10,089 research outputs found
Highly ionized atoms in cooling gas
The ionization of low density gas cooling from a high temperature was calculated. The evolution during the cooling is assumed to be isochoric, isobaric, or a combination of these cases. The calculations are used to predict the column densities and ultraviolet line luminosities of highly ionized atoms in cooling gas. In a model for cooling of a hot galactic corona, it is shown that the observed value of N(N V) can be produced in the cooling gas, while the predicted value of N(Si IV) falls short of the observed value by a factor of about 5. The same model predicts fluxes of ultraviolet emission lines that are a factor of 10 lower than the claimed detections of Feldman, Brune, and Henry. Predictions are made for ultraviolet lines in cooling flows in early-type galaxies and clusters of galaxies. It is shown that the column densities of interest vary over a fairly narrow range, while the emission line luminosities are simply proportional to the mass inflow rate
BATSE Soft Gamma-Ray Observations of GROJ0422+32
We report results of a comprehensive study of the soft gamma-ray (30 keV to
1.7 MeV) emission of GROJ0422+32 during its first known outburst in 1992. These
results were derived from the BATSE earth-occultation database with the JPL
data analysis package, EBOP (Enhanced BATSE Occultation Package). Results
presented here focus primarily on the long-term temporal and spectral
variability of the source emission associated with the outburst. The light
curves with 1-day resolution in six broad energy-bands show the high-energy
flux (>200 keV) led the low-energy flux (<200 keV) by ~5 days in reaching the
primary peak, but lagged the latter by ~7 days in starting the declining phase.
We confirm the "secondary maximum" of the low-energy (<200 keV) flux at TJD
8970-8981, ~120 days after the first maximum. Our data show that the "secondary
maximum" was also prominent in the 200-300 keV band, but became less pronounced
at higher energies. During this 200-day period, the spectrum evolved from a
power-law with photon index of 1.75 on TJD 8839, to a shape that can be
described by a Comptonized model or an exponential power law below 300 keV,
with a variable power-law tail above 300 keV. The spectrum remained roughly in
this two-component shape until ~9 November (TJD 8935) and then returned to the
initial power-law shape with an index of ~2 until the end of the period. The
correlation of the two spectral shapes with the high and low luminosities of
the soft gamma-ray emission is strongly reminiscent of that seen in Cygnus X-1.
We interpret these results in terms of the Advection Dominated Accretion Flow
(ADAF) model with possibly a "jet-like" region that persistently produced the
non-thermal power-law gamma rays observed throughout the event.Comment: 40 pages total, including 10 figures and 2 table
Chandra Observations of SN 2004et and the X-ray Emission of Type IIp Supernovae
We report the X-ray detection of the Type II-plateau supernova SN 2004et in
the spiral galaxy NGC 6946, using the Chandra X-Ray Observatory. The position
of the X-ray source was found to agree with the optical position within ~0.4
arcsec. Chandra also surveyed the region before the 2004 event, finding no
X-ray emission at the location of the progenitor. For the post-explosion
observations, a total of 202, 151, and 158 photons were detected in three
pointings, each ~29 ks in length, on 2004 October 22, November 6, and December
3, respectively. The spectrum of the first observation is best fit by a thermal
model with a temperature of kT=1.3 keV and a line-of-sight absorption of
N_H=1.0 x 10^{22} cm^{-2}. The inferred unabsorbed luminosity (0.4-8 keV) is
~4x10^{38} erg/s, adopting a distance of 5.5 Mpc. A comparison between hard and
soft counts on the first and third epochs indicates a softening over this time,
although there is an insufficient number of photons to constrain the variation
of temperature and absorption by spectral fitting. We model the emission as
arising from the reverse shock region in the interaction between the supernova
ejecta and the progenitor wind. For a Type IIP supernova with an extended
progenitor, the cool shell formed at the time of shock wave breakout from the
star can affect the initial evolution of the interaction shell and the
absorption of radiation from the reverse shock. The observed spectral softening
might be due to decreasing shell absorption. We find a pre-supernova mass loss
rate of (2-2.5)x 10^{-6} M_{\odot} /yr for a wind velocity of 10 kms, which is
in line with expectations for a Type IIP supernova.Comment: total 19 pages including 7 figures. ApJ, in press. See
http://spider.ipac.caltech.edu/staff/rho/preprint/SN2004etms.ps for the paper
including full resolution image
X-ray Spectroscopy of Candidate Ultracompact X-ray Binaries
We present high-resolution spectroscopy of the neutron star/low-mass X-ray
binaries (LMXBs) 4U 1850-087 and 4U 0513-40 as part of our continuing study of
known and candidate ultracompact binaries. The LMXB 4U 1850-087 is one of four
systems in which we had previously inferred an unusual Ne/O ratio in the
absorption along the line of sight, most likely from material local to the
binaries. However, our recent Chandra X-ray Observatory LETGS spectrum of 4U
1850-087 finds a Ne/O ratio by number of 0.22+/-0.05, smaller than previously
measured and consistent with the expected interstellar value. We propose that
variations in the Ne/O ratio due to source variability, as previously observed
in these sources, can explain the difference between the low- and
high-resolution spectral results for 4U 1850-087. Our XMM-Newton RGS
observation of 4U 0513-40 also shows no unusual abundance ratios in the
absorption along the line of sight. We also present spectral results from a
third candidate ultracompact binary, 4U 1822-000, whose spectrum is well fit by
an absorbed power-law + blackbody model with absorption consistent with the
expected interstellar value. Finally, we present the non-detection of a fourth
candidate ultracompact binary, 4U 1905+000, with an upper limit on the source
luminosity of < 1 x 10^{32} erg s^{-1}. Using archival data, we show that the
source has entered an extended quiescent state.Comment: 8 pages, 3 figures, accepted for publication to the Astrophysical
Journa
Gamma-Ray Burst Environments and Progenitors
Likely progenitors for the GRBs (gamma-ray bursts) are the mergers of compact
objects or the explosions of massive stars. These two cases have distinctive
environments for the GRB afterglow: the compact object explosions occur in the
ISM (interstellar medium) and those of massive stars occur in the preburst
stellar wind. We calculate the expected afterglow for a burst in a Wolf-Rayet
star wind and compare the results to those for constant, interstellar density.
The optical afterglow for the wind case is generally expected to decline more
steeply than in the constant density case, but this effect may be masked by
variations in electron spectral index, and the two cases have the same
evolution in the cooling regime. Observations of the concurrent radio and
optical/X-ray evolution are especially useful for distinguishing between the
two cases. The different rates of decline of the optical and X-ray afterglows
of GRB 990123 suggest constant density interaction for this case. We have
previously found strong evidence for wind interaction in SN 1998bw/GRB 980425
and here present a wind model for GRB 980519. We thus suggest that there are
both wind type GRB afterglows with massive star progenitors and ISM type
afterglows with compact binary star progenitors. The wind type bursts are
likely to be accompanied by a supernova, but not the ISM type.Comment: 11 pages, 1 figure, revised version, ApJ Letters, in pres
X-Ray Spectroscopy of the Low-Mass X-ray Binaries 2S 0918-549 and 4U1543-624: Evidence for Neon-Rich Degenerate Donors
We present high-resolution spectroscopy of the neutron-star/low-mass X-ray
binaries 2S 0918-549 and 4U 1543-624 with the High Energy Transmission Grating
Spectrometer onboard the Chandra X-ray Observatory and the Reflection Grating
Spectrometer onboard XMM-Newton. Previous low-resolution spectra of both
sources showed a broad line-like feature at 0.7 keV that was originally
attributed to unresolved line emission. We recently showed that this feature
could also be due to excess neutral Ne absorption, and this is confirmed by the
new high-resolution Chandra spectra. The Chandra spectra are each well fit by
an absorbed power-law + blackbody model with a modified Ne/O number ratio of
0.52+/-0.12 for 2S 0918-549 and 1.5+/-0.3 for 4U 1543-624, compared to the
interstellar-medium value of 0.18. The XMM spectrum of 2S 0918-549 is best fit
by an absorbed power-law model with a Ne/O number ratio of 0.46+/-0.03,
consistent with the Chandra result. On the other hand, the XMM spectrum of 4U
1543-624 is softer and less luminous than the Chandra spectrum and has a
best-fit Ne/O number ratio of 0.54+/-0.03. The difference between the measured
abundances and the expected interstellar ratio, as well as the variation of the
column densities of O and Ne in 4U 1543-624, supports the suggestion that there
is absorption local to these binaries. We propose that the variations in the O
and Ne column densities of 4U 1543-624 are caused by changes in the ionization
structure of the local absorbing material. It is important to understand the
effect of ionization on the measured absorption columns before the abundance of
the local material can be determined. This work supports our earlier suggestion
that 2S 0918-549 and 4U 1543-624 are ultracompact binaries with Ne-rich
companions.Comment: 11 pages, 5 figures, major revisions including addition of XMM
spectral analysis, accepted for publication in the Astrophysical Journal,
vol. 59
Influence of the r-mode instability on hypercritically accreting neutron stars
We have investigated an influence of the r-mode instability on
hypercritically accreting () neutron stars in
close binary systems during their common envelope phases based on the scenario
proposed by Bethe et al. \shortcite{bethe-brown-lee}. On the one hand neutron
stars are heated by the accreted matter at the stellar surface, but on the
other hand they are also cooled down by the neutrino radiation. At the same
time, the accreted matter transports its angular momentum and mass to the star.
We have studied the evolution of the stellar mass, temperature and rotational
frequency.
The gravitational-wave-driven instability of the r-mode oscillation strongly
suppresses spinning-up of the star, whose final rotational frequency is well
below the mass-shedding limit, typically as small as 10% of that of the
mass-shedding state. On a very short time scale the rotational frequency tends
to approach a certain constant value and saturates there as far as the amount
of the accreted mass does not exceed a certain limit to collapse to a black
hole. This implies that the similar mechanism of gravitational radiation as the
so-called Wagoner star may work in this process. The star is spun up by
accretion until the angular momentum loss by gravitational radiation balances
the accretion torque. The time-integrated dimensionless strain of the radiated
gravitational wave may be large enough to be detectable by the gravitational
wave detectors such as LIGO II.Comment: 6 pages, 3 figure
Time-dependence in Relativistic Collisionless Shocks: Theory of the Variable "Wisps" in the Crab Nebula
We describe results from time-dependent numerical modeling of the
collisionless reverse shock terminating the pulsar wind in the Crab Nebula. We
treat the upstream relativistic wind as composed of ions and electron-positron
plasma embedded in a toroidal magnetic field, flowing radially outward from the
pulsar in a sector around the rotational equator. The relativistic cyclotron
instability of the ion gyrational orbit downstream of the leading shock in the
electron-positron pairs launches outward propagating magnetosonic waves.
Because of the fresh supply of ions crossing the shock, this time-dependent
process achieves a limit-cycle, in which the waves are launched with
periodicity on the order of the ion Larmor time. Compressions in the magnetic
field and pair density associated with these waves, as well as their
propagation speed, semi-quantitatively reproduce the behavior of the wisp and
ring features described in recent observations obtained using the Hubble Space
Telescope and the Chandra X-Ray Observatory. By selecting the parameters of the
ion orbits to fit the spatial separation of the wisps, we predict the period of
time variability of the wisps that is consistent with the data. When coupled
with a mechanism for non-thermal acceleration of the pairs, the compressions in
the magnetic field and plasma density associated with the optical wisp
structure naturally account for the location of X-ray features in the Crab. We
also discuss the origin of the high energy ions and their acceleration in the
equatorial current sheet of the pulsar wind.Comment: 13 pages, 4 figures, accepted to ApJ. High-resolution figures and
mpeg movies available at http://astron.berkeley.edu/~anatoly/wisp
- âŠ