658 research outputs found
Gamma-ray burst host galaxies and the link to star-formation
We briefly review the current status of the study of long-duration gamma-ray
burst (GRB) host galaxies. GRB host galaxies are mainly interesting to study
for two reasons: 1) they may help us understand where and when massive stars
were formed throughout cosmic history, and 2) the properties of host galaxies
and the localisation within the hosts where GRBs are formed may give essential
clues to the precise nature of the progenitors. The main current problem is to
understand to what degree GRBs are biased tracers of star formation. If GRBs
are only formed by low-metallicity stars, then their host galaxies will not
give a representative view of where stars are formed in the Universe (at least
not a low redshifts). On the other hand, if there is no dependency on
metallicity then the nature of the host galaxies leads to the perhaps
surprising conclusion that most stars are formed in dwarf galaxies. In order to
resolve this issue and to fully exploit the potential of GRBs as probes of
star-forming galaxies throughout the observable universe it is mandatory that a
complete sample of bursts with redshifts and host galaxy detections is built.Comment: 9 pages, 3 figures. To appear in the proceedings of the Eleventh
Marcel Grossmann Meeting on General Relativity, eds. H. Kleinert, R. T.
Jantzen & R. Ruffini, World Scientific, Singapore, 200
The afterglow of GRB 021004: surfing on density waves
We present a model for the early optical afterglow of GRB 021004. This burst
had one of the earliest detected optical afterglows, allowing a dense optical
sampling. The lightcurve was peculiar, since bright bumps were superimposed to
the regular power-law decay observed in many other events. We show that, given
their time scale and shape, the bumps are due to the interaction of the
fireball with moderate enhancements in the ambient medium density. The
enhancements have a density contrast of order 10, modifying only slightly the
dynamics of the fireball, which therefore surfs on them rather than colliding
onto them. A relativistic reverse shock does not develop. Since the interaction
takes place when the fireball is still hyper-relativistic it is not possible to
understand if the overdensities are localized in clumps or are spherically
symmetric around the GRB progenitor. The monotonic decrease of the contrast of
successive rebrightenings suggests however the presence of clumps in an average
uniform environment. Such an interpretation, complemented by the detection of
several high velocity absorption systems in the optical spectrum, strongly
suggests that GRB 021004 exploded within the remnant of a previous explosion.Comment: 5 pages, 2 postscript figures, A&A reference adde
The optical afterglows and host galaxies of three short/hard gamma-ray bursts
Short GRBs are commonly thought to originate from the merging of double
compact object binaries but direct evidence for this scenario is still missing.
Optical observations of short GRBs allow us to measure redshifts, firmly
identify host galaxies, characterize their properties, and accurately localize
GRBs within them. Multiwavelength observations of GRB afterglows provide useful
information on the emission mechanisms at work. These are all key issues that
allow one to discriminate among different models of these elusive events. We
carried out photometric observations of the short/hard GRB 051227, GRB 061006,
and GRB 071227 with the ESO-VLT starting from several hours after the explosion
down to the host galaxy level several days later. For GRB 061006 and GRB 071227
we also obtained spectroscopic observations of the host galaxy. We compared the
results obtained from our optical observations with the available X-ray data of
these bursts. For all the three above bursts, we discovered optical afterglows
and firmly identified their host galaxies. About half a day after the burst,
the optical afterglows of GRB 051227 and GRB 061006 present a decay
significatly steeper than in the X-rays. In the case of GRB 051227, the optical
decay is so steep that it likely indicates different emission mechanisms in the
two wavelengths ranges. The three hosts are blue, star forming galaxies at
moderate redshifts and with metallicities comparable to the Solar one. The
projected offsets of the optical afterglows from their host galaxies centers
span a wide range, but all afterglows lie within the light of their hosts and
present evidence for local absorption in their X-ray spectra. We discuss our
findings in light of the current models of short GRB progenitors.Comment: Accepted for publication by A&A. 11 pages, 9 figures; v2: minor
changes and new version of Fig.
On the Distribution of Stellar Masses in Gamma-ray Burst Host Galaxies
We analyze Spitzer images of 30 long-duration gamma-ray burst (GRB) host galaxies. We estimate their total stellar masses (M_*) based on the rest-frame K-band luminosities (L_K_(rest)) and constrain their star formation rates (SFRs; not corrected for dust extinction) based on the rest-frame UV continua. Further, we compute a mean M_*/ L_K_(rest) = 0.45 M_☉/L_☉. We find that the hosts are low M_*, star-forming systems. The median M_* in our sample ( = 10^(9.7) M_☉) is lower than that of "field" galaxies (e.g., Gemini Deep Deep Survey). The range spanned by M_* is 10^7 M_☉ < M_* < 10^(11) M_☉, while the range spanned by the dust-uncorrected UV SFR is 10^(–2) M_☉ yr^(–1) < SFR < 10 M_☉ yr^(–1). There is no evidence for intrinsic evolution in the distribution of M_* with redshift. We show that extinction by dust must be present in at least 25% of the GRB hosts in our sample and suggest that this is a way to reconcile our finding of a relatively lower UV-based, specific SFR (φ ≡ SFR/M_*) with previous claims that GRBs have some of the highest φ values. We also examine the effect that the inability to resolve the star-forming regions in the hosts has on φ
The Redshift Distribution of the TOUGH Survey
We present the redshift results from a Very Large Telescope program aimed at
optimizing the legacy value of the Swift mission: to characterize a
homogeneous, X-ray selected, sample of 69 GRB host galaxies. 19 new redshifts
have been secured, resulting in a 83% (57/69) redshift completion, making the
survey the most comprehensive in terms of redshift completeness of any sample
to the full Swift depth, available to date. We present the cumulative redshift
distribution and derive a conservative, yet small, associated uncertainty. We
constrain the fraction of Swift GRBs at high redshift to a maximum of 10% (5%)
for z > 6 (z > 7). The mean redshift of the host sample is assessed to be >
2.2. Using this more complete sample, we confirm previous findings that the GRB
rate at high redshift (z > 3) appears to be in excess of predictions based on
assumptions that it should follow conventional determinations of the star
formation history of the universe, combined with an estimate of its likely
metallicity dependence. This suggests that either star formation at high
redshifts has been significantly underestimated, for example due to a dominant
contribution from faint, undetected galaxies, or that GRB production is
enhanced in the conditions of early star formation, beyond those usually
ascribed to lower metallicity.Comment: 7th Huntsville Gamma-Ray Burst Symposium, GRB 2013: paper 34 in eConf
Proceedings C130414
The Spectral Lag of GRB060505: A Likely Member of the Long Duration Class
Two long gamma-ray bursts, GRB 060505 and GRB 060614, occurred in nearby
galaxies at redshifts of 0.089 and 0.125 respectively. Due to their proximity
and durations, deep follow-up campaigns to search for supernovae (SNe) were
initiated. However none were found in either case, to limits more than two
orders of magnitude fainter than the prototypical GRB-associated SN, 1998bw. It
was suggested that the bursts, in spite of their durations (4 and 102 s),
belonged to the population of short GRBs which has been shown to be unrelated
to SNe. In the case of GRB 060614 this argument was based on a number of
indicators, including the negligible spectral lag, which is consistent with
that of short bursts. GRB 060505 has a shorter duration, but no spectral lag
was measured. We present the spectral lag measurements of GRB 060505 using
Suzakus Wide Area Monitor and the Swift Burst Alert Telescope. We find that the
lag is 0.36+/- 0.05 s, inconsistent with the lags of short bursts and
consistent with the properties of long bursts and SN-GRBs. These results
support the association of GRB 060505 with other low-luminosity GRBs also found
in star-forming galaxies and indicates that at least some massive stars may die
without bright SNe.Comment: Accepted by ApJL, 5 pages, 3 Figure
A nearby GRB host prototype for z~7 Lyman-break galaxies: Spitzer-IRS and X-shooter spectroscopy of the host galaxy of GRB031203
Gamma-ray burst (GRB) host galaxies have been studied extensively in optical
photometry and spectroscopy. Here we present the first mid-infrared spectrum of
a GRB host, HG031203. It is one of the nearest GRB hosts at z=0.1055, allowing
both low and high-resolution spectroscopy with Spitzer-IRS. Medium resolution
UV-to-K-band spectroscopy with the X-shooter spectrograph on the VLT is also
presented, along with Spitzer IRAC and MIPS photometry, as well as radio and
sub-mm observations. These data allow us to construct a UV-to-radio spectral
energy distribution with almost complete spectroscopic coverage from 0.3-35
micron of a GRB host galaxy for the first time, potentially valuable as a
template for future model comparisons. The IRS spectra show strong,
high-ionisation fine structure line emission indicative of a hard radiation
field in the galaxy, suggestive of strong ongoing star-formation and a very
young stellar population. The selection of HG031203 via the presence of a GRB
suggests that it might be a useful analogue of very young star-forming galaxies
in the early universe, and hints that local BCDs may be used as more reliable
analogues of star-formation in the early universe than typical local
starbursts. We look at the current debate on the ages of the dominant stellar
populations in z~7 and z~8 galaxies in this context. The nebular line emission
is so strong in HG031203, that at z~7, it can reproduce the spectral energy
distributions of z-band dropout galaxies with elevated IRAC 3.6 and 4.5 micron
fluxes without the need to invoke a 4000A break.Comment: Published in ApJ. 9 pages, 6 figures, emulateapj styl
Open issues in gamma-ray bursts: polarimetry and dark GRBs
We review some open problems in the physics of afterglows, namely their
polarization properties and the existence of dark/faint bursts. Polarization
studies yield precious insights in the physical structure and dynamical
evolution of GRB jets, revealing their magnetization properties and their
energy profile. Polarimetric observations of GRB 020813 already allowed to
exclude a homogeneous jet for this event. We then present observations of
faint/dark bursts, showing that some of them may be obscured by dust, while
others are possibly just intrinsically dim.Comment: 6 pages, 3 figures. Proceedings of the 4th Workshop "Gamma-Ray Bursts
in the Afterglow Era", Roma, 2004 October 18-22, eds. L. Piro, L. Amati, S.
Covino, and B. Gendre. Il Nuovo Cimento, in pres
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