20 research outputs found
Examining the evidence for dust destruction in GRB 980703
The effects that gamma-ray bursts have on their environments is an important
and outstanding issue. Dust destruction in particular has long been predicted
while observational evidence is difficult to obtain. We examine the evidence
for dust destruction by GRB 980703, in which various inconsistent measurements
of the host galaxy extinction have been made using the GRB afterglow emission.
We construct a spectral energy distribution from nIR to X-ray to measure the
extinction at early times and compare this with previous findings. We also
construct nIR/optical SEDs at intermediate epochs to examine a previously
reported decrease in extinction. The extinction is very high for a GRB host
galaxy. The earliest extinction measurement is likely to be lower than
previously estimated, and consistent with most later measurements. In a series
of SEDs we do not find any evidence of variable extinction. We therefore
conclude that there is no clear evidence of dust destruction in this case.Comment: 6 pages, 4 figures, accepted for publication in A&
The Swift capture of a long X-ray burst from XTE J1701-407
XTE J1701-407 is a new transient X-ray source discovered on June 8th, 2008.
More than one month later it showed a rare type of thermonuclear explosion: a
long type I X-ray burst. We report herein the results of our study of the
spectral and flux evolution during this burst, as well as the analysis of the
outburst in which it took place. We find an upper limit on the distance to the
source of 6.1 kpc by considering the maximum luminosity reached by the burst.
We measure a total fluence of 3.5*10^{-6} erg/cm^2 throughout the ~20 minutes
burst duration and a fluence of 2.6*10^{-3} erg/cm^2 during the first two
months of the outburst. We show that the flux decay is best fitted by a power
law (index ~1.6) along the tail of the burst. Finally, we discuss the
implications of the long burst properties, and the presence of a second and
shorter burst detected by Swift ten days later, for the composition of the
accreted material and the heating of the burning layer.Comment: MNRAS-Letters, accepted. Minor changes according to referee's report.
5 pages, 3 figure
Constraints on AGN accretion disc viscosity derived from continuum variability
We estimate a value of the viscosity parameter in AGN accretion discs for the
PG quasar sample. We assume that optical variability on time-scales of months
to years is caused by local instabilities in the inner accretion disc.
Comparing the observed variability time-scales to the thermal time-scales of
alpha-disc models we obtain constraints on the viscosity parameter for the
sample. We find that, at a given L/L_Edd, the entire sample is consistent with
a single value of the viscosity parameter, alpha. We obtain constraints of 0.01
< alpha < 0.03 for 0.01 < L/L_Edd < 1.0. This narrow range suggests that these
AGN are all seen in a single state, with a correspondingly narrow spread of
black hole masses or accretion rates. The value of alpha we derive is
consistent with predictions by current simulation s in which MHD turbulence is
the primary viscosity mechanism.Comment: 7 pages, 5 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
The Rapidly Flaring Afterglow of the Very Bright and Energetic GRB 070125
We report on multi-wavelength observations, ranging from the X-ray to radio
wave bands, of the IPN-localized gamma-ray burst GRB 070125. Spectroscopic
observations reveal the presence of absorption lines due to O I, Si II, and C
IV, implying a likely redshift of z = 1.547. The well-sampled light curves, in
particular from 0.5 to 4 days after the burst, suggest a jet break at 3.7 days,
corresponding to a jet opening angle of ~7.0 degrees, and implying an intrinsic
GRB energy in the 1 - 10,000 keV band of around E = (6.3 - 6.9)x 10^(51) erg
(based on the fluences measured by the gamma-ray detectors of the IPN network).
GRB 070125 is among the brightest afterglows observed to date. The spectral
energy distribution implies a host extinction of Av < 0.9 mag. Two
rebrightening episodes are observed, one with excellent time coverage, showing
an increase in flux of 56% in ~8000 seconds. The evolution of the afterglow
light curve is achromatic at all times. Late-time observations of the afterglow
do not show evidence for emission from an underlying host galaxy or supernova.
Any host galaxy would be subluminous, consistent with current GRB host-galaxy
samples. Evidence for strong Mg II absorption features is not found, which is
perhaps surprising in view of the relatively high redshift of this burst and
the high likelihood for such features along GRB-selected lines of sight.Comment: 50 pages, 9 figures, 5 tables Accepted to the Astrophysical Journa
The Highly Energetic Expansion of SN2010bh Associated with GRB 100316D
We present the spectroscopic and photometric evolution of the nearby (z =
0.059) spectroscopically confirmed type Ic supernova, SN 2010bh, associated
with the soft, long-duration gamma-ray burst (X-ray flash) GRB 100316D.
Intensive follow-up observations of SN 2010bh were performed at the ESO Very
Large Telescope (VLT) using the X-shooter and FORS2 instruments. Owing to the
detailed temporal coverage and the extended wavelength range (3000--24800 A),
we obtained an unprecedentedly rich spectral sequence among the hypernovae,
making SN 2010bh one of the best studied representatives of this SN class. We
find that SN 2010bh has a more rapid rise to maximum brightness (8.0 +/- 1.0
rest-frame days) and a fainter absolute peak luminosity (L_bol~3e42 erg/s) than
previously observed SN events associated with GRBs. Our estimate of the ejected
(56)Ni mass is 0.12 +/- 0.02 Msun. From the broad spectral features we measure
expansion velocities up to 47,000 km/s, higher than those of SNe 1998bw (GRB
980425) and 2006aj (GRB 060218). Helium absorption lines He I lambda5876 and He
I 1.083 microm, blueshifted by ~20,000--30,000 km/s and ~28,000--38,000 km/s,
respectively, may be present in the optical spectra. However, the lack of
coverage of the He I 2.058 microm line prevents us from confirming such
identifications. The nebular spectrum, taken at ~186 days after the explosion,
shows a broad but faint [O I] emission at 6340 A. The light-curve shape and
photospheric expansion velocities of SN 2010bh suggest that we witnessed a
highly energetic explosion with a small ejected mass (E_k ~ 1e52 erg and M_ej ~
3 Msun). The observed properties of SN 2010bh further extend the heterogeneity
of the class of GRB supernovae.Comment: 37 pages and 12 figures (one-column pre-print format), accepted for
publication in Ap