219 research outputs found
Discovery of Two Simultaneous Kilohertz Quasi-Periodic Oscillations in KS 1731-260
We have discovered two simultaneous quasi-periodic oscillations (QPOs) at
898.3+/-3.3 Hz and 1158.6+/-9.0 Hz in the 1996 August 1 observation of the
low-mass X-ray binary KS 1731-260 with the Rossi X-ray Timing Explorer. The rms
amplitude and FWHM of the lower frequency QPO were 5.3+/-0.7 % and 22+/-8 Hz,
whereas those of the higher frequency QPO were 5.2+/-1.0 % and 37+/-21 Hz. At
low inferred mass accretion rate both QPOs are visible, at slightly higher mass
accretion rate the lower frequency QPO disappears and the frequency of the
higher frequency QPO increases to ~1178 Hz. At the highest inferred mass
accretion rate this QPO is only marginally detectable (2.1 sigma) near 1207 Hz,
which is the highest frequency so far observed in an X-ray binary. The
frequency difference (260.3+/-9.6 Hz) between the QPOs is equal to half the
frequency of the oscillations observed in a type I burst in this source (at
523.92+/-0.05 Hz, Smith, Morgan and Bradt 1997). This suggests that the neutron
star spin frequency is 261.96 Hz (3.8 ms), and that the lower frequency QPO is
the beat between the higher frequency QPO, which could be a preferred orbital
frequency around the neutron star, and the neutron star spin. During the 1996
August 31 observation we detected an additional QPO at 26.9+/-2.3 Hz, with a
FWHM and rms amplitude of 11+/-5 Hz and 3.4+/-0.6 %.Comment: 6 pages including 3 figures, Astrophysical Journal Letters, in press
(issue 482
Rossi X-ray Timing Explorer Observations of the X-ray Pulsar EXO 1722-363 - a Candidate Eclipsing Supergiant System
Observations made of the X-ray pulsar EXO 1722-363 using the Proportional
Counter Array and All Sky Monitor on board the Rossi X-ray Timing Explorer
reveal the orbital period of this system to be 9.741 +/- 0.004 d from periodic
changes in the source flux. The detection of eclipses, together with the values
of the pulse and orbital periods, suggest that this source consists of a
neutron star accreting from the stellar wind of an early spectral type
supergiant companion. Pulse timing measurements were also obtained but do not
strongly constrain the system parameters. The X-ray spectra can be well fitted
with a model consisting of a power law with a high energy cutoff and, for some
spectra, a blackbody component with a temperature of approximately 0.85 keV.Comment: Accepted for publication in The Astrophysical Journal. 27 pages
including 10 figure
The Orbit and Position of the X-ray Pulsar XTE J1855-026 - an Eclipsing Supergiant System
A pulse timing orbit has been obtained for the X-ray binary XTE J1855-026
using observations made with the Proportional Counter Array on board the Rossi
X-ray Timing Explorer. The mass function obtained of ~16Mo together with the
detection of an extended near-total eclipse confirm that the primary star is a
supergiant as predicted. The orbital eccentricity is found to be very low with
a best fit value of 0.04 +/- 0.02. The orbital period is also refined to be
6.0724 +/- 0.0009 days using an improved and extended light curve obtained with
RXTE's All Sky Monitor. Observations with the ASCA satellite provide an
improved source location of R.A. = 18h 55m 31.3s}, decl. = -02o 36' 24.0"
(2000) with an estimated systematic uncertainty of less than 12". A
serendipitous new source, AX J1855.4-0232, was also discovered during the ASCA
observations.Comment: Accepted for publication in the Astrophysical Journa
Kilohertz QPO Peak Separation Is Not Constant in Scorpius X-1
We report on a series of twenty ~10^5 c/s, 0.125 msec time-resolution RXTE
observations of the Z source and low-mass X-ray binary Scorpius X-1. Twin
kilohertz quasi-periodic oscillation (QPO) peaks are obvious in nearly all
observations. We find that the peak separation is not constant, as expected in
some beat-frequency models, but instead varies from ~310 to ~230 Hz when the
centroid frequency of the higher-frequency peak varies from ~875 to ~1085 Hz.
We detect none of the additional QPO peaks at higher frequencies predicted in
the photon bubble model (PBM), with best-case upper limits on the peaks' power
ratio of 0.025. We do detect, simultaneously with the kHz QPO, additional QPO
peaks near 45 and 90 Hz whose frequency increases with mass accretion rate. We
interpret these as first and second harmonics of the so-called
horizontal-branch oscillations well known from other Z sources and usually
interpreted in terms of the magnetospheric beat-frequency model (BFM). We
conclude that the magnetospheric BFM and the PBM are now unlikely to explain
the kHz QPO in Sco X-1. In order to succeed in doing so, any BFM involving the
neutron star spin (unseen in Sco X-1) will have to postulate at least one
additional unseen frequency, beating with the spin to produce one of the kHz
peaks.Comment: 6 pages including 3 figure
Discovery of an Ultra-fast X-ray Pulsar in the Supernova Remnant N157B
We present the serendipitous discovery of 16 ms pulsed X-ray emission from
the Crab-like supernova remnant N157B in the Large Magellanic Cloud. This is
the fastest spinning pulsar associated with a supernova remnant (SNR).
Observations with the Rossi X-ray Timing Explorer (RXTE), centered on the field
containing SN1987A, reveal an X-ray pulsar with a narrow pulse profile.
Archival ASCA X-ray data confirm this detection and locate the pulsar within 1'
of the supernova remnant N157B, 14' from SN1987A. The pulsar manifests evidence
for glitch(es) between the RXTE and ASCA observations which span 3.5 years; the
mean linear spin-down rate is Pdot = 5.126 X 10E-14 s/s. The background
subtracted pulsed emission is similar to other Crab-like pulsars with a power
law of photon index of ~ 1.6. The characteristic spin-down age (~ 5000 years)
is consistent with the previous age estimate of the SNR. The inferred B-field
for a rotationally powered pulsar is ~ 1 X 10E12 Gauss. Our result confirms the
Crab-like nature of N157B; the pulsar is likely associated with a compact X-ray
source revealed by ROSAT HRI observations.Comment: 9 pages with 3 eps figure, LaTex (aas2pp2, psfig). Submitted to the
Astrophysical Journal Letter
A new Comptonization model for low-magnetized accreting neutron stars in low mass X-ray binaries
We developed a new model for the X-ray spectral fitting \xspec package which
takes into account the effects of both thermal and dynamical (i.e. bulk)
Comptonization. The model consists of two components: one is the direct
blackbody-like emission due to seed photons which are not subjected to
effective Compton scattering, while the other one is a convolution of the
Green's function of the energy operator with a blackbody-like seed photon
spectrum. When combined thermal and bulk effects are considered, the analytic
form of the Green's function may be obtained as a solution of the diffusion
Comptonization equation. Using data from the BeppoSAX, INTEGRAL and RXTE
satellites, we test our model on the spectra of a sample of six persistently
low magnetic field bright neutron star Low Mass X-ray Binaries, covering three
different spectral states. Particular attention is given to the transient
powerlaw-like hard X-ray (> 30 keV) tails that we interpret in the framework of
the bulk motion Comptonization process. We show that the values of the best-fit
delta-parameter, which represents the importance of bulk with respect to
thermal Comptonization, can be physically meaningful and can at least
qualitatively describe the physical conditions of the environment in the
innermost part of the system. Moreover, we show that in fitting the thermal
Comptonization spectra to the X-ray spectra of these systems, the best-fit
parameters of our model are in excellent agreement with those of COMPTT, a
broadly used and well established XSPEC model.Comment: 15 pages, 8 figures, accepted for publication in Ap
Correlation between 3:2 QPO pairs and Jets in Black Hole X-ray Binaries
We argue, following our earlier works (the "CEBZMC model"), that the
phenomenon of twin peak high frequency quasi-periodic oscillations (QPOs)
observed in black hole X-ray binaries is caused by magnetic coupling (MC)
between accretion disk and black hole (BH). Due to MC, two bright spots occur
at two separate radial locations r_{in} and r_{out} at the disk surface,
energized by a kind of the Blandford-Znajek mechanism (BZ). We assume,
following the Kluzniak-Abramowicz QPO resonance model, that Keplerian
frequencies at these two locations are in the 3:2 ratio. With this assumption,
we estimate the BH spins in several sources, including GRO J1655-40, GRS
1915+105, XTE J1550-564, H1743-322 and Sgr A*. We give an interpretation of the
"jet line" in the hardness-intensity plane discussing the parameter space
consisting of the BH spin and the power-law index for the variation of the
large-scale magnetic field in the disk. Furthermore, we propose a new scenario
for the spectral state transitions in BH X-ray binaries based on fluctuation in
densities of accreting plasma from a companion star.Comment: 17 pages, 6 figures, accepted by AP
A Giant Glitch in the Energetic 69 ms X-ray Pulsar AXS J161730-505505
We present new results on the recently discovered 69 ms X-ray pulsar AXS
J161730-505505, the sixth youngest sample of all known pulsars. We have
undertaken a comprehensive X-ray observing campaign of AXS J161730-505505 with
the ASCA, SAX, and XTE observatories and follow its long term spin-down history
between 1989 and 1999, using these, archival GINGA and ASCA data sets, and the
radio ephemeris. The spin-down is not simply described by a linear function as
originally thought, but instead we find evidence of a giant glitch (|Delta P/P|
> 10E-6) between 1993 August and 1997 September, perhaps the largest yet
observed from a young pulsar. The glitch is well described by steps in the
period and its first derivative accompanied by a persistent second derivative
similar to those in the Vela pulsar. The pulse profile of AXS J161730-505505
presents a single asymmetric peak which is maintained over all observation
epochs. The energy spectrum is also steady over time, characterized by a highly
absorbed power-law with a photon index 1.4 +/- 0.2, consistent with that found
for other young rotation powered pulsars.Comment: 6 pages with 2 figures, LaTex, emulateapj.sty. To appear in the
Astrophysical Journal Letter
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
Evidence for 1122 Hz X-Ray Burst Oscillations from the Neutron-Star X-Ray Transient XTE J1739-285
We report on millisecond variability from the X-ray transient XTE J1739-285.
We detected six X-ray type I bursts and found evidence for oscillations at 1122
+/- 0.3 Hz in the brightest X-ray burst. Taking into consideration the power in
the oscillations and the number of trials in the search, the detection is
significant at the 99.96% confidence level. If the oscillations are confirmed,
the oscillation frequency would suggest that XTE J1739-285 contains the fastest
rotating neutron star yet found. We also found millisecond quasiperiodic
oscillations in the persistent emission with frequencies ranging from 757 Hz to
862 Hz. Using the brightest burst, we derive an upper limit on the source
distance of about 10.6 kpc.Comment: To appear in ApJL, 4 page
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