Taking stock of SLSN and LGRB host galaxy comparison using a complete sample of LGRBs

Long gamma-ray bursts (LGRBs) and superluminous supernovae (SLSNe) are both explosive transients with very massive progenitor stars. Clues about the nature of the progenitors can be found by investigating environments in which such transients occur. While studies of LGRB host galaxies have a long history, dedicated observational campaigns have only recently resulted in a high enough number of photometrically and spectroscopically observed SLSN hosts to allow statistically significant analysis of their properties. In this paper we make a comparison of the host galaxies of hydrogen-poor (H-poor) SLSNe and the Swift/BAT6 sample of LGRBs. In contrast to previous studies we use a complete sample of LGRBs and we address a special attention to the comparison methodology and the selection of SLSN sample whose data have been compiled from the available literature. At intermediate redshifts (0.3 < z < 0.7) the two classes of transients select galaxies whose properties (stellar mass, luminosity, star-formation rate, specific star-formation rate and metallicity) do not differ on average significantly. Moreover, the host galaxies of both classes of objects follow the fundamental metallicity relation and the fundamental plane of metallicity. In contrast to previous studies we show that at intermediate redshifts the emission line equivalent widths of the two populations are essentially the same and that the previous claims regarding the higher fraction of SLSN hosts among the extreme emission line galaxies with respect to LGRBs are mostly due to a larger fraction of strong-line emitters among SLSN hosts at z < 0.3, where samples of LGRB hosts are small and poorly defined.Comment: 7 pages, 4 figures, accepted to Astronomy & Astrophysic

Optical and X-ray Rest-frame Light Curves of the BAT6 sample

We present the rest-frame light curves in the optical and X-ray bands of an unbiased and complete sample of Swift long Gamma-Ray Bursts (GRBs), namely the BAT6 sample. The unbiased BAT6 sample (consisting of 58 events) has the highest level of completeness in redshift ($\sim$ 95%), allowing us to compute the rest-frame X-ray and optical light curves for 55 and 47 objects, respectively. We compute the X-ray and optical luminosities accounting for any possible source of absorption (Galactic and intrinsic) that could affect the observed fluxes in these two bands. We compare the behaviour observed in the X-ray and in the optical bands to assess the relative contribution of the emission during the prompt and afterglow phases. We unarguably demonstrate that the GRBs rest-frame optical luminosity distribution is not bimodal, being rather clustered around the mean value Log(L$_{R}$) = 29.9 $\pm$ 0.8 when estimated at a rest frame time of 12 hr. This is in contrast with what found in previous works and confirms that the GRB population has an intrinsic unimodal luminosity distribution. For more than 70% of the events the rest-frame light curves in the X-ray and optical bands have a different evolution, indicating distinct emitting regions and/or mechanisms. The X-ray light curves normalised to the GRB isotropic energy (E$_{\rm iso}$), provide evidence for X-ray emission still powered by the prompt emission until late times ($\sim$ hours after the burst event). On the other hand, the same test performed for the E$_{\rm iso}$-normalised optical light curves shows that the optical emission is a better proxy of the afterglow emission from early to late times.Comment: Accepted for publication in A&A: 10 pages, 5 figures, 2 table

GRB orphan afterglows in present and future radio transient surveys

Orphan Afterglows (OA) are slow transients produced by Gamma Ray Bursts seen off-axis that become visible on timescales of days/years at optical/NIR and radio frequencies, when the prompt emission at high energies (X and gamma rays) has already ceased. Given the typically estimated jet opening angle of GRBs theta_jet ~ 3 deg, for each burst pointing to the Earth there should be a factor ~ 700 more GRBs pointing in other directions. Despite this, no secure OAs have been detected so far. Through a population synthesis code we study the emission properties of the population of OA at radio frequencies. OAs reach their emission peak on year-timescales and they last for a comparable amount of time. The typical peak fluxes (which depend on the observing frequency) are of few micro Jy in the radio band with only a few OA reaching the mJy level. These values are consistent with the upper limits on the radio flux of SN Ib/c observed at late times. We find that the OA radio number count distribution has a typical slope -1.7 at high fluxes and a flatter (-0.4) slope at low fluxes with a break at a frequency-dependent flux. Our predictions of the OA rates are consistent with the (upper) limits of recent radio surveys and archive searches for radio transients. Future radio surveys like VAST/ASKAP at 1.4 GHz should detect ~ 3x10^-3 OA deg^-2 yr-1, MeerKAT and EVLA at 8.4 GHz should see ~ 3x10^-1 OA deg-2 yr-1. The SKA, reaching the micro Jy flux limit, could see up to ~ 0.2-1.5 OA deg^-2 yr^-1. These rates also depend on the duration of the OA above a certain flux limit and we discuss this effect with respect to the survey cadence.Comment: (10 pages, 5 figures, 1 table) Accepted for publication by PAS

Herschel observations of gamma-ray burst host galaxies: implications for the topology of the dusty interstellar medium

Long-duration gamma-ray bursts (GRBs) are indisputably related to star formation, and their vast luminosity in gamma rays pin-points regions of star formation independent of galaxy mass. As such, GRBs provide a unique tool for studying star forming galaxies out to high-z independent of luminosity. Most of our understanding of the properties of GRB hosts (GRBHs) comes from optical and near-infrared (NIR) follow-up observations, and we therefore have relatively little knowledge of the fraction of dust-enshrouded star formation that resides within GRBHs. Currently ~20% of GRBs show evidence of significant amounts of dust along the line of sight to the afterglow through the host galaxy, and these GRBs tend to reside within redder and more massive galaxies than GRBs with optically bright afterglows. In this paper we present Herschel observations of five GRBHs with evidence of being dust-rich, targeted to understand the dust attenuation properties within GRBs better. Despite the sensitivity of our Herschel observations, only one galaxy in our sample was detected (GRBH 070306), for which we measure a total star formation rate (SFR) of ~100Mstar/yr, and which had a relatively high stellar mass (log[Mstar]=10.34+0.09/-0.04). Nevertheless, when considering a larger sample of GRBHs observed with Herschel, it is clear that stellar mass is not the only factor contributing to a Herschel detection, and significant dust extinction along the GRB sightline (A_{V,GRB}>1.5~mag) appears to be a considerably better tracer of GRBHs with high dust mass. This suggests that the extinguishing dust along the GRB line of sight lies predominantly within the host galaxy ISM, and thus those GRBs with A_{V,GRB}>1~mag but with no host galaxy Herschel detections are likely to have been predominantly extinguished by dust within an intervening dense cloud.Comment: 14 pages, 7 figures. Accepted for publication in A&

New light on gamma-ray burst host galaxies with Herschel

Until recently, dust emission has been detected in very few host galaxies of gamma-ray bursts (GRBHs). With Herschel, we have now observed 17 GRBHs up to redshift z~3 and detected seven of them at infrared (IR) wavelengths. This relatively high detection rate (41%) may be due to the composition of our sample which at a median redshift of 1.1 is dominated by the hosts of dark GRBs. Although the numbers are small, statistics suggest that dark GRBs are more likely to be detected in the IR than their optically-bright counterparts. Combining our IR data with optical, near-infrared, and radio data from our own datasets and from the literature, we have constructed spectral energy distributions (SEDs) which span up to 6 orders of magnitude in wavelength. By fitting the SEDs, we have obtained stellar masses, dust masses, star-formation rate (SFR), and extinctions for our sample galaxies. We find that GRBHs are galaxies that tend to have a high specfic SFR (sSFR), and like other star-forming galaxies, their ratios of dust-to-stellar mass are well correlated with sSFR. We incorporate our Herschel sample into a larger compilation of GRBHs, and compare this combined sample to SFR-weighted median stellar masses of the widest, deepest galaxy survey to date. This is done in order to establish whether or not GRBs can be used as an unbiased tracer of cosmic comoving SFR density (SFRD) in the universe. In contrast with previous results, this comparison shows that GRBHs are medium-sized galaxies with relatively high sSFRs; stellar masses and sSFRs of GRBHs as a function of redshift are similar to what is expected for star-forming galaxy populations at similar redshifts. We conclude that there is no strong evidence that GRBs are biased tracers of SFRD; thus they should be able to reliably probe the SFRD to early epochs.Comment: 18 pages, 9 figures, accepted for publication in A&A. Revised to include Fig. 6, mistakenly omitted in origina

A complete sample of bright Swift short Gamma-Ray Bursts

We present a carefully selected sample of short gamma-ray bursts (SGRBs) observed by the Swift satellite up to June 2013. Inspired by the criteria we used to build a similar sample of bright long GRBs (the BAT6 sample), we selected SGRBs with favorable 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% (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%) of binaries formed by dynamical capture (or experiencing large natal kicks). This sample is expected to significantly increase with further years of Swift activity.Comment: 17 pages, 8 figures, 7 tables. Accepted for publication in MNRA