135 research outputs found
The circumburst environment of a FRED GRB: study of the prompt emission and X-ray/optical afterglow of GRB 051111
We report a multi-wavelength analysis of the prompt emission and early
afterglow of GRB051111 and discuss its properties in the context of current
fireball models. The detection of GRB051111 by the Burst Alert Telescope
on-board Swift triggered early BVRi' observations with the 2-m robotic Faulkes
Telescope North in Hawaii, as well as X-ray observations with the Swift X-Ray
Telescope. The prompt gamma-ray emission shows a classical FRED profile. The
optical afterglow light curves are fitted with a broken power law, with
alpha_1=0.35 to alpha_2=1.35 and a break time around 12 minutes after the GRB.
Although contemporaneous X-ray observations were not taken, a power law
connection between the gamma-ray tail of the FRED temporal profile and the late
XRT flux decay is feasible. Alternatively, if the X-ray afterglow tracks the
optical decay, this would represent one of the first GRBs for which the
canonical steep-shallow-normal decay typical of early X-ray afterglows has been
monitored optically. We present a detailed analysis of the intrinsic
extinction, elemental abundances and spectral energy distribution. From the
absorption measured in the low X-ray band we find possible evidence for an
overabundance of some alpha elements such as oxygen, [O/Zn]=0.7+/-0.3, or,
alternatively, for a significant presence of molecular gas. The IR-to-X-ray
Spectral Energy Distribution measured at 80 minutes after the burst is
consistent with the cooling break lying between the optical and X-ray bands.
Extensive modelling of the intrinsic extinction suggests dust with big grains
or grey extinction profiles. The early optical break is due either to an energy
injection episode or, less probably, to a stratified wind environment for the
circumburst medium.Comment: accepted to A&A on Nov. 10 (14 pages, 8 figures
Dust extinction for an unbiased sample of GRB afterglows
In this paper we compute rest-frame extinctions for the afterglows of a
sample of gamma-ray bursts complete in redshift. The selection criteria of the
sample are based on observational high-energy parameters of the prompt emission
and therefore our sample should not be biased against dusty sight-lines. It is
therefore expected that our inferences hold for the general population of
gamma-ray bursts. Our main result is that the optical/near-infrared extinction
of gamma-ray burst afterglows in our sample does not follow a single
distribution. 87% of the events are absorbed by less than 2 mag, and 50% suffer
from less than 0.3-0.4 mag extinction. The remaining 13% of the afterglows are
highly absorbed. The true percentage of gamma-ray burst afterglows showing high
absorption could be even higher since a fair fraction of the events without
reliable redshift measurement are probably part of this class. These events may
be due to highly dusty molecular clouds/star forming regions associated with
the gamma-ray burst progenitor or along the afterglow line of sight, and/or to
massive dusty host galaxies. No clear evolution in the dust extinction
properties is evident within the redshift range of our sample, although the
largest extinctions are at z~1.5-2, close to the expected peak of the star
formation rate. Those events classified as dark are characterized, on average,
by a higher extinction than typical events in the sample. A correlation between
optical/near-infrared extinction and hydrogen-equivalent column density based
on X-ray studies is shown although the observed NH appears to be well in excess
compared to those observed in the Local Group. Dust extinction does not seem to
correlate with GRB energetics or luminosity.Comment: 18 pages, 7 figures, 10 tables, MNRAS, in pres
Evidence for energy injection and a fine-tuned central engine at optical wavelengths in GRB 070419A
We present a comprehensive multiwavelength temporal and spectral analysis of
the FRED GRB 070419A. The early-time emission in the -ray and X-ray
bands can be explained by a central engine active for at least 250 s, while at
late times the X-ray light curve displays a simple power-law decay. In
contrast, the observed behaviour in the optical band is complex (from 10 up
to 10 s). We investigate the light curve behaviour in the context of the
standard forward/reverse shock model; associating the peak in the optical light
curve at 450 s with the fireball deceleration time results in a Lorenz
factor at this time. In contrast, the shallow optical
decay between 450 and 1500 s remains problematic, requiring a reverse shock
component whose typical frequency is above the optical band at the optical peak
time for it to be explained within the standard model. This predicts an
increasing flux density for the forward shock component until t 4
10 s, inconsistent with the observed decay of the optical emission
from t 10 s. A highly magnetized fireball is also ruled out due to
unrealistic microphysic parameters and predicted light curve behaviour that is
not observed. We conclude that a long-lived central engine with a finely tuned
energy injection rate and a sudden cessation of the injection is required to
create the observed light curves - consistent with the same conditions that are
invoked to explain the plateau phase of canonical X-ray light curves of GRBs.Comment: 9 pages, 10 figures, accepted for publication in MNRA
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
UBVRI observations of the flickering of RS Ophiuchi at Quiescence
We report observations of the flickering variability of the recurrent nova RS
Oph at quiescence on the basis of simultaneous observations in 5 bands (UBVRI).
RS Oph has flickering source with (U-B)_0=-0.62 \pm 0.07, (B-V)_0=0.15 \pm
0.10, (V-R)_0=0.25 \pm 0.05. We find for the flickering source a temperature
T_fl = 9500 \pm 500 K, and luminosity L_fl = 50 - 150 L_sun (using a distance
of d=1.6kpc). We also find that on a (U-B) vs (B-V) diagram the flickering of
the symbiotic stars differs from that of the cataclysmic variables. The
possible source of the flickering is discussed. The data are available upon
request from the authors and on the web
www.astro.bas.bg/~rz/RSOph.UBVRI.2010.MNRAS.tar.gz.Comment: 7 pages, MNRAS (accepted
Lowly Polarized Light from a Highly Magnetized Jet of GRB 190114C
We report multicolor optical imaging and polarimetry observations of the afterglow of the first TeV-detected gamma-ray burst (GRB), GRB 190114C, using the RINGO3 and MASTER II polarimeters. Observations begin 31 s after the onset of the GRB and continue until ~7000 s postburst. The light curves reveal a chromatic break at ~400â500 s, with initial temporal decay α = 1.669 ± 0.013 flattening to α ~ 1 postbreak, which we model as a combination of reverse and forward shock components with magnetization parameter R B ~ 70. The observed polarization degree decreases from 7.7% ± 1.1% to 2%â4% 52â109 s postburst and remains steady at this level for the subsequent ~2000 s at a constant position angle. Broadband spectral energy distribution modeling of the afterglow confirms that GRB 190114C is highly obscured (A v,HG = 1.49 ± 0.12 mag; cmâ2). We interpret the measured afterglow polarization as intrinsically low and dominated by dust âin contrast to the P > 10% measured previously for other GRB reverse shocksâwith a small contribution from polarized prompt photons in the first minute. We test whether first- and higher-order inverse Compton scattering in a magnetized reverse shock can explain the low optical polarization and subteraelectronvolt emission but conclude that neither is explained in the reverse shock inverse Compton model. Instead, the unexpectedly low intrinsic polarization degree in GRB 190114C can be explained if large-scale jet magnetic fields are distorted on timescales prior to reverse shock emission
Rise and fall of the X-ray flash 080330: an off-axis jet?
Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO). DOI: 10.1051/0004-6361/200911719Context. X-ray flashes (XRFs) are a class of gamma-ray bursts (GRBs) with a peak energy of the time-integrated spectrum, , typically below 30 keV, whereas classical GRBs have of a few hundreds of keV. Apart from and the systematically lower luminosity, the properties of XRFs, such as their duration or spectral indices, are typical of the classical GRBs. Yet, the nature of XRFs and their differences from GRBs are not understood. In addition, there is no consensus on the interpretation of the shallow decay phase observed in most X-ray afterglows of both XRFs and GRBs. Aims. We examine in detail the case of XRF 080330 discovered by Swift at redshift 1.51. This burst is representative of the XRF class and exhibits an X-ray shallow decay. The rich broadband (from NIR to UV) photometric data set we collected during this phase makes it an ideal candidate for testing the off-axis jet interpretation proposed to explain both the softness of XRFs and the shallow decay phase. Methods. We present prompt -ray, early and late NIR/visible/UV and X-ray observations of the XRF 080330. We derive a spectral energy distribution from NIR to X-ray bands across the shallow/plateau phase and describe the temporal evolution of the multi-wavelength afterglow within the context of the standard afterglow model. Results. The multiwavelength evolution of the afterglow is achromatic from ~102 s to ~8104 s. The energy spectrum from NIR to X-ray is reproduced well by a simple power-law, , with = 0.790.01 and negligible rest-frame dust extinction. The light curve can be modelled by either a piecewise power-law or the combination of a smoothly broken power law with an initial rise up to ~600 s, a plateau lasting up to ~2 ks, followed by a gradual steepening to a power-law decay index of ~2 until 82 ks. At this point, a bump appears to be modelled well with a second component, while the corresponding optical energy spectrum, , reddens by = 0.260.06. Conclusions. A single-component jet viewed off-axis can explain the light curve of XRF 080330, the late-time reddening being due to the reverse shock of an energy injection episode and its being an XRF. Other possibilities, such as the optical rise marking the pre-deceleration of the fireball within a wind environment, cannot be excluded definitely, but appear to be contrived. We exclude the possibility of a dust decreasing column density being swept up by the fireball as explaining the rise of the afterglow.Peer reviewe
GRB 080319B: A Naked-Eye Stellar Blast from the Distant Universe
Long duration gamma-ray bursts (GRBs) release copious amounts of energy
across the entire electromagnetic spectrum, and so provide a window into the
process of black hole formation from the collapse of a massive star. Over the
last forty years, our understanding of the GRB phenomenon has progressed
dramatically; nevertheless, fortuitous circumstances occasionally arise that
provide access to a regime not yet probed. GRB 080319B presented such an
opportunity, with extraordinarily bright prompt optical emission that peaked at
a visual magnitude of 5.3, making it briefly visible with the naked eye. It was
captured in exquisite detail by wide-field telescopes, imaging the burst
location from before the time of the explosion. The combination of these unique
optical data with simultaneous gamma-ray observations provides powerful
diagnostics of the detailed physics of this explosion within seconds of its
formation. Here we show that the prompt optical and gamma-ray emissions from
this event likely arise from different spectral components within the same
physical region located at a large distance from the source, implying an
extremely relativistic outflow. The chromatic behaviour of the broadband
afterglow is consistent with viewing the GRB down the very narrow inner core of
a two-component jet that is expanding into a wind-like environment consistent
with the massive star origin of long GRBs. These circumstances can explain the
extreme properties of this GRB.Comment: 43 pages, 18 figures, 3 tables, submitted to Nature May 11, 200
Constraining GRB Emission Physics with Extensive Early-Time, Multiband Follow-up
Understanding the origin and diversity of emission processes responsible for
Gamma-ray Bursts (GRBs) remains a pressing challenge. While prompt and
contemporaneous panchromatic observations have the potential to test
predictions of the internal-external shock model, extensive multiband imaging
has been conducted for only a few GRBs. We present rich, early-time, multiband
datasets for two \swift\ events, GRB 110205A and GRB 110213A. The former shows
optical emission since the early stages of the prompt phase, followed by the
steep rising in flux up to ~1000s after the burst ( with
). We discuss this feature in the context of the
reverse-shock scenario and interpret the following single power-law decay as
being forward-shock dominated. Polarization measurements, obtained with the
RINGO2 instrument mounted on the Liverpool Telescope, also provide hints on the
nature of the emitting ejecta. The latter event, instead, displays a very
peculiar optical to near-infrared lightcurve, with two achromatic peaks. In
this case, while the first peak is probably due to the onset of the afterglow,
we interpret the second peak to be produced by newly injected material,
signifying a late-time activity of the central engine.Comment: 48 pages,11 figures, 24 tables. Accepted to The Astrophysical Journa
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