~200 Mpc (i.e. the design aLIGO range for the detection of gravitational waves produced by NS−NS merger events), that there should be 0.007−0.03 SGRBs yr-1 detectable as γ-ray events. Assuming current estimates of NS−NS merger rates and that all NS−NS mergers lead to a SGRB event, we derive a conservative estimate of the average opening angle of SGRBs ⟨ θjet ⟩ ~ 3°−6°. The luminosity function implies a prompt emission average luminosity ⟨L⟩ ~ 1.5 × 1052 erg s-1, higher by nearly two orders of magnitude than previous findings in the literature, which greatly enhances the chance of observing SGRB “orphan” afterglows. Effort should go in the direction of finding and identifying such orphan afterglows as counterparts of GW events.
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