99 research outputs found
Hypocoercivity of linear kinetic equations via Harris's Theorem
We study convergence to equilibrium of the linear relaxation Boltzmann (also
known as linear BGK) and the linear Boltzmann equations either on the torus
or on the whole space with a confining potential. We present
explicit convergence results in total variation or weighted total variation
norms (alternatively or weighted norms). The convergence rates are
exponential when the equations are posed on the torus, or with a confining
potential growing at least quadratically at infinity. Moreover, we give
algebraic convergence rates when subquadratic potentials considered. We use a
method from the theory of Markov processes known as Harris's Theorem
A deep search for the host galaxies of GRBs with no detected optical afterglow
Gamma-Ray Bursts can provide information about star formation at high
redshifts. Even in the absence of a optical/near-infrared/radio afterglow, the
high detection rate of X-ray afterglows by swift/XRT and its localization
precision of 2-3 arcsec facilitates the identification and study of GRB host
galaxies. We focus on the search for the host galaxies of a sample of 17 bursts
with XRT error circles but no detected long-wavelength afterglow. Three of
these events can also be classified as truly dark bursts: the observed upper
limit on the optical flux of the afterglow was less than expected based on the
X-ray flux. Our study is based on deep R and K-band observations performed with
ESO/VLT instruments, supported by GROND and NEWFIRM. To be conservative, we
searched for host galaxies in an area with a radius twice the 90% swift/XRT
error circle. For 15 of the 17 bursts we find at least one galaxy inside the
doubled XRT error circle. In seven cases we discover extremely red objects in
the error circles. The most remarkable case is the host of GRB 080207 which as
a colour of R-K~4.7 mag (AB), one of the reddest galaxies ever associated with
a GRB. As a by-product of our study we identify the optical afterglow of GRB
070517A. Optically dim afterglows result from cosmological Lyman drop out and
dust extinction, but the former process is only equired for a minority of cases
(<1/3). Extinction by dust in the host galaxies might explain all other events.
Thereby, a seemingly non-negligible fraction of these hosts are globally
dust-enshrouded, extremely red galaxies. This suggests that bursts with
optically dim afterglows trace a subpopulation of massive starburst galaxies,
which are markedly different from the main body of the GRB host galaxy
population, namely the blue, subluminous, compact galaxies.Comment: 29 pages, 31 figures, accepted for publication in A&
GRB 071028B, a burst behind large amounts of dust in an unabsorbed galaxy
We report on the discovery and properties of the fading afterglow and
underlying host galaxy of GRB 071028B, thereby facilitating a detailed
comparison between these two. Observations were performed with the Gamma-ray
Burst Optical and Near-infrared Detector at the 2.2 m telescope on the La Silla
Paranal Observatory in Chile. We conducted five observations from 1.9 d to
227.2 d after the trigger and obtained deep images in the g'r'i'z' and JHKs
bands. Based on accurate seven-channel photometry covering the optical to
near-infrared wavelength range, we derive a photometric redshift of z = 0.94
+0.05 -0.10 for the unabsorbed host galaxy of GRB 071028B. In contrast, we show
that the afterglow with an intrinsic extinction of AV(SB) = (0.70 +/- 0.11) mag
is moderately absorbed and requires a relatively flat extinction curve.
According to the reported Swift/BAT observations, the energetics yield an
isotropic energy release of E(gamma,iso.,rest) = (1.4 +2.4 -0.7) x 10^51 erg.Comment: 8 pages, 5 figures, accepted for publication in A&
On the nature of the extremely fast optical rebrightening of the afterglow of GRB 081029
Context. After the launch of the Swift satellite, the Gamma-Ray Burst (GRB)
optical light-curve smoothness paradigm has been questioned thanks to the
faster and better sampled optical follow-up, which has unveiled a very complex
behaviour. This complexity is triggering the interest of the whole GRB
community. The GROND multi-channel imager is used to study optical and
near-infrared (NIR) afterglows of GRBs with unprecedented optical and
near-infrared temporal and spectral resolution. The GRB 081029 has a very
prominent optical rebrightening event and is an outstanding example of the
application of the multi-channel imager to GRB afterglows. Aims. Here we
exploit the rich GROND multi-colour follow-up of GRB 081029 combined with XRT
observations to study the nature of late-time rebrightenings that appear in the
optical-NIR light-curves of some GRB afterglows. Methods. We analyse the
optical and NIR observations obtained with the seven-channel Gamma-Ray burst
Optical and Near-infrared Detector (GROND) at the 2.2 m MPI/ESO telescope and
the X-ray data obtained with the XRT telescope on board the Swift observatory.
The multi-wavelength temporal and spectral evolution is discussed in the
framework of different physical models. Results. The extremely steep optical
and NIR rebrightening observed in GRB 081029 cannot be explained in the
framework of the standard forward shock afterglow model. The absence of a
contemporaneous X-ray rebrightening and the evidence of a strong spectral
evolution in the optical-NIR bands during the rise suggest two separate
components that dominate in the early and late-time lightcurves, respectively.
The steepness of the optical rise cannot be explained even in the framework of
the alternative scenarios proposed in the literature unless a late-time
activity of the central engine is assumed.Comment: 9 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
A photometric redshift of for the \agile GRB 080514B
Aims: The AGILE gamma-ray burst GRB 080514B is the first burst with detected
emission above 30 MeV and an optical afterglow. However, no spectroscopic
redshift for this burst is known.
Methods: We compiled ground-based photometric optical/NIR and millimeter data
from several observatories, including the multi-channel imager GROND, as well
as ultraviolet \swift UVOT and X-ray XRT observations. The spectral energy
distribution of the optical/NIR afterglow shows a sharp drop in the \swift UVOT
UV filters that can be utilized for the estimation of a redshift.
Results: Fitting the SED from the \swift UVOT band to the band, we
estimate a photometric redshift of , consistent with the
pseudo redshift reported by Pelangeon & Atteia (2008) based on the gamma-ray
data.
Conclusions: The afterglow properties of GRB 080514B do not differ from those
exhibited by the global sample of long bursts, supporting the view that
afterglow properties are basically independent of prompt emission properties.Comment: submitted to A&A letter
CHANDRA Detection of 16 New X-Ray Sources
We have detected 18 sources over 6 sigma threshold within two regions
8.3X16.9 arcmin^2 and 8.3X33.6 arcmin^2 in the vicinity of the point with
alpha=03h31m02.45s (J2000) and delta=+43degree47arcmin58.5arcsec (J2000) using
a CHANDRA ACIS (S+I) observation. Two of the sources were detected before with
HRI and one source could be closely identified with a star in the
optical catalog, USNO A-2. We have also studied source spectra applying four
spectral models to the data. Most of the sources can be classified as
Cataclysmic Variable, Low Mass X-ray Binary or single star candidates due to
their spectral characteristics and luminosities. We also searched for the
extragalactic origin for these 18 sources. The source count rates vary between
5.8X10^{-4}- 4.7X10^{-3} counts/s. Due to low count rates temporal
characteristics of the sources can not be studied effectively.Comment: 7 pages, 1 figures, accepted to Astronomy and Astrophysic
Hypocoercivity of linear kinetic equations via Harris's Theorem
We study convergence to equilibrium of the linear relaxation Boltzmann (also known as linear BGK) and the linear Boltzmann equations either on the torus (x,v)∈Td×Rd or on the whole space
(x,v)∈Rd×Rd with a confining potential. We present explicit convergence results in total variation or weighted total variation norms (alternatively L1 or weightedL1 norms). The convergence rates are exponential when the equations are posed on the torus, or with a confining potential growing at least quadratically at infinity. Moreover, we give algebraic convergence rates when subquadratic potentials considered. We use a method from the theory of Markov processes known as Harris's Theorem
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
A strong optical flare before the rising afterglow of GRB 080129
We report on GROND observations of a 40 sec duration (rest-frame) optical
flare from GRB 080129 at redshift 4.349. The rise- and decay time follow a
power law with indices +12 and -8, respectively, inconsistent with a reverse
shock and a factor 10 faster than variability caused by ISM interaction.
While optical flares have been seen in the past (e.g. GRB 990123, 041219B,
060111B and 080319B), for the first time, our observations not only resolve the
optical flare into sub-components, but also provide a spectral energy
distribution from the optical to the near-infrared once every minute. The delay
of the flare relative to the GRB, its spectral energy distribution as well as
the ratio of pulse widths suggest it to arise from residual collisions in GRB
outflows \cite{liw08}.If this interpretation is correct and can be supported by
more detailed modelling or observation in further GRBs, the delay measurement
provides an independent, determination of the Lorentz factor of the outflow.Comment: accepted for publ. in ApJ, 5 Fig
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