590 research outputs found
BeppoSAX observations of the black hole candidates LMC X-1 and LMC X-3
We describe BeppoSAX observations of the black hole candidates LMC X--1 and
LMC X--3 performed in Oct. 1997. Both sources can be modelled by a multicolor
accretion disk spectrum, with temperature keV. However, there is some
evidence that a thin emitting component coexists with the thick disk at these
temperatures. In the direction of LMC X--1, we detected a significant emission
above 10 keV, which we suspect originates from the nearby source PSR 0540-69.
For LMC X--1, we estimate an absorbing column density of cm, which is almost ten times larger than that found for LMC
X--3. In both sources, we find no indication of emission or absorption features
whatsoever.Comment: 4 pages, 2 figures. Accepted for pubblication in the Proc. of 32nd
Cospar scientific assembly, Nagoya, 13-15 July 199
Evidence for a Molecular Cloud Origin for Gamma-Ray Bursts: Implications for the Nature of Star Formation in the Universe
It appears that the majority of rapidly-, well-localized gamma-ray bursts
with undetected, or dark, optical afterglows, or `dark bursts' for short, occur
in clouds of size R > 10L_{49}^{1/2} pc and mass M > 3x10^5L_{49} M_{sun},
where L is the isotropic-equivalent peak luminosity of the optical flash. We
show that clouds of this size and mass cannot be modeled as a gas that is bound
by pressure equilibrium with a warm or hot phase of the interstellar medium
(i.e., a diffuse cloud): Such a cloud would be unstable to gravitational
collapse, resulting in the collapse and fragmentation of the cloud until a
burst of star formation re-establishes pressure equilibrium within the
fragments, and the fragments are bound by self-gravity (i.e., a molecular
cloud). Consequently, dark bursts probably occur in molecular clouds, in which
case dark bursts are probably a byproduct of this burst of star formation if
the molecular cloud formed recently, and/or the result of lingering or latter
generation star formation if the molecular cloud formed some time ago. We then
show that if bursts occur in Galactic-like molecular clouds, the column
densities of which might be universal, the number of dark bursts can be
comparable to the number of bursts with detected optical afterglows: This is
what is observed, which suggests that the bursts with detected optical
afterglows might also occur in molecular clouds. We confirm this by modeling
and constraining the distribution of column densities, measured from absorption
of the X-ray afterglow, of the bursts with detected optical afterglows: We find
that this distribution is consistent with the expectation for bursts that occur
in molecular clouds, and is not consistent with the expectation for bursts that
occur in diffuse clouds. More...Comment: Accepted to The Astrophysical Journal, 22 pages, 6 figures, LaTe
Radio Monitoring of the January 11, 1997 Gamma-Ray Burst
We report on a comprehensive radio monitoring program of the bright gamma-ray
burster GRB970111. These VLA observations were made at a frequency of 1.4 GHz
and span a range of post-burst timescales between 28 hours and one month.
Despite extensive sampling at sub-milliJansky sensitivities, no radio source
was detected above 0.5 mJy in the current best error box (~14 arcmin^2) for
GRB970111. A highly unusual radio source, VLA J1528.7+1945, was seen to drop in
flux density by a factor of two in our monitoring period but it lies outside
the error box and thus it is unlikely to be related to GRB970111. Cosmological
fireball models of gamma-ray bursts make predictions of late-time emission
occurring at longer wavelengths. The absence of a flaring or fading radio
counterpart to GRB970111 provides strong constraints on these models.Comment: ApJ Let (accepted
Multifrequency observations of XTE J0421+560/CI Cam in outburst
We report on two X-ray observations of the transient source XTE J0421+560
performed by BeppoSAX, and on a series of observations performed by the 0.7m
Teramo-Normale Telescope. Outburst peak occurrence time and duration depend on
photon energy: the outburst peak is achieved first in the X-ray band, then in
the optical and finally in the radio. An exponential decay law fits well the
X-ray data except in the TOO2 0.5-1.0 keV band, where erratic time variability
is detected. During TOO1 the e-folding time scale decreases with energy up to ~
20 keV, when it achieves a saturation; during TOO2 it decreases up to ~ 2 keV
and then increases. This change is correlated with a spectral change,
characterized by the onset of a soft (< 2 keV) component in TOO2 (Orr et al.
1998). This component might originate from the relativistic jets, while the
hard component is more likely associated to processes occurring in the
circumstellar matter and/or near the compact object. Optical observations show
that the object appears intrinsically red even during the outburst. The nature
of the compact object is discussed.Comment: Four pages. Accepted for publication in A&A Lette
Identification of an Extended Accretion Disk Corona in the Hercules X-1 Low State: Moderate Optical Depth, Precise Density Determination, and Verification of CNO Abundances
We identify an accretion disk atmosphere and corona from the high resolution
X-ray spectrum of Hercules X-1, and we determine its detailed physical
properties. More than two dozen recombination emission lines (from Fe XXVI at
1.78 A to N VI at 29.08 A) and Fe K-alpha, K-beta fluorescence lines were
detected in a 50 ks observation with the Chandra High-Energy Transmission
Grating Spectrometer (HETGS). They allow us to measure the density,
temperature, spatial distribution, elemental composition, and kinematics of the
plasma. We exclude HZ Her as the source of the recombination emission. We
compare accretion disk model atmospheres with the observed spectrum in order to
constrain the stratification of density and ionization, disk atmosphere area,
elemental composition, and energetics. The atmospheric spectrum observed during
the low state is photoionized by the main-on X-ray continuum, indicating that
the disk is observed edge-on during the low state. We infer the mean number of
scatterings N of Ly-alpha and Ly-beta line photons from H-like ions. We derive
N < 69 for O VIII Ly_alpha_1, which rules out the presence of a mechanism
modeled by Sako (2003) to enhance N VII emission via a line overlap with O
VIII. The line optical depth diagnostics are consistent with a flattened
atmosphere. Our spectral analysis, the disk atmosphere model, and the presence
of intense N VII and N VI lines (plus N V in the UV), confirm the
over-abundance of nitrogen relative to other metals, which was shown to be
indicative of CNO cycle processing in a massive progenitor.Comment: 38 pages, 14 figures, accepted for publication in Ap
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