69 research outputs found
Indication of Two Classes in the Swift Short Gamma-Ray Bursts from the XRT X-Ray Afterglow Light Curves
We present the discovery of two distinct classes in the Swift short duration
gamma-ray bursts (S-GRBs) from the X-Ray Telescope (XRT) X-ray afterglow light
curve. We find that about 40% of the Swift S-GRBs have an X-ray afterglow light
curves which only lasts less than 10000 seconds after the burst trigger
(hereafter short-lived S-GRBs). On the other hand, another 60% of S-GRBs have a
long lasting X-ray afterglow light curve which resembles the long duration
gamma-ray bursts. We also find that none of the short-lived S-GRBs shows the
extended emission in the Burst Alert Telescope (BAT) energy range. We compare
the burst properties for both the prompt emission and the afterglow, and
discuss the possibility of different progenitors for the Swift short GRBs.Comment: 3 pages, 6 figures, Submitted to Proceedings of Huntsville 2008
Symposium on GRB
An Extended Burst Tail from SGR 1900+14 with a Thermal X-ray Spectrum
The Soft Gamma Repeater, SGR 1900+14, entered a new phase of activity in
April 2001 initiated by the intermediate flare recorded on April 18. Ten days
following this flare, we discovered an abrupt increase in the source flux
between consecutive RXTE orbits. This X-ray flux excess decayed over the next
several minutes and was subsequently linked to a high fluence burst from SGR
1900+14 recorded by other spacecraft (Ulysses and KONUS) while the SGR was
Earth-occulted for RXTE. We present here spectral and temporal analysis of both
the burst of 28 April and the long X-ray tail following it. We find strong
evidence of an exclusively thermal X-ray tail in this event and bring this
evidence to bear on other bursts and flares from SGR 1900+14 which have shown
extended X-ray excesses (e.g. 1998 August 29). We include in this comparison a
discussion of the physical origins of SGR bursts and extended X-ray tails.Comment: 27 pages, 13 figures, ApJ submissio
CMBPol Mission Concept Study: Foreground Science Knowledge and Prospects
We report on our knowledge of Galactic foregrounds, as well as on how a CMB
satellite mission aiming at detecting a primordial B-mode signal (CMBPol) will
contribute to improving it. We review the observational and analysis techniques
used to constrain the structure of the Galactic magnetic field, whose presence
is responsible for the polarization of Galactic emissions. Although our current
understanding of the magnetized interstellar medium is somewhat limited,
dramatic improvements in our knowledge of its properties are expected by the
time CMBPol flies. Thanks to high resolution and high sensitivity instruments
observing the whole sky at frequencies between 30 GHz and 850 GHz, CMBPol will
not only improve this picture by observing the synchrotron emission from our
galaxy, but also help constrain dust models. Polarized emission from
interstellar dust indeed dominates over any other signal in CMBPol's highest
frequency channels. Observations at these wavelengths, combined with
ground-based studies of starlight polarization, will therefore enable us to
improve our understanding of dust properties and of the mechanism(s)
responsible for the alignment of dust grains with the Galactic magnetic field.
CMBPol will also shed new light on observations that are presently not well
understood. Morphological studies of anomalous dust and synchrotron emissions
will indeed constrain their natures and properties, while searching for
fluctuations in the emission from heliospheric dust will test our understanding
of the circumheliospheric interstellar medium. Finally, acquiring more
information on the properties of extra-Galactic sources will be necessary in
order to maximize the cosmological constraints extracted from CMBPol's
observations of CMB lensing. (abridged)Comment: 43 pages, 7 figures, 2 table
Chandra Observations of the Faintest Low-Mass X-ray Binaries
There exists a group of persistently faint galactic X-ray sources that, based
on their location in the galaxy, high L_x/L_opt, association with X-ray bursts,
and absence of low frequency X-ray pulsations, are thought to be low-mass X-ray
binaries (LMXBs). We present results from Chandra observations for eight of
these systems: 4U 1708-408, 2S 1711-339, KS 1739-304, SLX 1735-269, GRS
1736-297, SLX 1746-331, 1E 1746.7-3224, and 4U 1812-12. Locations for all
sources, excluding GRS 1736-297, SLX 1746-331, and KS 1739-304 (which were not
detected) were improved to 0.6" error circles (90% confidence). Our
observations support earlier findings of transient behavior of GRS 1736-297, KS
1739-304, SLX 1746-331, and 2S 1711-339 (which we detect in one of two
observations). Energy spectra for 4U 1708-408, 2S 1711-339, SLX 1735-269, 1E
1746.7-3224, and 4U 1812-12 are hard, with power law indices typically 1.4-2.1,
which are consistent with typical faint LMXB spectra.Comment: 15 pages, 3 figures. Accepted by Ap
Spectral Lags of Gamma-Ray Bursts from Primordial Black Hole (PBH) Evaporations
Primordial Black Holes (PBHs), which may have been created in the early
Universe, are predicted to be detectable by their Hawking radiation. PBHs with
an initial mass of 5.0 * 10^14 g should be expiring today with a burst of high
energy particles. Evaporating PBHs in the solar neighborhood are candidate
Gamma-Ray Bursts (GRBs) progenitors. We propose spectral lag, which is the
temporal delay between the high energy photon pulse and the low energy photon
pulse, as a possible method to detect PBH evaporation events with the Fermi
Gamma-ray Space Telescope Observatory.Comment: 3 pages; Published in the proceedings of Huntsville 2008 symposium on
GRBs; Indices in Equation 7 and 8 correcte
Temporal and Spectral Characteristics of Short Bursts from the Soft Gamma Repeaters 1806-20 and 1900+14
We study the temporal and coarse spectral properties of 268 bursts from SGR
1806-20 and 679 bursts from SGR 1900+14, all observed with the Rossi X-Ray
Timing Explorer/Proportional Counter Array. Hardness ratios and temporal
parameters, such as T_90 durations and tau_90 emission times are determined for
these bursts. We find a lognormal distribution of burst durations, ranging over
more than two orders of magnitude: T_90 ~ 10^-2 to >~ 1 s, with a peak at ~ 0.1
s. The burst light curves tend to be asymmetrical, with more than half of all
events showing rise times t_r < 0.3 T_90. We find that there exists a
correlation between the duration and fluence of bursts from both sources. We
also find a significant anti-correlation between hardness ratio and fluence for
SGR 1806-20 bursts and a marginal anti-correlation for SGR 1900+14 events.
Finally, we discuss possible physical implications of these results within the
framework of the magnetar model.Comment: 24 pages, 11 figures, accepted for publication in Ap
The optical afterglow of the short gamma-ray burst GRB 050709
It has long been known that there are two classes of gamma-ray bursts (GRBs),
mainly distinguished by their durations. The breakthrough in our understanding
of long-duration GRBs (those lasting more than ~2 s), which ultimately linked
them with energetic Type Ic supernovae, came from the discovery of their
long-lived X-ray and optical afterglows, when precise and rapid localizations
of the sources could finally be obtained. X-ray localizations have recently
become available for short (duration <2 s) GRBs, which have evaded optical
detection for more than 30 years. Here we report the first discovery of
transient optical emission (R-band magnitude ~23) associated with a short
burst; GRB 050709. The optical afterglow was localized with subarcsecond
accuracy, and lies in the outskirts of a blue dwarf galaxy. The optical and
X-ray afterglow properties 34 h after the GRB are reminiscent of the afterglows
of long GRBs, which are attributable to synchrotron emission from
ultrarelativistic ejecta. We did not, however, detect a supernova, as found in
most nearby long GRB afterglows, which suggests a different origin for the
short GRBs.Comment: 11 pages, 3 figures, press material at http://www.astro.ku.dk/dark
The optical counterpart to gamma-ray burst GRB970228 observed using the Hubble Space Telescope
Although more than 2,000 astronomical gamma-ray bursts (GRBs) have been
detected, and numerous models proposed to explain their occurrence, they have
remained enigmatic owing to the lack of an obvious counterpart at other
wavelengths. The recent ground-based detection of a transient source in the
vicinity of GRB 970228 may therefore have provided a breakthrough. The optical
counterpart appears to be embedded in an extended source which, if a galaxy as
has been suggested, would lend weight to those models that place GRBs at
cosmological distances. Here we report the observations using the Hubble Space
Telescope of the transient counterpart and extended source 26 and 39 days after
the initial gamma-ray outburst. We find that the counterpart has faded since
the initial detection (and continues to fade), but the extended source exhibits
no significant change in brightness between the two dates of observations
reported here. The size and apparent constancy between the two epochs of HST
observations imply that it is extragalactic, but its faintness makes a
definitive statement about its nature difficult. Nevertheless, the decay
profile of the transient source is consistent with a popular impulsive-fireball
model, which assumes a merger between two neutron stars in a distant galaxy.Comment: 11 pages + 2 figures. To appear in Nature (29 May 1997 issue
NuSTAR Observations of X-Ray Bursts from the Magnetar 1E 1048.1-5937
We report the detection of eight bright X-ray bursts from the 6.5 s magnetar 1E 1048.15937, during a 2013 July observation campaign with the Nuclear Spectroscopic Telescope Array. We study the morphological and spectral properties of these bursts and their evolution with time. The bursts resulted in count rate increases by orders of magnitude, sometimes limited by the detector dead time, and showed blackbody spectra with kT is approx. 6-8 keV in the T90 duration of 1-4 s, similar to earlier bursts detected from the source. We find that the spectra during the tail of the bursts can be modeled with an absorbed blackbody with temperature decreasing with flux. The burst flux decays followed a power law of index 0.8-0.9. In the burst tail spectra, we detect a is approx. 13 keV emission feature, similar to those reported in previous bursts from this source as well as from other magnetars observed with the Rossi X-ray Timing Explorer.We explore possible origins of the spectral feature such as proton cyclotron emission, which implies a magnetic field strength of B is approx. 210(exp15) G in the emission region. However, the consistency of the energy of the feature in different objects requires further explanation
Prompt and Afterglow Emission Properties of Gamma-Ray Bursts with Spectroscopically Identified Supernovae
We present a detailed spectral analysis of the prompt and afterglow emission
of four nearby long-soft gamma-ray bursts (GRBs 980425, 030329, 031203, and
060218) that were spectroscopically found to be associated with type Ic
supernovae, and compare them to the general GRB population. For each event, we
investigate the spectral and luminosity evolution, and estimate the total
energy budget based upon broadband observations. The observational inventory
for these events has become rich enough to allow estimates of their energy
content in relativistic and sub-relativistic form. The result is a global
portrait of the effects of the physical processes responsible for producing
long-soft GRBs. In particular, we find that the values of the energy released
in mildly relativistic outflows appears to have a significantly smaller scatter
than those found in highly relativistic ejecta. This is consistent with a
picture in which the energy released inside the progenitor star is roughly
standard, while the fraction of that energy that ends up in highly relativistic
ejecta outside the star can vary dramatically between different events.Comment: 55 pages including 23 figures and 8 tables. Accepted for publication
in ApJ. Replaced with the accepted versio
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