4,064 research outputs found
Letter from Geraldine Ferraro to the Red Cross of San Juan de Dios in Costa Rica
Letter from Geraldine Ferraro to the Red Cross of San Juan de Dios in Costa Rica. Letter has handwritten notes.https://ir.lawnet.fordham.edu/vice_presidential_campaign_correspondence_1984_international/1342/thumbnail.jp
The complete catalogue of gamma-ray bursts observed by the Wide Field Cameras on board BeppoSAX
We present the complete on-line catalogue of gamma-ray bursts observed by the
two Wide Field Cameras on board \sax in the period 1996-2002. Our aim is to
provide the community with the largest published data set of GRB's prompt
emission X-ray light curves and other useful data. This catalogue
(BS-GRBWFCcat) contains data on 77 bursts and a collection of the X-ray light
curves of 56 GRB discovered or noticed shortly after the event and of other
additional bursts detected in subsequent searches. Light curves are given in
the three X-ray energy bands (2-5, 5-10, 10-26 keV). The catalogue can be
accessed from the home web page of the ASI Science Data Center-ASDC
(http://www.asdc.asi.it)Comment: 4 pages, 3 figure
Letter from the Red Cross of San Juan de Dios in Costa Rica to Geraldine Ferraro
Letter from the Red Cross of San Juan de Dios in Costa Rica to Geraldine Ferraro.https://ir.lawnet.fordham.edu/vice_presidential_campaign_correspondence_1984_international/1341/thumbnail.jp
Broad band X-ray spectral properties of Gamma-ray bursts with BeppoSAX
In about one year, five gamma-ray bursts were simultaneously observed with
the Wide Field Cameras and Gamma Ray Burst Monitor aboard the BeppoSAX
satellite. From some of them X-ray afterglow emission has been clearly detected
with the same satellite. In order to understand how GRB emission is related to
the X-ray afterglow, we are performing a systematic study of the spectral
properties of these events. We report here preliminary results of this study.Comment: 5 pages, 3 figures. To appear in the Proceedings of the 4th
Huntsville Gamma-ray Burst Symposiu
The prompt emission of GRB990712 with BeppoSAX: evidence of a transient X-ray emission feature
We report on the prompt X- and gamma-ray observations of GRB990712 with the
BeppoSAX Gamma-Ray Burst Monitor and Wide Field Camera No. 2. Due to Sun
constraints, we could not perform a follow-up observation with the BeppoSAX
Narrow Field Instruments. The light curve of the prompt emission shows two
pulses and a total duration of about 40s in X-rays. In gamma-rays the event is
even shorter. The 2-700 keV spectral emission with time shows a discontinuity
in the peak energy Ep of the E F(E) spectrum: Ep is above our energy passband
during the first pulse and goes down to ~10 keV during the second pulse.
Another peculiarity is noted in this event for the first time: the possible
evidence of a 2s duration emission feature during the tail of the first pulse.
The feature is consistent with either a Gaussian profile with centroid energy
of 4.5 keV or a blackbody spectrum with kTbb ~1.3 keV. We discuss the possible
origin of the feature. The most attractive possibility is that we are observing
the thermal emission of a baryon-loaded expanding fireball, when it becomes
optically thin.Comment: 18 pages, 4 figures, published in The Astrophysical Journal Letters,
relevant changes in the Discussion (section 4) with respect to previous
versio
The prompt X-ray emission of GRB011211: possible evidence of a transient absorption feature
We report on observation results of the prompt X- and gamma-ray emission from
GRB011211. This event was detected with the Gamma-Ray Burst Monitor and one of
the Wide Field Cameras aboard the BeppoSAX satellite. The optical counterpart
to the GRB was soon identified and its redshift determined (z = 2.140), while
with the XMM-Newton satellite, the X-ray afterglow emission was detected.
Evidence of soft X-ray emission lines was reported by Reeves et al. (2002), but
not confirmed by other authors. In investigating the spectral evolution of the
prompt emission we find the possible evidence of a transient absorption feature
at 6.9^{+0.6}_{-0.5} keV during the rise of the primary event. The significance
of the feature is derived with non parametric tests and numerical simulations,
finding a chance probability which ranges from 3x10^{-3} down to 4x10^{-4}. The
feature shows a Gaussian profile and an equivalent width of 1.2^{+0.5}_{-0.6}
keV. We discuss our results and their possible interpretation.Comment: 23 pages, 3 Tables, 6 Figures. Accepted for publication in
Astrophysical Journa
A comparative study of the X-ray afterglow properties of optically bright and dark GRBs
We have examined the complete set of X-ray afterglow observations of dark and
optically bright GRBs performed by BeppoSAX until February 2001. X-ray
afterglows are detected in 90% of the cases. We do not find significant
differences in the X-ray spectral shape, in particular no higher X-ray
absorption in GRBs without optical transient (dark GRBs) compared to GRBs with
optical transient (OTGRBs). Rather, we find that the 1.6-10 keV flux of OTGRBs
is on average about 5 times larger than that of the dark GRBs. A K-S test shows
that this difference is significant at 99.8% probability. Under the assumption
that dark and OTGRB have similar spectra, this could suggest that the first are
uncaught in the optical band because they are just faint sources. In order to
test this hypothesis, we have determined the optical-to-X ray flux ratios of
the sample. OTGRBs show a remarkably narrow distribution of flux ratios, which
corresponds to an average optical-to-x spectral index 0.794\pm 0.054. We find
that, while 75% of dark GRBs have flux ratio upper limits still consistent with
those of OT GRBs, the remaining 25% are 4 - 10 times weaker in optical than in
X-rays. The significance of this result is equal to or higher than 2.6 sigma.
If this sub-population of dark GRBs were constituted by objects assimilable to
OTGRBs, they should have shown optical fluxes higher than upper limits actually
found. We discuss the possible causes of their behaviour, including a possible
occurrence in high density clouds or origin at very high redshift and a
connection with ancient, Population III stars.Comment: 19 pages, 5 figures. To be published in The Astrophysical Journa
Star Formation History and Chemical Evolution of the Sextans Dwarf Spheroidal Galaxy
We present the star formation history and chemical evolution of the Sextans
dSph dwarf galaxy as a function of galactocentric distance. We derive these
from the photometry of stars in the field using the SMART
model developed by Yuk & Lee (2007, ApJ, 668, 876) and adopting a closed-box
model for chemical evolution. For the adopted age of Sextans 15 Gyr, we find
that 84% of the stars formed prior to 11 Gyr ago, significant star formation
extends from 15 to 11 Gyr ago ( 65% of the stars formed 13 to 15 Gyr ago
while 25% formed 11 to 13 Gyr ago), detectable star formation continued
to at least 8 Gyr ago, the star formation history is more extended in the
central regions than the outskirts, and the difference in star formation rates
between the central and outer regions is most marked 11 to 13 Gyr ago. Whether
blue straggler stars are interpreted as intermediate age main sequence stars
affects conclusions regarding the star formation history for times 4 to 8 Gyr
ago, but this is at most only a trace population. We find that the metallicity
of the stars increased rapidly up to [Fe/H]=--1.6 in the central region and to
[Fe/H]=--1.8 in the outer region within the first Gyr, and has varied slowly
since then. The abundance ratios of several elements derived in this study are
in good agreement with the observational data based on the high resolution
spectroscopy in the literature. We conclude that the primary driver for the
radial gradient of the stellar population in this galaxy is the star formation
history, which self-consistently drives the chemical enrichment history.Comment: 36 pages, 14 figures, To appear in the ApJ, 200
Iron line signatures in X-ray afterglows of GRB by BeppoSAX
We report the possible detection (99.3% of statistical significance) of
redshifted Fe iron line emission in the X-ray afterglow of Gamma-ray burst
GRB970508 observed by BeppoSAX. Its energy is consistent with the redshift of
the putative host galaxy determined from optical spectroscopy. In contrast to
the fairly clean environment expected in the merging of two neutron stars, the
observed line properties would imply that the site of the burst is embedded in
a large mass of material (>0.5 \Ms), consistent with pre-explosion ejecta of
a very massive star. This material could be related with the outburst observed
in the afterglow 1 day after the GRB and with the spectral variations measured
during this phase. We did not find evidence of Fe line in two other GRB with
known redshift (GB971214 and GB980613), but we note that the upper limits are
of the same order of the intensity measured in GB97508 and that none of these
afterglows shows rebursting activity.Comment: 2 pages, 1 figure, proc. of Gamma-Ray Bursts in the Afterglow Era,
Nov. 1998, Rome, F. Frontera & L. Piro ed.s., A&A Suppl. Ser., in pres
Puzzling thermonuclear burst behaviour from the transient low-mass X-ray binary IGR J17473-2721
We investigate the thermonuclear bursting behaviour of IGR J17473-2721, an
X-ray transient that in 2008 underwent a six month long outburst, starting
(unusually) with an X-ray burst. We detected a total of 57 thermonuclear bursts
throughout the outburst with AGILE, Swift, RXTE, and INTEGRAL. The wide range
of inferred accretion rates (between <1% and about 20% of the Eddington
accretion rate m-dot_Edd) spanned during the outburst allows us to study
changes in the nuclear burning processes and to identify up to seven different
phases. The burst rate increased gradually with the accretion rate until it
dropped (at a persistent flux corresponding to about 15% of m-dot_Edd) a few
days before the outburst peak, after which bursts were not detected for a
month. As the persistent emission subsequently decreased, the bursting activity
resumed with a much lower rate than during the outburst rise. This hysteresis
may arise from the thermal effect of the accretion on the surface nuclear
burning processes, and the timescale is roughly consistent with that expected
for the neutron star crust thermal response. On the other hand, an undetected
superburst, occurring within a data gap near the outburst peak, could have
produced a similar quenching of burst activity.Comment: 18 pages, 12 figures, 1 table, accepted for publication in MNRA
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