286 research outputs found
Dust in the wind: the role of recent mass loss in long gamma-ray bursts
We study the late-time (t>0.5 days) X-ray afterglows of nearby (z<0.5) long
Gamma-Ray Bursts (GRB) with Swift and identify a population of explosions with
slowly decaying, super-soft (photon index Gamma_x>3) X-ray emission that is
inconsistent with forward shock synchrotron radiation associated with the
afterglow. These explosions also show larger-than-average intrinsic absorption
(NH_x,i >6d21 cm-2) and prompt gamma-ray emission with extremely long duration
(T_90>1000 s). Chance association of these three rare properties (i.e. large
NH_x,i, super-soft Gamma_x and extreme duration) in the same class of
explosions is statistically unlikely. We associate these properties with the
turbulent mass-loss history of the progenitor star that enriched and shaped the
circum-burst medium. We identify a natural connection between NH_x,i Gamma_x
and T_90 in these sources by suggesting that the late-time super-soft X-rays
originate from radiation reprocessed by material lost to the environment by the
stellar progenitor before exploding, (either in the form of a dust echo or as
reprocessed radiation from a long-lived GRB remnant), and that the interaction
of the explosion's shock/jet with the complex medium is the source of the
extremely long prompt emission. However, current observations do not allow us
to exclude the possibility that super-soft X-ray emitters originate from
peculiar stellar progenitors with large radii that only form in very dusty
environments.Comment: 6 pages, Submitted to Ap
Precursors of short gamma-ray bursts
We carried out a systematic search of precursors on the sample of short GRBs
observed by Swift. We found that ~8-10% of short GRBs display such early
episode of emission. One burst (GRB 090510) shows two precursor events, the
former ~13 s and the latter ~0.5 s before the GRB. We did not find any
substantial difference between the precursor and the main GRB emission, and
between short GRBs with and without precursors. We discuss possible mechanisms
to reproduce the observed precursor emission within the scenario of compact
object mergers. The implications of our results on quantum gravity constraints
are also discussed.Comment: 8 pages, 2 figures, accepted for publication in The Astrophysical
Journa
Variable Ly alpha sheds light on the environment surrounding GRB 090426
Long duration gamma-ray bursts are commonly associated with the deaths of
massive stars. Spectroscopic studies using the afterglow as a light source
provide a unique opportunity to unveil the medium surrounding it, probing the
densest region of their galaxies. This material is usually in a low ionisation
state and at large distances from the burst site, hence representing the normal
interstellar medium in the galaxy. Here we present the case of GRB 090426 at
z=2.609, whose optical spectrum indicates an almost fully ionised medium
together with a low column density of neutral hydrogen. For the first time, we
also observe variations in the Ly alpha absorption line. Photoionisation
modeling shows that we are probing material from the vicinity of the burst (~80
pc). The host galaxy is a complex of two luminous interacting galaxies, which
might suggest that this burst could have occurred in an isolated star-forming
region outside its host galaxy created in the interaction of the two galaxies.Comment: 11 pages, 9 figures, submitted to MNRA
Swift multi-wavelength observations of the bright flaring burst GRB051117A
We report on the temporal and spectral characteristics of the early X-ray
emission from the Gamma Ray Burst 051117A as observed by Swift. The superb
quality of the early X-ray light-curve and spectra of this source, one of the
brightest seen by the X-ray Telescope at such early times, allows an
unprecedented look at the spectral and temporal evolution of the prompt and
early afterglow emission for this GRB and allows us to place stringent limits
on the detection of lines. The X-ray light-curve at early times is
characteristic of a shot-noise process, with individual shots well-modelled by
a fast-rise and exponential decay spanning a broad range in rise-times and
decay rates. A temporal spectral analysis of the early light-curve shows that
the photon index and source intensity are highly correlated with the spectrum
being significantly harder when brighter, consistent with the movement of the
peak of the Band function to lower energies following individual flares. The
high quality spectrum obtained from the first orbit of WT mode data, enables us
to place a 3 sigma upper limit on the strength of any emission line features of
EW < 15 eV, assuming a narrow emission-line of 100 eV at the peak of the
effective area (abridged).Comment: Accepted 15/3/2007 - To appear in A&
On the jet structure and magnetic field configuration of GRB 020813
The polarization curve of GRB 020813 is discussed and compared to different
models for the structure, evolution and magnetisation properties of the jet and
the interstellar medium onto which the fireball impacts. GRB 020813 is best
suited for this kind of analysis for the smoothness of its afterglow light
curve, ensuring the applicability of current models. The polarization dataset
allows us to rule out the standard GRB jet, in which the energy and Lorentz
factor have a well defined value inside the jet opening angle and the magnetic
field is generated at the shock front. We explore alternative models finding
that a structured jet or a jet with a toroidal component of the magnetic field
can fit equally well the polarization curve. Stronger conclusions cannot be
drawn due to the incomplete sampling of the polarization curve. A more dense
sampling, especially at early times, is required to pin down the structure of
the jet and the geometry of its magnetic field.Comment: 7 pages, 5 postscript figures, minor revisions according to the
referee comments. A&A in pres
The First Survey of X-ray Flares from Gamma Ray Bursts Observed by Swift: Spectral Properties and Energetics
Observations of gamma ray bursts (GRBs) with Swift produced the initially
surprising result that many bursts have large X-ray flares superimposed on the
underlying afterglow. The flares were sometimes intense, had rapid rise and
decay phases, and occurred late relative to the ``prompt'' phase. Some
remarkable flares are observed with fluence comparable to the prompt GRB
fluence. Many GRBs have several flares, which are sometimes overlapping. Short,
intense, repetitive, and late flaring can be most easily understood within the
context of the standard fireball model with the internal engine that powers the
prompt GRB emission in an active state at late times. However, other models for
flares have been proposed. Flare origin can be investigated by comparing the
flare spectra to that of the afterglow and the initial prompt emission. In this
work, we have analyzed all significant X-ray flares from the first 110 GRBs
observed by Swift. From this sample 33 GRBs were found to have significant
X-ray flares, with 77 flares that were detected above the 3 level. In
addition to temporal analysis presented in a companion paper, a variety of
spectral models have been fit to each flare. In some cases, we find that the
spectral fits favor a Band function model, which is more akin to the prompt
emission than to that of an afterglow. We find that the average fluence of the
flares is 2.4e-7 erg/cm^2/s in the 0.2-10 keV energy band, which is
approximately a factor of ten below the average prompt GRB fluence. These
results, when combined with those presented in the companion paper on temporal
properties of flares, supports the hypothesis that most X-ray flares are
late-time activity of the internal engine that spawned the initial GRB; not an
afterglow related effect.Comment: accepted by ApJ; 39 pages with 14 figures and 7 table
Luminous infrared galaxies as possible sources of the UHE cosmic rays
Ultra High Energy (UHE) particles coming from discrete extragalactic sources
are potential candidates for EAS events above a few tens of EeV. In particular,
galaxies with huge infrared luminosity triggered by collision and merging
processes are possible sites of UHECR acceleration. Using the PSCz catalogue of
IR galaxies we calculate a large scale anisotropy of UHE protons originating in
the population of the luminous infrared galaxies (LIRGs). Small angle particle
scattering in weak irregular extragalactic magnetic fields as well as
deflection by regular Galactic field are taken into account. We give analytical
formulae for deflection angles with included energy losses on cosmic microwave
background (CMB). The hypotheses of the anisotropic and isotropic distributions
of the experimental data above 40 EeV from AGASA are checked, using various
statistical tests. We show that on the basis of the small scale clustering
analysis there is a much better correlation of the UHECRs data below GZK
cut-off with the predictions of the LIRG origin than with those of isotropy. We
derive analytical formulae for a probability of a given number of doublets,
triplets and quadruplets for any density distribution of independent events on
the sky. The famous AGASA UHE triple event is found to be very well correlated
on the sky with the brightest extragalactic infrared source within 70 Mpc -
merger galaxies Arp 299 (NGC 3690 + IC 694).Comment: 17 pages, 10 figures, accepted for publ: Journal of Physics
A New Frequency-Luminosity Relation for Long GRBs?
We have studied power density spectra (PDS) of 206 long Gamma-Ray Bursts
(GRBs). We fitted the PDS with a simple power-law and extracted the exponent of
the power-law (alpha) and the noise-crossing threshold frequency (f_th). We
find that the distribution of the extracted alpha peaks around -1.4 and that of
f_th around 1 Hz. In addition, based on a sub-set of 58 bursts with known
redshifts, we show that the redshift-corrected threshold frequency is
positively correlated with the isotropic peak luminosity. The correlation
coefficient is 0.57 +/- 0.03.Comment: 9 pages, 17 figures, 1 table; Accepted for publication in MNRA
The X-ray emission lines in GRB afterglows: the evidence for the two-component jet model
Recently, X-ray emission lines have been observed in X-ray afterglows of
several -ray bursts. It is a major breakthrough for understanding the
nature of the progenitors. It is proposed that the X-ray emission lines can be
well explained by the Geometry-Dominated models, but in these models the
illuminating angle is much larger than that of the collimated jet of the
-ray bursts(GRBs). For GRB 011211, we obtain the illuminating angle is
about , while the angle of GRB jet is only ,
so we propose that the outflow of the GRBs with emission lines should have two
distinct components. The wide component illuminates the reprocessing material,
and produces the emission lines, while the narrow one produces the -ray
bursts. The observations show that the energy for producing the emission lines
is higher than that of the GRBs. In this case, when the wide component
dominates the afterglows, a bump will appear in the GRBs afterglows. For GRB
011211, the emergence time of the bump is less than 0.05 days after the GRB, it
is obviously too early for the observation to catch it. With the presence of
the X-ray emission lines there should also be a bright emission component
between the UV and the soft X-rays. These features can be tested by the
satellite in the near future.Comment: 10 pags, 1 figure, ChJAA in pres
The resolved fraction of the Cosmic X-ray Background
We present the X-ray source number counts in two energy bands (0.5-2 and 2-10
keV) from a very large source sample: we combine data of six different surveys,
both shallow wide field and deep pencil beam, performed with three different
satellites (ROSAT, Chandra and XMM-Newton). The sample covers with good
statistics the largest possible flux range so far: [2.4*10^-17 - 10^-11] cgs in
the soft band and [2.1*10^-16 - 8*10^{-12}]cgs in the hard band. Integrating
the flux distributions over this range and taking into account the (small)
contribution of the brightest sources we derive the flux density generated by
discrete sources in both bands. After a critical review of the literature
values of the total Cosmic X--Ray Background (CXB) we conclude that, with the
present data, the 94.3%, and 88.8% of the soft and hard CXB can be ascribed to
discrete source emission. If we extrapolate the analytical form of the Log
N--Log S distribution beyond the flux limit of our catalog in the soft band we
find that the flux from discrete sources at ~3*10^-18 cgs is consistent with
the entire CXB, whereas in the hard band it accounts for only 93% of the total
CXB at most, hinting for a faint and obscured population to arise at even
fainter fluxes.Comment: Accepted for publication in Ap
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