313 research outputs found
A 695-Hz quasi-periodic oscillation in the low-mass X-ray binary EXO 0748-676
We report the discovery of a 695-Hz quasi-periodic oscillation (QPO) in data
taken with the Rossi X-ray Timing Explorer of the low-mass X-ray binary (LMXB)
EXO 0748-676. This makes EXO 0748-676 the second dipping LMXB, after 4U
1915-05, that shows kHz QPOs. Comparison with other sources suggests that the
QPO corresponds to the lower frequency peak of the kHz QPO pair often observed
in other LMXBs. The QPO was found in the only observation done during an
outburst of the source in early 1996. This observation is also the only one in
which the ~1 Hz QPO recently found in EXO 0748-676 is not present.Comment: 6 pages, accepted for publication in Part 1 of The Astrophysical
Journa
The X-ray Properties of Low-Frequency Quasi-Periodic Oscillations from GRS 1915+105 up to 120 keV
We present a study of the properties of strong 0.8-3.0 Hz quasi-periodic
oscillations (QPOs) that occurred during 1997 RXTE observations of the
microquasar GRS 1915+105 in the low-hard state. The high count rates allow us
to track individual QPO peaks, and we exploit this to develop a QPO folding
technique. In contrast to previous QPO studies with RXTE, we emphasize the high
energy QPO properties and report the detection of a QPO in the 60-124 keV
energy band. Our technique allows us, for the first time, to measure the phase
of the QPO harmonics relative to the fundamental. Variation in this phase
difference leads to changes in the shape of the QPO profile with energy and
over time. The strength of the QPO fundamental increases up to 19 keV, but the
data do not suggest that the strength continues to increase above this energy.
In some cases, the QPO amplitudes in the 30-60 keV and 60-124 keV energy bands
are significantly less than in the 13-19 keV and 19-29 keV energy bands. We
also use our technique to measure the phase lag of the QPO fundamental and
harmonics. In the case where negative phase lags are detected for the
fundamental, positive phase lags are detected for the first harmonic.Comment: Submitted to ApJ, Refereed, 9 page
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
Precise Measurements of the Kilohertz Quasi-Periodic Oscillations in 4U 1728-34
We have analyzed seventeen observations of the low-mass X-ray binary and
atoll source 4U 1728-34, carried out by the Rossi X-ray Timing Explorer in 1996
and 1997. We obtain precise measurements of the frequencies of the two
simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) in this source.
We show that the frequency separation between the two QPO, , is
always significantly smaller than the frequency of the nearly-coherent
oscillations seen in this source during X-ray bursts, even at the lowest
inferred mass accretion rate, when seems to reach its maximum
value. We also find that decreases significantly, from Hz to Hz, as the frequency of the lower frequency kHz QPO
increases from 615 to 895 Hz. This is the first time that variations of the kHz
QPO peak separation are measured in a source which shows nearly-coherent
oscillations during bursts.Comment: Accepted for publication in The Astrophysical Journal Letters. Uses
AAS LaTex v4.0 (5 pages plus 4 postscript figures
The Weak Blue Bump of H2106-099 and AGN De-Reddening
We present multi-frequency spectra of the Seyfert 1 galaxy H2106-099, from
radio to hard X-rays, spanning over a decade of observations. The hard X-ray
(2-20 keV) spectrum measured with Ginga had a Log slope of -0.80 +/- 0.02 on
1988 May 18 and -1.02 +/- 0.10 on 1988 May 22 / 23 UT, with no observed flux
changes. Other measurements showed variability and unusual spectral features:
The V band flux changed by a factor of 1.8 (> 10 sigma) in six weeks. Only
moderate optical Fe II emission is present, but strong [FeVII] and [Fe X] lines
are present in some epochs. The Balmer lines show > 25% variations in flux
relative to the mean, and He I changed by more than 100% relative to the mean
in <~ six years. The most surprising finds came from the composite UV through
near-IR spectrum: If the spectrum is de-reddened by the galactic extinction
value (from 21 cm observations), a residual 2175 Angstrom absorption feature is
present. Additional de-reddening to correct the feature yields E(B-V)=0.07 mag
due to material outside our galaxy, most probably associated with the AGN or
its host galaxy. No other clear indications of reddening are observed in this
object, suggesting that blue bump strength measurements in low and intermediate
red-shift AGN could be incorrect if derived without UV observations of the
region near 2175 Angstrom in the AGN frame. After all reddening corrections are
performed, the log slope of H2106-099 from the near IR (~12500 Angstrom) to the
UV (~1400 Angstrom), -0.94 +/- 0.05, is steep compared to other AGN, suggesting
that the blue bump in this object is intrinsically weak. Weak blue bumps are,
therefore, not always an artifact caused by reddening.Comment: Three parts: A. 33 pages text, B. one landscape table, C. 8 figure
X 1908+075: An X-ray Binary with a 4.4 day Period
X 1908+075 is an optically unidentified and highly absorbed X-ray source that
appears in early surveys such as Uhuru, OSO-7, Ariel V, HEAO-1, and the EXOSAT
Galactic Plane Survey. These surveys measured a source intensity in the range
of 2-12 mCrab at 2-10 keV, and the position was localized to ~ 0.5 degrees. We
use the Rossi X-ray Timing Explorer (RXTE) All Sky Monitor (ASM) to confirm our
expectation that a particular Einstein IPC detection (1E 1908.4+0730) provides
the correct position for X 1908+075. The analysis of the coded mask shadows
from the ASM for the position of 1E 1908.4+0730 yields a persistent intensity ~
8 mCrab (1.5-12 keV) over a 3 year interval beginning in 1996 February.
Furthermore, we detect a period of 4.400 +- 0.001 days with a false alarm
probability < 1.0e-7 . The folded light curve is roughly sinusoidal, with an
amplitude that is 22 % of the mean flux. The X-ray period may be attributed to
the scattering and absorption of X-rays through a stellar wind combined with
the orbital motion in a binary system. We suggest that X 1908+075 is an X-ray
binary with a high mass companion star.Comment: 6 pages, two-column,"emulateapj" style, submitted to Ap
Discovery of Microsecond Soft Lags in the X-Ray Emission of the Atoll Source 4U1636-536
Exploiting the presence of kilohertz quasi-periodic oscillations (QPOs) in
the timing power spectrum, we find that the soft x-ray emission of the
neutron-star X-ray binary and atoll source 4U1636-536 modulated at the QPO
frequency lags behind that of the hard x-ray emission. Emission in the 3.8-6.4
keV band is delayed by 25.0 +/- 3.3 microseconds relative to the 9.3-69 keV
band. The delay increases in magnitude with increasing energy. Our results are
consistent with those of Vaughan et al. (1997), when the sign is corrected
(Vaughan et al. 1998), for the atoll source 4U1608-52. The soft lag could be
produced by Comptonization of hard photons injected into a cooler electron
cloud or by intrinsic spectral softening of the emission during each
oscillation cycle.Comment: Accepted to the Astrophysical Journal Letters, 4 page
Detection of Low-Hard State Spectral and Timing Signatures from the Black Hole X-Ray Transient XTE J1650-500 at Low X-Ray Luminosities
Using the Chandra X-ray Observatory and the Rossi X-ray Timing Explorer, we
have studied the black hole candidate (BHC) X-ray transient XTE J1650-500 near
the end of its 2001-2002 outburst after its transition to the low-hard state at
X-ray luminosities down to L = 1.5E34 erg/s (1-9 keV, assuming a source
distance of 4 kpc). Our results include a characterization of the spectral and
timing properties. At the lowest sampled luminosity, we used an 18 ks Chandra
observation to measure the power spectrum at low frequencies. For the 3 epochs
at which we obtained Chandra/RXTE observations, the 0.5-20 keV energy spectrum
is consistent with a spectral model consisting of a power-law with interstellar
absorption. We detect evolution in the power-law photon index from 1.66 +/-
0.05 to 1.93 +/- 0.13 (90% confidence errors), indicating that the source
softens at low luminosities. The power spectra are characterized by strong
(20-35% fractional rms) band-limited noise, which we model as a zero-centered
Lorentzian. Including results from an RXTE study of XTE J1650-500 near the
transition to the low-hard state by Kalemci et al. (2003), the half-width of
the zero-centered Lorentzian (roughly where the band-limited noise cuts off)
drops from 4 Hz at L = 7E36 erg/s (1-9 keV, absorbed) to 0.067 +/- 0.007 Hz at
L = 9E34 erg/s to 0.0035 +/- 0.0010 Hz at the lowest luminosity. While the
spectral and timing parameters evolve with luminosity, it is notable that the
general shapes of the energy and power spectra remain the same, indicating that
the source stays in the low-hard state. This implies that the X-ray emitting
region of the system likely keeps the same overall structure, while the
luminosity changes by a factor of 470. We discuss how these results may
constrain theoretical black hole accretion models.Comment: 11 pages, accepted by ApJ after minor revision
On the Spin History of the X-ray Pulsar in Kes 73: Further Evidence For an Utramagnetized Neutron Star
In previous papers, we presented the discovery of a 12-s X-ray pulsar in the
supernova remnant Kes 73, providing the first direct evidence for an
ultramagnetized neutron star, a magnetar, with an equivalent dipole field of
nearly twenty times the quantum critical magnetic field. Our conclusions were
based on two epochs of measurement of the spin, along with an age estimate of
the host supernova remnant. Herein, we present a spin chronology of the pulsar
using additional GINGA, ASCA, XTE, & SAX datasets spanning over a decade.
Timing and spectral analysis confirms our initial results and severely limit an
accretion origin for the observed flux. Over the 10 year baseline, the pulsar
is found to undergo a rapid, constant spindown, while maintaining a steady flux
and an invariant pulse profile. Within the measurement uncertainties, no
systematic departures from a linear spin-down are found - departures as in the
case of glitches or simply stochastic fluctuations in the pulse
times-of-arrival (e.g. red timing noise). We suggest that this pulsar is akin
to the soft gamma-ray repeaters, however, it is remarkably stable and has yet
to display similar outbursts; future gamma-ray activity from this object is
likely.Comment: 6 pages with 3 embedded figures, LaTex, emulateapj.sty. Submitted to
the ApJ Letter
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
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