130 research outputs found
Confirmation of the \eps -- \eiso (Amati) relation from the X-ray flash XRF 050416A observed by Swift/BAT
We report Swift Burst Alert Telescope (BAT) observations of the X-ray Flash
(XRF) XRF 050416A. The fluence ratio between the 15-25 keV and 25-50 keV energy
bands of this event is 1.5, thus making it the softest gamma-ray burst (GRB)
observed by BAT so far. The spectrum is well fitted by the Band function with
E^{\rm obs}_{\rm peak} of 15.0_{-2.7}^{+2.3} keV. Assuming the redshift of the
host galaxy (z = 0.6535), the isotropic-equivalent radiated energy E_{\rm iso}
and the peak energy at the GRB rest frame (E^{\rm src}_{\rm peak}) of XRF
050416A are not only consistent with the correlation found by Amati et al. and
extended to XRFs by Sakamoto et al., but also fill-in the gap of this relation
around the 30 - 80 keV range of E^{\rm src}_{\rm peak}. This result tightens
the validity of the E^{\rm src}_{\rm peak} - E_{\rm iso} relation from XRFs to
GRBs.
We also find that the jet break time estimated using the empirical relation
between E^{\rm src}_{\rm peak} and the collimation corrected energy E_{\gamma}
is inconsistent with the afterglow observation by Swift X-ray Telescope. This
could be due to the extra external shock emission overlaid around the jet break
time or to the non existence of a jet break feature for XRF, which might be a
further challenging for GRB jet emission, models and XRF/GRB unification
scenarios.Comment: 16 pages, 4 figures; accepted for publication in ApJ
On the Early Time X-ray Spectra of Swift Afterglows I: Evidence for Anomalous Soft X-ray Emission
We have conducted a thorough and blind search for emission lines in >70 Swift
X-ray afterglows of total exposure ~10^7s. We find that most afterglows are
consistent with pure power-laws plus extinction. Significant outliers to the
population exist at the 5-10% level and have anomalously soft, possibly thermal
spectra. Four bursts are singled out via possible detections of 2-5 lines: GRBs
060218, 060202, 050822, and 050714B. Alternatively, a blackbody model with
kT~0.1-0.5 keV can describe the soft emission in each afterglow. The most
significant soft component detections in the full data set of ~2000 spectra
correspond to GRB060218/SN2006aj, with line significances ranging up to
\~20-sigma. A thermal plasma model fit to the data indicates that the flux is
primarily due to L-shell transitions of Fe at ~ solar abundance. We associate
(>4-sigma significant) line triggers in the 3 other events with K-shell
transitions in light metals. We favor a model where the possible line emission
in these afterglows arises from the mildly relativistic cocoon of matter
surrounding the GRB jet as it penetrates and exits the surface of the
progenitor star. The emitting material in each burst is at a similar distance
\~10^12--10^13 cm, a similar density ~10^17 cm^-3, and subject to a similar
flux of ionizing radiation. The lines may correlate with the X-ray flaring. For
the blackbody interpretation, the soft flux may arise from break out of the GRB
shock or plasma cocoon from the progenitor stellar wind, as recently suggested
for GRB060218 (Campana et al. 2006). Due to the low z of GRB060218, bursts
faint in Gamma-rays with fluxes dominated by this soft X-ray component could
outnumber classical GRBs 100-1.Comment: 32 pages, 10 tables, 17 figures, To Appear in ApJ v656, February 20,
200
The First Swift BAT Gamma-Ray Burst Catalog
We present the first Swift Burst Alert Telescope (BAT) catalog of gamma-ray
bursts (GRBs), which contains bursts detected by the BAT between 2004 December
19 and 2007 June 16. This catalog (hereafter BAT1 catalog) contains burst
trigger time, location, 90% error radius, duration, fluence, peak flux, and
time averaged spectral parameters for each of 237 GRBs, as measured by the BAT.
The BAT-determined position reported here is within 1.75' of the Swift X-ray
Telescope (XRT)-determined position for 90% of these GRBs. The BAT T_90 and
T_50 durations peak at 80 and 20 seconds, respectively. From the
fluence-fluence correlation, we conclude that about 60% of the observed peak
energies, Epeak, of BAT GRBs could be less than 100 keV. We confirm that GRB
fluence to hardness and GRB peak flux to hardness are correlated for BAT bursts
in analogous ways to previous missions' results. The correlation between the
photon index in a simple power-law model and Epeak is also confirmed. We also
report the current status for the on-orbit BAT calibrations based on
observations of the Crab Nebula.Comment: 63 pages, 23 figures, Accepted in ApJS, Corrected for the BAT ground
position, the image significance, and the error radius of GRB 051105, Five
machine-readable tables are available at
http://swift.gsfc.nasa.gov/docs/swift/results/bat1_catalog
Ferromagnetic redshift of the optical gap in GdN
We report measurements of the optical gap in a GdN film at temperatures from
300 to 6K, covering both the paramagnetic and ferromagnetic phases. The gap is
1.31eV in the paramagnetic phase and red-shifts to 0.9eV in the spin-split
bands below the Curie temperature. The paramagnetic gap is larger than was
suggested by very early experiments, and has permitted us to refine a
(LSDA+U)-computed band structure. The band structure was computed in the full
translation symmetry of the ferromagnetic ground state, assigning the
paramagnetic-state gap as the average of the majority- and minority-spin gaps
in the ferromagnetic state. That procedure has been further tested by a band
structure in a 32-atom supercell with randomly-oriented spins. After fitting
only the paramagnetic gap the refined band structure then reproduces our
measured gaps in both phases by direct transitions at the X point.Comment: 5 pages, 4 figure
The Spectral Lag of GRB060505: A Likely Member of the Long Duration Class
Two long gamma-ray bursts, GRB 060505 and GRB 060614, occurred in nearby
galaxies at redshifts of 0.089 and 0.125 respectively. Due to their proximity
and durations, deep follow-up campaigns to search for supernovae (SNe) were
initiated. However none were found in either case, to limits more than two
orders of magnitude fainter than the prototypical GRB-associated SN, 1998bw. It
was suggested that the bursts, in spite of their durations (4 and 102 s),
belonged to the population of short GRBs which has been shown to be unrelated
to SNe. In the case of GRB 060614 this argument was based on a number of
indicators, including the negligible spectral lag, which is consistent with
that of short bursts. GRB 060505 has a shorter duration, but no spectral lag
was measured. We present the spectral lag measurements of GRB 060505 using
Suzakus Wide Area Monitor and the Swift Burst Alert Telescope. We find that the
lag is 0.36+/- 0.05 s, inconsistent with the lags of short bursts and
consistent with the properties of long bursts and SN-GRBs. These results
support the association of GRB 060505 with other low-luminosity GRBs also found
in star-forming galaxies and indicates that at least some massive stars may die
without bright SNe.Comment: Accepted by ApJL, 5 pages, 3 Figure
The BAT-Swift Science Software
The BAT instrument tells the Swift satellite where to point to make immediate
follow-up observations of GRBs. The science software on board must efficiently
process gamma-ray events coming in at up to 34 kHz, identify rate increases
that could be due to GRBs while disregarding those from known sources, and
produce images to accurately and rapidly locate new Gamma-ray sources.Comment: 4 pages, no figures, to appear in Santa Fe proceedings "Gamma-Ray
Bursts: 30 Years of Discovery", Fenimore and Galassi (eds), AIP, 200
The Burst Alert Telescope (BAT) on the Swift MIDEX Mission
The Burst Alert Telescope (BAT) is one of 3 instruments on the Swift MIDEX
spacecraft to study gamma-ray bursts (GRBs). The BAT first detects the GRB and
localizes the burst direction to an accuracy of 1-4 arcmin within 20 sec after
the start of the event. The GRB trigger initiates an autonomous spacecraft slew
to point the two narrow field-of-view (FOV) instruments at the burst location
within 20-70 sec so to make follow-up x-ray and optical observations. The BAT
is a wide-FOV, coded-aperture instrument with a CdZnTe detector plane. The
detector plane is composed of 32,768 pieces of CdZnTe (4x4x2mm), and the
coded-aperture mask is composed of approximately 52,000 pieces of lead
(5x5x1mm) with a 1-m separation between mask and detector plane. The BAT
operates over the 15-150 keV energy range with approximately 7 keV resolution,
a sensitivity of approximately 10E-8 erg*cm^-2*s^-1, and a 1.4 sr (half-coded)
FOV. We expect to detect >100 GRBs/yr for a 2-year mission. The BAT also
performs an all-sky hard x-ray survey with a sensitivity of approximately 2
mCrab (systematic limit) and it serves as a hard x-ray transient monitor.Comment: 18 Pages, 12 Figures, To be published in Space Science Review
Correlative Analysis of GRBs detected by Swift, Konus and HETE
Swift has now detected a large enough sample of gamma-ray bursts (GRBs) to allow correlation studies of burst parameters. Such studies of earlier data sets have yielded important results leading to further understanding of burst parameters and classifications. This work focuses on seventeen Swift bursts that have also been detected either by Konus-Wind or HETE-II, providing high energy spectra and fits to E(sub peak). Eight of these bursts have spectroscopic redshifts and for others we can estimate redshifts using the variability/luminosity relationship. We can also compare E(sub peak) with E(sub iso), and for those bursts for which a jet break was observed in the afterglow we can derive E(sub g) and test the relationship between E(peak) and E(sub gamma). For all bursts we can derive durations and hardness ratios from the prompt emission
In search of progenitors for supernova-less GRBs 060505 and 060614: re-examination of their afterglows
GRB060505 and GRB060614 are nearby long-duration gamma-ray bursts (LGRBs)
without accompanying supernovae (SNe) down to very strict limits. They thereby
challenge the conventional LGRB-SN connection and naturally give rise to the
question: are there other peculiar features in their afterglows which would
help shed light on their progenitors? To answer this question, we combine new
observational data with published data and investigate the multi-band temporal
and spectral properties of the two afterglows. We find that both afterglows can
be well interpreted within the framework of the jetted standard external shock
wave model, and that the afterglow parameters for both bursts fall well within
the range observed for other LGRBs. Hence, from the properties of the
afterglows there is nothing to suggest that these bursts should have another
progenitor than other LGRBs. Recently, Swift-discovered GRB080503 also has the
spike + tail structure during its prompt gamma-ray emission seemingly similar
to GRB060614. We analyse the prompt emission of this burst and find that this
GRB is actually a hard-spike + hard-tail burst with a spectral lag of
0.80.4 s during its tail emission. Thus, the properties of the prompt
emission of GRB060614 and GRB080503 are clearly different, motivating further
thinking of GRB classification. Finally we note that, whereas the progenitor of
the two SN-less bursts remains uncertain, the core-collapse origin for the
SN-less bursts would be quite certain if a wind-like environment can be
observationally established, e.g, from an optical decay faster than the X-ray
decay in the afterglow's slow cooling phase.Comment: 15 pages, 7 figures, 4 tables, ApJ in press; added Fig. 7 of the
lag-luminosity relatio
Epeak estimator for Gamma-Ray Bursts Observed by the Swift Burst Alert Telescope
We report a correlation based on a spectral simulation study of the prompt
emission spectra of gamma-ray bursts (GRBs) detected by the Swift Burst Alert
Telescope (BAT). The correlation is between the Epeak energy, which is the peak
energy in the \nu F_\nu spectrum, and the photon index (\Gamma) derived from a
simple power-law model. The Epeak - \Gamma relation, assuming the typical
smoothly broken power-law spectrum of GRBs, is \log Epeak = 3.258 - 0.829\Gamma
(1.3 < \Gamma < 2.3). We take into account not only a range of Epeak energies
and fluences, but also distributions for both the low-energy photon index and
the high-energy photon index in the smoothly broken power-law model. The
distribution of burst durations in the BAT GRB sample is also included in the
simulation. Our correlation is consistent with the index observed by BAT and
Epeak measured by the BAT, and by other GRB instruments. Since about 85% of
GRBs observed by the BAT are acceptably fit with the simple power-law model
because of the relatively narrow energy range of the BAT, this relationship can
be used to estimate Epeak when it is located within the BAT energy range.Comment: 27 pages, 31 figures, accepted for publication in Ap
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