2,513 research outputs found
Stellar Wind Accretion in GX301-2: Evidence for a High-density Stream
The X-ray binary system GX301-2 consists of a neutron star in an eccentric
orbit accreting from the massive early-type star WRAY 977. It has previously
been shown that the X-ray orbital light curve is consistent with existence of a
gas stream flowing out from Wray 977 in addition to its strong stellar wind.
Here, X-ray monitoring observations by the Rossi X-ray Timing Explorer (RXTE)/
All-Sky-Monitor (ASM) and pointed observations by the RXTE/ Proportional
Counter Array (PCA) over the past decade are analyzed. We analyze both the flux
and column density dependence on orbital phase. The wind and stream dynamics
are calculated for various system inclinations, companion rotation rates and
wind velocities, as well as parametrized by the stream width and density. These
calculations are used as inputs to determine both the expected accretion
luminosity and the column density along the line-of-sight to the neutron star.
The model luminosity and column density are compared to observed flux and
column density vs. orbital phase, to constrain the properties of the stellar
wind and the gas stream. We find that the change between bright and medium
intensity levels is primarily due to decreased mass loss in the stellar wind,
but the change between medium and dim intensity levels is primarily due to
decreased stream density. The mass-loss rate in the stream exceeds that in the
stellar wind by a factor of 2.5. The quality of the model fits is significantly
better for lower inclinations, favoring a mass for WRAY 977 of 53 to 62 Msun.Comment: 19 pages, 6 figure
Radio Spectrum and Distance of the SNR HB9
New images are presented of the supernova remnant (SNR) HB9 based on 408 MHz
and 1420 MHz continuum emission and HI-line emission data of the Canadian
Galactic Plane Survey (CGPS) by the Dominion Radio Astrophysical Observatory
(DRAO). Two methods of spectral index analysis for HB9 are presented and
compared: one removes compact sources at both frequencies but is limited to the
resolution of the 408 MHz image; the other removes compact sources only in the
1420 MHz image so is effective at higher spatial resolution. The second allows
more detailed spectral index variation studies than the first. The two T-T plot
methods and new integrated flux densities give spectral index
(S_{\nu}\propto\nu^{-alpha}) for the whole of HB9 of 0.48+-0.03; and
0.47+-0.06, respectively. These are lower than previous spectral index for HB9
(alpha=0.61). Spatial variations of spectral index are derived using the second
method and yield a steeper spectral index for interior regions than for the
rim. This can be explained by a standard curved interstellar electron energy
spectrum combined with lower interior magnetic field compared to that near the
outer shock, which results in a larger proportion of steep spectrum emission
for lines-of-sight through the central body of the SNR. HI observations show
structures probably associated with the SNR in the radial velocity range -3 to
-9 km/s and suggest a distance of 0.8+-0.4 kpc for the SNR. This is consistent
with the distance to the radio pulsar 0458+46, offset from the center of HB9 by
23'. However the pulsar spindown and kinematic ages are significantly greater
than estimates of the SNR age: the Sedov age for HB9 is 6600 yr and the
evaporative cloud model yields ages of 4000-7,000 yr.Comment: 8 pages, 6 figures, 1 table, accepted by A&
A Monte Carlo Study of the 6.4 keV Emission at the Galactic Center
Strong fluorescent Fe line emission at 6.4 keV has been observed from the Sgr
B2 giant molecular cloud located in the Galactic Center region. The large
equivalent width of this line and the lack of an apparent illuminating nearby
object indicate that a time-dependent source, currently in a low-activity
state, is causing the fluorescent emission. It has been suggested that this
illuminator is the massive black hole candidate, Sgr A*, whose X-ray luminosity
has declined by an unprecedented six orders of magnitude over the past 300
years. We here report the results of our Monte Carlo simulations for producing
this line under a variety of source configurations and characteristics. These
indicate that the source may in fact be embedded within Sgr B2, although
external sources give a slightly better fit to the data. The weakened
distinction between the internal and external illuminators is due in part to
the instrument response function, which accounts for an enhanced equivalent
width of the line by folding some of the continuum radiation in with the
intrinsic line intensity. We also point out that although the spectrum may be
largely produced by K emission in cold gas, there is some evidence in
the data to suggest the presence of warm (~10^5 K) emitting material near the
cold cloud.Comment: 11 pages, 4 figure
The HI absorption distance of HESS J1943+213 favours its extragalactic nature
The H.E.S.S. collaboration (Abramowski et al. 2011) dicovered a new TeV
point-like source HESS J1943+213 in the Galactic plane and suggested three
possible low-energy-band counterparts: a -ray binary, a pulsar wind
nebula (PWN), or a BL Lacertae object. We measure the distance to the radio
counterpart G57.76-1.29 of HESS J1943+213. We analyze Very Large Array
observations to obtain a reliable HI absorption spectrum.The resulting distance
limit is 16 kpc. This distance strongly supports that HESS J1943+213 is
an extragalactic source, consistent with the preferred counterpart of the HESS
collaboration.Comment: 3 figures, 2 pages, A&A accepte
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