2,999 research outputs found
Strong Aperiodic X-ray Variability and Quasi-Periodic Oscillation in X-ray Nova XTE J1550-564
We report the discovery of strong aperiodic X-ray variability and
quasi-periodic oscillation (QPO) in the X-ray light curves of a new X-ray Nova,
XTE J1550-564, and the evolution of the observed temporal properties during the
rise of the recent X-ray outburst. The power spectral analysis of the first
observation reveals strong aperiodic X-ray variability of the source (~28%), as
well as the presence of a QPO at ~82 mHz with fractional rms amplitude ~14%
over the 2-60 keV energy range. Also apparent is the first harmonic of the QPO
with the amplitude ~9%. As the X-ray flux increases, the source tends to become
less variable, and the QPO frequency increases rapidly, from 82 mHz to 4 Hz,
over the flux (2-50 keV) range of 1.73-5.75 x 10^{-8} ergs cm^{-2} s^{-1}. The
amplitude of the fundamental component of the QPO varies little, while that of
the harmonic follows a decreasing trend. The fundamental component strengthens
toward high energies, while its harmonic weakens. Initially, the power spectrum
is roughly flat at low frequencies and turns into a power law at high
frequencies, with the QPO harmonic sitting roughly at the break. In later
observations, however, the high-frequency portion of the continuum can actually
be better described by a broken power law (as opposed to a simple power law).
This effect becomes more apparent at higher energies. The overall amplitude of
the continuum shows a similar energy dependence to that of the fundamental
component of the QPO. Strong rapid X-ray variability, as well as hard energy
spectrum, makes XTE J1550-564 a good black hole candidate. We compare its
temporal properties with those of other black hole candidates.Comment: 12 pages, including 5 figures. To appear in ApJ Letters, vol. 512
(1999
Discovery of pulsations in the X-ray transient 4U 1901+03
We describe observations of the 2003 outburst of the hard-spectrum X-ray
transient 4U 1901+03 with the Rossi X-ray Timing Explorer. The outburst was
first detected in 2003 February by the All-Sky Monitor, and reached a peak
2.5-25 keV flux of 8x10^-9 ergs/cm^2/s (around 240 mCrab). The only other known
outburst occurred 32.2 yr earlier, likely the longest presently known
recurrence time for any X-ray transient. Proportional Counter Array (PCA)
observations over the 5-month duration of the 2003 outburst revealed a 2.763 s
pulsar in a 22.58 d orbit. The detection of pulsations down to a flux of
3x10^-11 ergs/cm^2/s (2.5-25 keV), along with the inferred long-term accretion
rate of 8.1x10^-11 M_sun/yr (assuming a distance of 10 kpc) suggests that the
surface magnetic field strength is below ~5x10^11 G. The corresponding
cyclotron energy is thus below 4 keV, consistent with the non-detection of
resonance features at high energies. Although we could not unambiguously
identify the optical counterpart, the lack of a bright IR candidate within the
1' RXTE error circle rules out a supergiant mass donor. The neutron star in 4U
1901+03 probably accretes from the wind of a main-sequence O-B star, like most
other high-mass binary X-ray pulsars. The almost circular orbit e=0.036
confirms the system's membership in a growing class of wide, low-eccentricity
systems in which the neutron stars may have received much smaller kicks as a
result of their natal supernova explosions.Comment: 7 pages, 6 figures, accepted by ApJ. Very minor addition in response
to referee's comment; updated author affiliatio
Soft Phase Lags of Pulsed Emission from the Millisecond X-ray Pulsar SAX J1808.4-3658
We report the discovery of phase shifts between X-ray pulses at different
energies in the newly discovered millisecond (ms) X-ray pulsar SAX
J1808.4-3658. The results show that low-energy pulses lag high-energy pulses by
as much as 0.2 ms (or 8% of the pulse period). The measurements
were made in two different ways: (1) computing cross power spectra between
different energy bands, and (2) cross-correlating the folded pulse profiles in
different energy bands; consistent results were obtained. We speculate that the
observed soft lags might be related to the lateral expansion and subsequent
cooling of a ``hot spot'' on the neutron star surface in which the pulsed X-ray
emission originates. Also presented is the possibility of producing soft lags
via Compton down scattering of hard X-ray photons from the hot spot in the cool
surrounding atmosphere. We will discuss possible X-ray production mechanisms
for SAX J1808.4-3658 and constraints on the emission environment, based on the
observed soft lags, pulse profiles, and energy spectrum.Comment: 11 pages, including four figures. To appear in ApJ Letter
The long-term evolution of the spin, pulse shape, and orbit of the accretion-powered millisecond pulsar SAX J1808.4-3658
We present a 7 yr timing study of the 2.5 ms X-ray pulsar SAX J1808.4-3658,
an X-ray transient with a recurrence time of ~2 yr, using data from the Rossi
X-ray Timing Explorer covering 4 transient outbursts (1998-2005). We verify
that the 401 Hz pulsation traces the spin frequency fundamental and not a
harmonic. Substantial pulse shape variability, both stochastic and systematic,
was observed during each outburst. Analysis of the systematic pulse shape
changes suggests that, as an outburst dims, the X-ray "hot spot" on the pulsar
surface drifts longitudinally and a second hot spot may appear. The overall
pulse shape variability limits the ability to measure spin frequency evolution
within a given X-ray outburst (and calls previous nudot measurements of this
source into question), with typical upper limits of |nudot| < 2.5x10^{-14} Hz/s
(2 sigma). However, combining data from all the outbursts shows with high (6
sigma) significance that the pulsar is undergoing long-term spin down at a rate
nudot = (-5.6+/-2.0)x10^{-16} Hz/s, with most of the spin evolution occurring
during X-ray quiescence. We discuss the possible contributions of magnetic
propeller torques, magnetic dipole radiation, and gravitational radiation to
the measured spin down, setting an upper limit of B < 1.5x10^8 G for the
pulsar's surface dipole magnetic field and and Q/I < 5x10^{-9} for the
fractional mass quadrupole moment. We also measured an orbital period
derivative of Pdot = (3.5+/-0.2)x10^{-12} s/s. This surprising large Pdot is
reminiscent of the large and quasi-cyclic orbital period variation observed in
the so-called "black widow" millisecond radio pulsars, supporting speculation
that SAX J1808.4-3658 may turn on as a radio pulsar during quiescence. In an
appendix we derive an improved (0.15 arcsec) source position from optical data.Comment: 22 pages, 10 figures; accepted for publication in 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
SIMULTANEOUS OBSERVATIONS OF GIANT PULSES FROM THE CRAB PULSAR, WITH THE MURCHISON WIDEFIELD ARRAY AND PARKES RADIO TELESCOPE: IMPLICATIONS FOR THE GIANT PULSE EMISSION MECHANISM
We report on observations of giant pulses from the Crab pulsar performed simultaneously with the Parkes radio telescope and the incoherent combination of the Murchison Widefield Array (MWA) antenna tiles. The observations were performed over a duration of approximately one hour at a center frequency of 1382 MHz with 340 MHz bandwidth at Parkes, and at a center frequency of 193 MHz with 15 MHz bandwidth at the MWA. Our analysis has led to the detection of 55 giant pulses at the MWA and 2075 at Parkes above a threshold of 3.5σ and 6.5σ, respectively. We detected 51% of the MWA giant pulses at the Parkes radio telescope, with spectral indices in the range of -3.6 > α > -4.9 (S[subscript v] ∝ v[superscript α]). We present a Monte Carlo analysis supporting the conjecture that the giant pulse emission in the Crab is intrinsically broadband, the less than 100% correlation being due to the relative sensitivities of the two instruments and the width of the spectral index distribution. Our observations are consistent with the hypothesis that the spectral index of giant pulses is drawn from normal distribution of standard deviation 0.6, but with a mean that displays an evolution with frequency from −3.00 at 1382 MHz, to −2.85 at 192 MHz
RXTE Discovery of Coherent Millisecond Pulsations during an X-ray Burst from KS 1731-260
A highly coherent 523.92+-0.05 Hz periodic X-ray signal has been observed
during a type I X-ray burst from the low-mass X-ray binary system KS 1731-260
with the PCA on RXTE. The spectral evolution of the burst indicates
photospheric-radius expansion and contraction. The 524 Hz signal occurred at
the end of the contraction phase, lasted for ~2 s, was highly coherent (Q >~
900), and had a pulse fraction (ratio of sinusoidal amplitude to mean count
rate) of 6.2+-0.6%. KS 1731-260 is one of only three systems that have
exhibited high-coherence millisecond oscillations during X-ray bursts and the
first reported where the pulsations are associated with photospheric
contraction. These coherent signals may be interpreted as a direct indication
of the neutron star spin.Comment: 15 pages plus 4 Postscript figures; AASTeX format; submitted to
Astrophysical Journal Letter
HST/NICMOS Observations of Fast Infrared Flickering in the Microquasar GRS 1915+105
We report infrared observations of the microquasar GRS 1915+105 using the
NICMOS instrument of the Hubble Space Telescope during 9 visits in April-June
2003. During epochs of high X-ray/radio activity near the beginning and end of
this period, we find that the \um infrared flux is generally low ( mJy) and relatively steady. However, during the X-ray/radio ``plateau''
state between these epochs, we find that the infrared flux is significantly
higher ( mJy), and strongly variable. In particular, we find events
with amplitudes % occurring on timescales of s
(e-folding timescales of s). These flickering timescales are several
times faster than any previously-observed infrared variability in GRS 1915+105
and the IR variations exceed corresponding X-ray variations at the same () timescale. These results suggest an entirely new type of infrared
variability from this object. Based on the properties of this flickering, we
conclude that it arises in the plateau-state jet outflow itself, at a distance
AU from the accretion disk. We discuss the implications of this work and
the potential of further flickering observations for understanding jet
formation around black holes.Comment: 19 pages, incl. 4 figures; accepted for publication in Ap
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