511 research outputs found
X-ray Shots of Cyg X-1
X-ray shots of Cyg X-1 in different energy bands and spectral states have
been studied with PCA/RXTE observations. The detailed shot structure is
obtained by superposing many shots with one millisecond time bin through
aligning their peaks with an improved algorithm. In general, the shots are
composed of a slow rise and fast decay. The shot structures in the different
states are different. The duration of shot in the high state is shorter than
that in the low and transition states. The shot profile in the high energy band
is more asymmetric and narrower than that in the low energy band. The average
hardness of shot is lower than that of steady emission in the transition and
low states but higher than that in the high state. The time lags between the
shots in higher and lower energy bands have been found in the different states.
In transition states, the time lag is the largest among the different states of
Cyg X-1, and it is the smallest in the low state. The implications of the
observed shot features for shot models are discussed.Comment: Latex, 20 pages, 12 figures(included), to be published in Ap
Neutron Star Masses and Radii as Inferred from kilo-Hertz QPOs
Kilo-Hertz (kHz) Quasi-periodic oscillations (QPOs) have been discovered in
the X-ray fluxes of 8 low-mass X-ray binaries (LMXBs) with the Rossi X-ray
Timing Explorer (RXTE). The characteristics of these QPOs are remarkably
similar from one source to another. In particular, the highest observed QPO
frequencies for 6 of the 8 sources fall in a very narrow range: 1,066 to 1,171
Hz. This is the more remarkable when one considers that these sources are
thought to have very different luminosities and magnetic fields, and produce
very different count rates in the RXTE detectors. Therefore it is highly
unlikely that this near constancy of the highest observed frequencies is due to
some unknown selection effect or instrumental bias. In this letter we propose
that the highest observed QPO frequency can be taken as the orbital frequency
of the marginally stable orbit. This leads to the conclusions that the neutron
stars in these LMXBs are inside their marginally stable orbits and have masses
in the vicinity of 2.0 solar masses. This mass is consistent with the
hypothesis that these neutron stars were born with about 1.4 solar masses and
have been accreting matter at a fraction of the Eddington limit for 100 million
years.Comment: 7 pages, uses aas2pp4.sty, Accepted by ApJ
X-ray Observations and Infrared Identification of the Transient 7.8 s X-ray Binary Pulsar XTE J1829-098
XMM-Newton and Chandra observations of the transient 7.8 s pulsar XTE
J1829-098 are used to characterize its pulse shape and spectrum, and to
facilitate a search for an optical or infrared counterpart. In outburst, the
absorbed, hard X-ray spectrum with Gamma = 0.76+/-0.13 and N_H = (6.0+/-0.6) x
10^{22} cm^{-2} is typical of X-ray binary pulsars. The precise Chandra
localization in a faint state leads to the identification of a probable
infrared counterpart at R.A. = 18h29m43.98s, decl. = -09o51'23.0" (J2000.0)
with magnitudes K=12.7, H=13.9, I>21.9, and R>23.2. If this is a highly
reddened O or B star, we estimate a distance of 10 kpc, at which the maximum
observed X-ray luminosity is 2x10^{36} ergs s^{-1}, typical of Be X-ray
transients or wind-fed systems. The minimum observed luminosity is
3x10^{32}(d/10 kpc)^2 ergs s^{-1}. We cannot rule out the possibility that the
companion is a red giant. The two known X-ray outbursts of XTE J1829-098 are
separated by ~1.3 yr, which may be the orbital period or a multiple of it, with
the neutron star in an eccentric orbit. We also studied a late M-giant
long-period variable that we found only 9" from the X-ray position. It has a
pulsation period of ~1.5 yr, but is not the companion of the X-ray source.Comment: 6 pages, 7 figures. To appear in The Astrophysical Journa
An X-Ray Dip in the X-Ray Transient 4U 1630-47
An x-ray dip was observed during a 1996 Rossi X-Ray Timing Explorer
observation of the recurrent x-ray transient 4U 1630-47. During the dip, the
2-60 keV x-ray flux drops by a factor of about three, and, at the lowest point
of the dip, the x-ray spectrum is considerably softer than at non-dip times. We
find that the 4U 1630-47 dip is best explained by absorption of the inner part
of an accretion disk, while the outer part of the disk is unaffected. The
spectral evolution during the dip is adequately described by the variation of a
single parameter, the column density obscuring the inner disk.Comment: 13 pages, 4 figures, Accepted for publication in Ap
Exploring the spreading layer of GX 9+9 using RXTE and INTEGRAL
We have fitted ~200 RXTE and INTEGRAL spectra of the neutron star LMXB GX 9+9
from 2002-2007 with a model consisting of a disc blackbody and another
blackbody representing the spreading layer (SL), i.e. an extended accretion
zone on the NS surface as opposed to the more traditional disc-like boundary
layer. Contrary to theory, the SL temperature was seen to increase towards low
SL luminosities, while the approximate angular extent had a nearly linear
luminosity dependency. Comptonization was not required to adequately fit these
spectra. Together with the ~70 degree upper bound of inclination implied by the
lack of eclipses, the best-fitting normalization of the accretion disc
blackbody component implies a distance of ~10 kpc, instead of the usually
quoted 5 kpc.Comment: 14 pages, 10 figures, accepted for publication in MNRA
On the two types of steady hard X-ray states of GRS 1915+105
Using the data of 5 years of RXTE observations we investigate the X-ray
spectral and timing properties of GRS 1915+105 during the hard steady states.
According to the results of our simultaneous X-ray spectral and timing analysis
the behavior the source during the hard steady states can be reduced to a
couple of major distinct types. i) Type I states: The dominant hard component
of the energy spectrum has characteristic quasi- exponential cut-off at 50-120
keV. The broad-band power density spectrum of the source shows significant high
frequency noise component with a cut-off at 60-80 Hz. ii) Type II states: The
hard spectral component has a break in its slope at ~12-20 keV. The high
frequency part of the power density spectrum fades quickly lacking significant
variability at frequencies higher than ~30 Hz. These two types of the X-ray
hard states are also clearly distinguished by their properties in the radio
band: while during the type I observations the source tends to be
'radio-quiet', the type II observations are characterized by high level of
radio flux ('plateau' radio states). In this work we demonstrate aforementioned
differences using the data of 12 representative hard steady state observations.
We conclude that the difference between these two types can be probably
explained in terms of different structure of the accretion flow in the
immediate vicinity of the compact object due to presence of relativistic
outflow of matter.Comment: 16 pages, including 3 figures, submitted to Astrophysical Journal
Letter
Strong Field Gravity and X-Ray Observations of 4U1820-30
The behavior of quasi-periodic oscillations (QPOs) at frequencies near 1 kHz
in the x-ray emission from the neutron star x-ray binary 4U1820-30 has been
interpreted as evidence for the existence of the marginally stable orbit, a key
prediction of strong-field general relativity. The signature of the marginally
stable orbit is a saturation in QPO frequency, assumed to track inner disk
radius, versus mass accretion rate. Previous studies of 4U1820-30 have used
x-ray count rate as an indicator of mass accretion rate. However, x-ray count
rate is known to not correlate robustly with mass accretion rate or QPO
frequency in other sources. Here, we examine the QPO frequency dependence on
two other indicators of mass accretion rate: energy flux and x-ray spectral
shape. Using either of these indicators, we find that the QPO frequency
saturates at high mass accretion rates. We interpret this as strong evidence
for the existence of the marginally stable orbit.Comment: accepted to the Astrophysical Journal Letters, 7 page
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