1,741 research outputs found
Production of Millisecond Dips in Sco X-1 Count Rates by Dead Time Effects
Chang et al. (2006) reported millisecond duration dips in the X-ray intensity
of Sco X-1 and attributed them to occultations of the source by small
trans-Neptunian objects (TNOs). We have found multiple lines of evidence that
these dips are not astronomical in origin, but rather the result of high-energy
charged particle events in the RXTE PCA detectors. Our analysis of the RXTE
data indicates that at most 10% of the observed dips in Sco X-1 could be due to
occultations by TNOs, and, furthermore, we find no positive or supporting
evidence for any of them being due to TNOs. We therefore believe that it is a
mistake to conclude that any TNOs have been detected via occultation of Sco
X-1.Comment: Submitted to ApJ; uses emulateapj.cls, 8 pages with 8 figure
Use of Gas Electron Multiplier (GEM) Detectors for an Advanced X-ray Monitor
We describe a concept for a NASA SMEX Mission in which Gas Electron
Multiplier (GEM) detectors, developed at CERN, are adapted for use in X-ray
astronomy. These detectors can be used to obtain moderately large detector area
and two-dimensional photon positions with sub mm accuracy in the range of 1.5
to 15 keV. We describe an application of GEMs with xenon gas, coded mask
cameras, and simple circuits for measuring event positions and for
anticoincidence rejection of particle events. The cameras are arranged to cover
most of the celestial sphere, providing high sensitivity and throughput for a
wide variety of cosmic explosions. At longer timescales, persistent X-ray
sources would be monitored with unprecedented levels of coverage. The
sensitivity to faint X-ray sources on a one-day timescale would be improved by
a factor of 6 over the capability of the RXTE All Sky Monitor.Comment: 10 pages, 5 figs., in X-Ray and Gamma Ray Instrumentation for
Astronomy XI, SPIE conference, San Diego, Aug. 200
A Frequency Glitch in an Accreting Pulsar
Frequency glitches have been observed so far only in radio pulsars and
anomalous X-ray pulsars. Here we present evidence of a glitch in a neutron star
accreting from a Be companion. The transient KS 1947+300 reappeared in 2000
October as a moderately strong X-ray source that exhibited 18.7 s pulsations,
leading to an identification with the BATSE source GRO J1948+32, last detected
in 1994. We have analyzed Rossi X-ray Timing Explorer observations taken during
the 2000-01 outburst, as well as additional observations taken during a smaller
outburst in July 2002. Orbital Doppler shifts are apparent in the temporal
variation of the pulse frequency. A joint fit of the RXTE data together with
BATSE measurements from an outburst in 1994 yields the orbital period P_orb =
40.415+/-0.010 d, the projected orbital radius a_x sin i = 137+/-3 lt-s, and
the eccentricity e = 0.033+/-0.013. This degree of eccentricity is unexpectedly
low for such a wide orbit. Pulse timing results also show that the intrinsic
pulse frequency increased from 53.30 to 53.47 mHz at a rate approximately
proportional to the X-ray flux. This is about the degree of spin up expected
from the accretion torques that must be present when the X-ray luminosity
reaches ~10^38 ergs/s. On one occasion during the 2000-01 outburst, the pulse
frequency increased by ~1.8x10^-6 Hz in less than approximately 10 hr over and
above the mean trend seen around that time, without any indication of a
correspondingly large increase in X-ray flux. The fractional change in
frequency of 3.7x10^-5 during this event is significantly larger than the
values observed in the glitches in radio pulsars and anomalous X-ray pulsars.
We discuss other similarities and differences between these events.Comment: 10 pages, 5 figures; expanded spectral section, including a figure
with representative spectra, and other minor changes following referee's
comments. Accepted by Ap
X-ray Nova XTE J1550-564: RXTE Spectral Observations
Excellent coverage of the 1998 outburst of the X-ray Nova XTE J1550-564 was
provided by the Rossi X-ray Timing Explorer. XTE J1550-564 exhibited an intense
(6.8 Crab) flare on 1998 September 19 (UT), making it the brightest new X-ray
source observed with RXTE. We present a spectral analysis utilizing 60
Proportional Counter Array spectra from 2.5-20 keV spanning 71 days, and a
nearly continuous All Sky Monitor light curve. The spectra were fit to a model
including multicolor blackbody disk and power-law components. XTE J1550-564 is
observed in the very high, high/soft, and intermediate canonical outburst
states of Black Hole X-ray Novae.Comment: 14 pages including 1 table and 4 figures, Accepted to ApJ Letter
Complete RXTE Spectral Observations of the Black Hole X-ray Nova XTE J1550-564
We report on the X-ray spectral behavior of the exceptionally bright X-ray
nova XTE J1550-564 during its 1998-99 outburst. Our study is based on 209
pointed observations using the PCA and HEXTE instruments onboard the Rossi
X-ray Timing Explorer spanning 250 days and covering the entire double-peaked
eruption that occurred from 1998 September until 1999 May. The spectra are fit
to a model including multicolor blackbody disk and power-law components. The
source is observed in the very high and high/soft outburst states of black hole
X-ray novae. During the very high state, when the power-law component dominated
the spectrum, the inner disk radius is observed to vary by more than an order
of magnitude; the radius decreased by a factor of 16 in one day during a 6.8
Crab flare. If the larger of these observed radii is taken to be the last
stable orbit, then the smaller observed radius would imply that the inner edge
of the disk is inside the event horizon! However, we conclude that the apparent
variations of the inner disk radius observed during periods of increased
power-law emission are probably caused by the failure of the multicolor
disk/power-law model; the actual physical radius of the inner disk may remain
fairly constant. This interpretation is supported by the fact that the observed
inner disk radius remains approximately constant over 120 days in the high
state, when the power-law component is weak, even though the disk flux and
total flux vary by an order of magnitude. The mass of the black hole inferred
by equating the approximately constant inner disk radius observed in the
high/soft state with the last stable orbit for a Schwarzschild black hole is
M_BH = 7.4 M_sun (D/6 kpc) (cos i)^{-1/2}.Comment: Submitted to ApJ, 20 pages including 6 figures + 4 large table
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