178 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
Unveiling the population of orphan Gamma Ray Bursts
Gamma Ray Bursts are detectable in the gamma-ray band if their jets are
oriented towards the observer. However, for each GRB with a typical theta_jet,
there should be ~2/theta_jet^2 bursts whose emission cone is oriented elsewhere
in space. These off-axis bursts can be eventually detected when, due to the
deceleration of their relativistic jets, the beaming angle becomes comparable
to the viewing angle. Orphan Afterglows (OA) should outnumber the current
population of bursts detected in the gamma-ray band even if they have not been
conclusively observed so far at any frequency. We compute the expected flux of
the population of orphan afterglows in the mm, optical and X-ray bands through
a population synthesis code of GRBs and the standard afterglow emission model.
We estimate the detection rate of OA by on-going and forthcoming surveys. The
average duration of OA as transients above a given limiting flux is derived and
described with analytical expressions: in general OA should appear as daily
transients in optical surveys and as monthly/yearly transients in the mm/radio
band. We find that ~ 2 OA yr^-1 could already be detected by Gaia and up to 20
OA yr^-1 could be observed by the ZTF survey. A larger number of 50 OA yr^-1
should be detected by LSST in the optical band. For the X-ray band, ~ 26 OA
yr^-1 could be detected by the eROSITA. For the large population of OA
detectable by LSST, the X-ray and optical follow up of the light curve (for the
brightest cases) and/or the extensive follow up of their emission in the mm and
radio band could be the key to disentangle their GRB nature from other
extragalactic transients of comparable flux density.Comment: 9 pages, 4 figures, 2 tables. Accepted for publication by Astronomy
and Astrophysic
PHENOMENOLOGY OF REVERSE-SHOCK EMISSION IN THE OPTICAL AFTERGLOWS OF GAMMA-RAY BURSTS
We use a parent sample of 118 gamma-ray burst (GRB) afterglows, with known redshift and host galaxy extinction, to separate afterglows with and without signatures of dominant reverse-shock emission and to determine which physical conditions lead to a prominent reverse-shock emission. We identify 10 GRBs with reverse shock signatures - GRBs 990123, 021004, 021211, 060908, 061126, 080319B, 081007, 090102, 090424 and 130427A. By modeling their optical afterglows with reverse and forward shock analytic light curves and using Monte Carlo simulations, we estimate the parameter space of the physical quantities describing the ejecta and circumburst medium. We find that physical properties cover a wide parameter space and do not seem to cluster around any preferential values. Comparing the rest-frame optical, X-ray and high-energy properties of the larger sample of non-RS-dominated GRBs, we show that the early-time (< 1ks) optical spectral luminosity, X-ray
afterglow luminosity and -ray energy output of our reverse-shock dominated sample do not differ significantly from the general population at early times. However, the GRBs with dominant reverse shock emission have fainter than average optical forward-shock emission at late time (> 10 ks). We
find that GRBs with an identifiable reverse shock component show high magnetization parameter RB = "B,r/"B,f � 2 − 104. Our results are in agreement with the mildly magnetized baryonic jet model of GRBs
Spectrophotometric analysis of GRB afterglow extinction curves with X-shooter
In this work we use gamma-ray burst (GRB) afterglow spectra observed with the
VLT/X-shooter spectrograph to measure rest-frame extinction in GRB
lines-of-sight by modeling the broadband near-infrared (NIR) to X-ray afterglow
spectral energy distributions (SEDs). Our sample consists of nine Swift GRBs,
eight of them belonging to the long-duration and one to the short-duration
class. Dust is modeled using the average extinction curves of the Milky Way and
the two Magellanic Clouds. We derive the rest-frame extinction of the entire
sample, which fall in the range .
Moreover, the SMC extinction curve is the preferred extinction curve template
for the majority of our sample, a result which is in agreement with those
commonly observed in GRB lines-of-sights. In one analysed case (GRB 120119A),
the common extinction curve templates fail to reproduce the observed
extinction. To illustrate the advantage of using the high-quality X-shooter
afterglow SEDs over the photometric SEDs, we repeat the modeling using the
broadband SEDs with the NIR-to-UV photometric measurements instead of the
spectra. The main result is that the spectroscopic data, thanks to a
combination of excellent resolution and coverage of the blue part of the SED,
are more successful in constraining the extinction curves and therefore the
dust properties in GRB hosts with respect to photometric measurements. In all
cases but one the extinction curve of one template is preferred over the
others. We show that the modeled values of the extinction and the spectral
slope, obtained through spectroscopic and photometric SED analysis, can differ
significantly for individual events. Finally we stress that, regardless of the
resolution of the optical-to-NIR data, the SED modeling gives reliable results
only when the fit is performed on a SED covering a broader spectral region.Comment: 17 pages, 7 figures, 4 tables, accepted for publication in Astronomy
& Astrophysic
Radio flares from gamma-ray bursts
We present predictions of centimeter and millimeter radio emission from reverse shocks in the early afterglows of gamma-ray bursts with the goal of determining their detectability with current and future radio facilities. Using a range of GRB properties, such as peak optical brightness and time, isotropic equivalent gamma-ray energy and redshift, we simulate radio light curves in a framework generalized for any circumburst medium structure and including a parametrization of the shell thickness regime that is more realistic than the simple assumption of thick- or thin-shell approximations. Building on earlier work by Mundell et al. (2007) and Melandri et al. (2010) in which the typical frequency of the reverse shock was suggested to lie at radio, rather than optical wavelengths at early times, we show that the brightest and most distinct reverse-shock radio signatures are detectable up to 0.1 -- 1 day after the burst, emphasizing the need for rapid radio follow-up. Detection is easier for bursts with later optical peaks, high isotropic energies, lower circumburst medium densities, and at observing frequencies that are less prone to synchrotron self-absorption effects - typically above a few GHz. Given recent detections of polarized prompt gamma-ray and optical reverse-shock emission, we suggest that detection of polarized radio/mm emission will unambiguously confirm the presence of low-frequency reverse shocks at early time
GRB host galaxies with VLT/X-Shooter: properties at 0.8 < z < 1.3
Long gamma-ray bursts (LGRBs) are associated with the death of massive stars.
Their host galaxies therefore represent a unique class of objects tracing star
formation across the observable Universe. Indeed, recently accumulated evidence
shows that GRB hosts do not differ substantially from general population of
galaxies at high (z > 2) redshifts. However, it has been long recognised that
the properties of z < 1.5 hosts, compared to general star-forming population,
are unusual. To better understand the reasons for the supposed difference in
LGRB hosts properties at z < 1.5, we obtained VLT/X- Shooter spectra of six
hosts lying in the redshift range of 0.8 < z < 1.3. Some of these hosts have
been observed before, yet we still lack well constrained information on their
characteristics such as metallicity, dust extinction and star formation rate.
We search for emission lines in the VLT/X-Shooter spectra of the hosts and
measure their fluxes. We perform a detailed analysis, estimating host average
extinction, star-formation rates, metallicities and electron densities where
possible. Measured quantities of our hosts are compared to a larger sample of
previously observed GRB hosts at z < 2. Star-formation rates and metallicities
are measured for all the hosts analyzed in this paper and metallicities are
well determined for 4 hosts. The mass-metallicity relation, the fundamental
metallicity relation and SFRs derived from our hosts occupy similar parameter
space as other host galaxies investigated so-far at the same redshift. We
therefore conclude that GRB hosts in our sample support the found discrepancy
between the properties of low-redshift GRB hosts and the general population of
star- forming galaxies.Comment: 13 pages, 6 figures, accepted for publication in MNRA
The high-redshift gamma-ray burst GRB140515A
High-redshift gamma-ray bursts have several advantages for the study of the
distant universe, providing unique information about the structure and
properties of the galaxies in which they exploded. Spectroscopic identification
with large ground-based telescopes has improved our knowledge of the class of
such distant events. We present the multi-wavelength analysis of the high-
Swift gamma-ray burst GRB140515A (). The best estimate of the
neutral hydrogen fraction of the intergalactic medium (IGM) towards the burst
is . The spectral absorption lines detected for this event
are the weakest lines ever observed in gamma-ray burst afterglows, suggesting
that GRB140515A exploded in a very low density environment. Its circum-burst
medium is characterised by an average extinction (A) that
seems to be typical of events. The observed multi-band light curves
are explained either with a very flat injected spectrum () or with a
multi-component emission (). In the second case a long-lasting central
engine activity is needed in order to explain the late time X-ray emission. The
possible origin of GRB140515A from a Pop III (or from a Pop II stars with local
environment enriched by Pop III) massive star is unlikely.Comment: 10 pages, 8 figures, 5 tables, submitted to Astronomy & Astrophysic
A quiescent galaxy at the position of the long GRB 050219A
Long-duration gamma-ray bursts (LGRBs) are produced by the collapse of very
massive stars. Due to the short lifetime of their progenitors, LGRBs pinpoint
star-forming galaxies. We present here a multi-band search for the host galaxy
of the long dark GRB 050219A within the enhanced Swift/XRT error circle. We
used spectroscopic observations acquired with VLT/X-shooter to determine the
redshift and star-formation rate of the putative host galaxy. We compared the
results with the optical/IR spectral energy distribution obtained with
different facilities. Surprisingly, the host galaxy is a old and quiescent
early-type galaxy at z = 0.211 characterised by an unprecedentedly low specific
star-formation rate. It is the first LGRB host to be also an early-type
post-starburst galaxy. This is further evidence that GRBs can explode in all
kind of galaxies, with the only requirement being an episode of star-formation.Comment: 11 pages, 10 figures, 3 tables; accepted for publication in Astronomy
& Astrophysic
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