1,409 research outputs found
Physical parameters of GRB 970508 and GRB 971214 from their afterglow synchrotron emission
We have calculated synchrotron spectra of relativistic blast waves, and find
predicted characteristic frequencies that are more than an order of magnitude
different from previous calculations. For the case of an adiabatically
expanding blast wave, which is applicable to observed gamma-ray burst (GRB)
afterglows at late times, we give expressions to infer the physical properties
of the afterglow from the measured spectral features.
We show that enough data exist for GRB970508 to compute unambiguously the
ambient density, n=0.03/cm**3, and the blast wave energy per unit solid angle,
E=3E52 erg/4pi sr. We also compute the energy density in electrons and magnetic
field. We find that they are 12% and 9%, respectively, of the nucleon energy
density and thus confirm for the first time that both are close to but below
equipartition.
For GRB971214, we discuss the break found in its spectrum by Ramaprakash et
al. (1998). It can be interpreted either as the peak frequency or as the
cooling frequency; both interpretations have some problems, but on balance the
break is more likely to be the cooling frequency. Even when we assume this, our
ignorance of the self-absorption frequency and presence or absence of beaming
make it impossible to constrain the physical parameters of GRB971214 very well.Comment: very strongly revised analysis of GRB971214 and discussion, submitted
to ApJ, 11 pages LaTeX, 4 figures, uses emulateapj.sty (included
GRB 970228 Revisited: Evidence for a Supernova in the Light Curve and La te Spectral Energy Distribution of the Afterglow
At the time of its discovery, the optical and X-ray afterglow of GRB 970228
appeared to be a ringing endorsement of the previously untried relativistic
fireball model of gamma-ray burst (GRB) afterglows, but now that nearly a dozen
optical afterglows to GRBs have been observed, the wavering light curve and
reddening spectrum of this afterglow make it perhaps the most difficult of the
observed afterglows to reconcile with the fireball model. In this Letter, we
argue that this afterglow's unusual temporal and spectral properties can be
attributed to a supernova that overtook the light curve nearly two weeks after
the GRB. This is the strongest case yet for a GRB/supernova connection. It
strengthens the case that a supernova also dominated the late afterglow of GRB
980326, and the case that GRB 980425 is related to SN 1998bw.Comment: Accepted to The Astrophysical Journal (Letters), 14 pages, LaTe
Intrinsic spectra and energetics of BeppoSAX Gamma-Ray Bursts with known redshifts
We present the main results of a study of spectral and energetics properties
of twelve gamma-ray bursts (GRBs) with redshift estimates. All GRBs in our
sample were detected by BeppoSAX in a broad energy range (2-700 keV). From the
redshift estimates and the good-quality BeppoSAX time-integrated spectra we
deduce the main properties of GRBs in their cosmological rest frames. All
spectra in our sample are satisfactorily represented by the Band model with no
significant soft X-ray excesses or spectral absorptions. We find a positive
correlation between the estimated total (isotropic) energies in the 1-10000 keV
energy range (E_rad) and redshifts z. Interestingly, more luminous GRBs are
characterized also by larger peak energies E_p of their EF(E) spectra.
Furthermore, more distant GRBs appear to be systematically harder in the X-ray
band compared to GRBs with lower redshifts. We discuss how selection and data
truncation effects could bias our results and give possible explanations for
the correlations that we found.Comment: 10 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Valence change of praseodymium in Pr0.5Ca0.5CoO3 investigated by x-ray absorption spectroscopy
X-ray absorption spectroscopy measurements in Pr0.5Ca0.5CoO3 were performed
at the Pr M4,5, Pr L3, and Ca L2,3 absorption edges as a function of
temperature below 300 K. Ca spectra show no changes down to 10 K while a
noticeable thermally dependent evolution takes place at the Pr edges across the
metal-insulator transition. Spectral changes are analyzed by different methods,
including multiple scattering simulations, which provide quantitative details
on an electron loss at Pr 4f orbitals. We conclude that in the insulating phase
a fraction [15(+5)%] of Pr3+ undergoes a further oxidation to adopt a
hybridized configuration composed of an admixture of atomic-like 4f1 states
(Pr4+) and f- symmetry states on the O 2p valence band (Pr3+L states)
indicative of a strong 4f- 2p interaction.Comment: 19 pages (.doc), 4 figures, Phys. Rev. B, in pres
Disappearance of Hard X-ray Emission in the Last BeppoSAX Observation of the Z Source GX 349+2
We report on the results from two BeppoSAX observations of the Z source GX
349+2 performed in February 2001 and covering the broad energy range 0.12-200
keV. The light curve obtained from these observations shows a large flaring
activity, the count rate varying from ~130 to ~260 counts/s, indicating that
the source was in the flaring branch during these observations. The average
spectrum is well described by a soft blackbody and a Comptonized component. To
well fit the energy spectrum three gaussian lines are needed at 1.2 keV, 2.6
keV, and 6.7 keV with corresponding equivalent widths of 13 eV, 10 eV, and 39
eV, probably associated to L-shell emission of Fe XXIV, Ly-alpha S XVI, and Fe
XXV, respectively. These lines may be produced at different distances from the
neutron star, which increase when the count rate of the source increases. An
absorption edge is also needed at 9 keV with an optical depth of ~3 10^{-2}.
From the Color-Color Diagram (CD) we selected five zones from which we
extracted the corresponding energy spectra. The temperatures of the blackbody
and of the Comptonized component tend to increase when the intensity of the
source increases. We discuss our results comparing them to those obtained from
a previous BeppoSAX observation, performed in March 2000, during which the
source was a similar position of its Z-track. In particular we find that,
although the source showed similar spectral states in the 2000 and the 2001
observations, a hard tail, that was significantly detected in March 2000, is
not observed in these recent observations.Comment: Accepted for publication on Ap
Lapex: A Phoswich balloon experiment for hard X-ray astronomy
Satellite and balloon observations have shown that several classes of celestial objects are hard ( 15 keV) energy band with a sensitivity of approx 10 mCrab has been performed with the UCSD/MIT instrument (A4) on board the HEAO 1 satellite. About 70 X-ray sources were detected, including galactic and extragalactic objects. Hard X-ray emission has been detected in the Galaxy from X-ray pulsars. Extragalactic sources of hard X-ray emission include clusters of galaxies, QSOs, BL Lac objects, Seyfert galaxies. The essential characteristics of the Large Area Phoswich Experiment (LAPEX) for crowded sky field observations are described. It has: (1) a broad energy band of operation (20-300 keV); (2) a 3 sigma sensitivity of about 1 mCrab in 10,000 s of live observing time; and (3) imaging capabilities with an angular resolution of about 20'
- …