341 research outputs found
Testing the Epeak - Eiso relation for GRBs detected by Swift and Suzaku-WAM
One of the most prominent, yet controversial associations derived from the
ensemble of prompt-phase observations of gamma-ray bursts (GRBs) is the
apparent correlation in the source frame between the peak energy Epeak) of the
nu-F(nu) spectrum and the isotropic radiated energy, Eiso. Since most gamma-ray
bursts (GRBs) have Epeak above the energy range (15-150 keV) of the Burst Alert
Telescope (BAT) on Swift, determining accurate Epeak values for large numbers
of Swift bursts has been difficult. However, by combining data from Swift/BAT
and the Suzaku Wide-band All-Sky Monitor (WAM), which covers the energy range
from 50-5000 keV, for bursts which are simultaneously detected, one can
accurately fit Epeak and Eiso and test the relationship between them for the
Swift sample. Between the launch of Suzaku in July 2005 and the end of April
2009, there were 48 gamma-ray bursts (GRBs) which triggered both Swift/BAT and
WAM and an additional 48 bursts which triggered Swift and were detected by WAM,
but did not trigger. A BAT-WAM team has cross-calibrated the two instruments
using GRBs, and we are now able to perform joint fits on these bursts to
determine their spectral parameters. For those bursts with spectroscopic
redshifts, we can also calculate the isotropic energy. Here we present the
results of joint Swift/BAT-Suzaku/WAM spectral fits for 91 of the bursts
detected by the two instruments. We show that the distribution of spectral fit
parameters is consistent with distributions from earlier missions and confirm
that Swift bursts are consistent with earlier reported relationships between
Epeak and isotropic energy. We show through time-resolved spectroscopy that
individual burst pulses are also consistent with this relationship.Comment: Accepted for publication in the Astrophysical Journa
Probing the ambient medium of GRB 090618 with XMM-Newton observations
Long Gamma-ray Bursts (GRBs) signal the death of massive stars. The afterglow
emission can be used to probe the progenitor ambient through a detailed study
of the absorption pattern imprinted by the circumburst material as well as the
host galaxy interstellar medium on the continuum spectrum. This has been done
at optical wavelengths with impressive results. Similar studies can in
principle be carried out in the X-ray band, allowing us to shed light on the
material metallicity, composition and distance of the absorber. We start
exploiting this route through high resolution spectroscopy XMM-Newton
observations of GRB 090618. We find a high metallicity absorbing medium (Z> 0.2
Zsun) with possible enhancements of S and Ne with respect to the other elements
(improving the fit at a level of >3.4 sigma). Including the metallicity effects
on the X-ray column density determination, the X-ray and optical evaluations of
the absorption are in agreement for a Small Magellanic Cloud extinction curve.Comment: Accepted for publication on MNRAS (7 pages, 6 figures
Super-solar metallicity at the position of the ultra-long GRB130925A
Over the last decade there has been immense progress in the follow-up of
short and long GRBs, resulting in a significant rise in the detection rate of
X-ray and optical afterglows, in the determination of GRB redshifts, and of the
identification of the underlying host galaxies. Nevertheless, our theoretical
understanding on the progenitors and central engines powering these vast
explosions is lagging behind, and a newly identified class of `ultra-long' GRBs
has fuelled speculation on the existence of a new channel of GRB formation. In
this paper we present high signal-to-noise X-shooter observations of the host
galaxy of GRB130925A, which is the fourth unambiguously identified ultra-long
GRB, with prompt gamma-ray emission detected for ~20ks. The GRB line of sight
was close to the host galaxy nucleus, and our spectroscopic observations cover
both this region along the bulge/disk of the galaxy, in addition to a bright
star-forming region within the outskirts of the galaxy. From our broad
wavelength coverage we obtain accurate metallicity and dust-extinction
measurements at both the galaxy nucleus, and an outer star-forming region, and
measure a super-solar metallicity at both locations, placing this galaxy within
the 10-20% most metal-rich GRB host galaxies. Such a high metal enrichment has
implications on the progenitor models of both long and ultra-long GRBs,
although the edge-on orientation of the host galaxy does not allow us to rule
out a large metallicity variation along our line of sight. The spatially
resolved spectroscopic data presented in this paper offer important insight
into variations in the metal and dust abundance within GRB host galaxies. They
also illustrate the need for IFU observations on a larger sample of GRB host
galaxies at varies metallicities to provide a more quantitative view on the
relation between the GRB circumburst and the galaxy-whole properties.Comment: 9 pages, 3 figures, A&A in press, matches published versio
The Two-Component Afterglow of Swift GRB 050802
This paper investigates GRB 050802, one of the best examples of a it Swift
gamma-ray burst afterglow that shows a break in the X-ray lightcurve, while the
optical counterpart decays as a single power-law. This burst has an optically
bright afterglow of 16.5 magnitude, detected throughout the 170-650nm spectral
range of the UVOT on-board Swift. Observations began with the XRT and UVOT
telescopes 286s after the initial trigger and continued for 1.2 x 10^6s. The
X-ray lightcurve consists of three power-law segments: a rise until 420s,
followed by a slow decay with alpha_2 = 0.63 +/- 0.03 until 5000s, after which,
the lightcurve decays faster with a slope of alpha_3 = 1.59 +/- 0.03. The
optical lightcurve decays as a single power-law with alpha_O = 0.82 +/- 0.03
throughout the observation. The X-ray data on their own are consistent with the
break at 5000s being due to the end of energy injection. Modelling the optical
to X-ray spectral energy distribution, we find that the optical afterglow can
not be produced by the same component as the X-ray emission at late times,
ruling out a single component afterglow. We therefore considered two-component
jet models and find that the X-ray and optical emission is best reproduced by a
model in which both components are energy injected for the duration of the
observed afterglow and the X-ray break at 5000s is due to a jet break in the
narrow component. This bright, well-observed burst is likely a guide for
interpreting the surprising finding of Swift that bursts seldom display
achromatic jet breaks.Comment: 13 pages, 5 figures, accepted MNRA
GRB 081028 and its late-time afterglow re-brightening
‘The definitive version is available at www3.interscience.wiley.com '. Copyright Royal Astronomical SocietySwift captured for the first time a smoothly rising X-ray re-brightening of clear non-flaring origin after the steep decay in a long gamma-ray burst (GRB): GRB 081028. A rising phase is likely present in all GRBs but is usually hidden by the prompt tail emission and constitutes the first manifestation of what is later to give rise to the shallow decay phase. Contemporaneous optical observations reveal a rapid evolution of the injection frequency of a fast cooling synchrotron spectrum through the optical band, which disfavours the afterglow onset (start of the forward shock emission along our line of sight when the outflow is decelerated) as the origin of the observed re-brightening. We investigate alternative scenarios and find that the observations are consistent with the predictions for a narrow jet viewed off-axis. The high on-axis energy budget implied by this interpretation suggests different physical origins of the prompt and (late) afterglow emission. Strong spectral softening takes place from the prompt to the steep decay phase: we track the evolution of the spectral peak energy from the γ-rays to the X-rays and highlight the problems of the high latitude and adiabatic cooling interpretations. Notably, a softening of both the high and low spectral slopes with time is also observed. We discuss the low on-axis radiative efficiency of GRB 081028 comparing its properties against a sample of Swift long GRBs with secure Eγ,iso measurements.Peer reviewe
GRB 081203A: Swift UVOT captures the earliest ultraviolet spectrum of a gamma-ray burst
We present the earliest ultraviolet (UV) spectrum of a gamma-ray burst (GRB) as observed with the Swift Ultra-Violet/Optical Telescope (UVOT). The GRB 081203A spectrum was observed for 50 s with the UV-grism starting 251 s after the Swift-Burst-Alert-Telescope (BAT) trigger. During this time, the GRB was ≈13.4 mag (u filter) and was still rising to its peak optical brightness. In the UV-grism spectrum, we find a damped Lyα line, Lyβ and the Lyman continuum break at a redshift z= 2.05 ± 0.01. A model fit to the Lyman absorption implies a gas column density of log NH i= 22.0 ± 0.1 cm−2, which is typical of GRB host galaxies with damped Lyα absorbers. This observation of GRB 081203A demonstrates that for brighter GRBs (v≈ 14 mag) with moderate redshift (0.5 < z < 3.5) the UVOT is able to provide redshifts, and probe for damped Lyα absorbers within 4–6 min from the time of the Swift-BAT trigger
Swift detects a remarkable gamma-ray burst, GRB 060614, that introduces a new classification scheme
Gamma ray bursts (GRBs) are known to come in two duration classes, separated
at ~2 s. Long bursts originate from star forming regions in galaxies, have
accompanying supernovae (SNe) when near enough to observe and are likely caused
by massive-star collapsars. Recent observations show that short bursts
originate in regions within their host galaxies with lower star formation rates
consistent with binary neutron star (NS) or NS - black hole (BH) mergers.
Moreover, although their hosts are predominantly nearby galaxies, no SNe have
been so far associated with short GRBs. We report here on the bright, nearby
GRB 060614 that does not fit in either class. Its ~102 s duration groups it
with long GRBs, while its temporal lag and peak luminosity fall entirely within
the short GRB subclass. Moreover, very deep optical observations exclude an
accompanying supernova, similar to short GRBs. This combination of a long
duration event without accompanying SN poses a challenge to both a collapsar
and merging NS interpretation and opens the door on a new GRB classification
scheme that straddles both long and short bursts.Comment: 13 pages, 2 figures, accepted in Natur
Prompt optical observations of GRB050319 with the Swift UVOT
The UVOT telescope on the Swift observatory has detected optical afterglow
emission from GRB 050319. The flux declines with a power law slope of alpha =
-0.57 between the start of observations some 230 seconds after the burst onset
(90s after the burst trigger) until it faded below the sensitivity threshold of
the instrument after ~5 x 10^4s. There is no evidence for the rapidly declining
component in the early light curve that is seen at the same time in the X-ray
band. The afterglow is not detected in UVOT shortward of the B-band, suggesting
a redshift of about 3.5. The optical V-band emission lies on the extension of
the X-ray spectrum, with an optical to X-ray slope of beta = -0.8. The
relatively flat decay rate of the burst suggests that the central engine
continues to inject energy into the fireball for as long as a few x 10^4s after
the burst.Comment: 16 pages, 4 figures, accepted by Ap
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