1,849 research outputs found
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 ( 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) = 29.9 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), provide evidence for X-ray emission still
powered by the prompt emission until late times ( hours after the burst
event). On the other hand, the same test performed for the E-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
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