9 research outputs found
High variability in Vela X-1: giant flares and off states
We investigate the spectral and temporal behavior of the high mass X-ray
binary Vela X-1 during a phase of high activity, with special focus on the
observed giant flares and off states. INTEGRAL observed Vela X-1 in a long
almost uninterrupted observation for two weeks in 2003 Nov/Dec. The data were
analyzed with OSA 7.0 and FTOOLS 6.2. We derive the pulse period, light curves,
spectra, hardness ratios, and hardness intensity diagrams, and study the
eclipse. In addition to an already high activity level, Vela X-1 exhibited
several intense flares, the brightest ones reaching a maximum intensity of more
than 5 Crab in the 20-40 keV band and several off states where the source was
no longer detected by INTEGRAL. We determine the pulse period to be
283.5320+/-0.0002 s, which is stable throughout the entire observation.
Analyzing the eclipses provided an improvement in the ephemeris. Spectral
analysis of the flares indicates that there appear to be two types of flares:
relatively brief flares, which can be extremely intense and show spectral
softening, in contrast to high intensity states, which are longer and show no
softening. Both flares and off states are interpreted as being due to a
strongly structured wind of the optical companion. When Vela X-1 encounters a
cavity with strongly reduced density, the flux will drop triggering the onset
of the propeller effect, which inhibits further accretion, giving rise to off
states. The sudden decrease in the density of the material required to trigger
the propeller effect in Vela X-1 is of the same order as predicted by
theoretical papers about the densities in OB star winds. A similarly structured
wind can produce giant flares when Vela X-1 encounters a dense blob in the
wind.Comment: 15 pages, 11 figures, accepted for publication in Astronomy and
Astrophysics as of August 2, 200
Highly Structured Wind in Vela X-1
We present an in-depth analysis of the spectral and temporal behavior of a long almost uninterrupted INTEGRAL observation of Vela X-1 in Nov/Dec 2003. In addition to an already high activity level, Vela X-1 exhibited several very intense flares with a maximum intensity of more than 5 Crab in the 20 40 keV band. Furthermore Vela X-1 exhibited several off states where the source became undetectable with ISGRI. We interpret flares and off states as being due to the strongly structured wind of the optical companion: when Vela X-1 encounters a cavity in the wind with strongly reduced density, the flux will drop, thus potentially triggering the onset of the propeller effect which inhibits further accretion, thus giving rise to the off states. The required drop in density to trigger the propeller effect in Vela X-1 is of the same order as predicted by theoretical papers for the densities in the OB star winds. The same structured wind can give rise to the giant flares when Vela X-1 encounters a dense blob in the wind. Further temporal analysis revealed that a short lived QPO with a period of 6800 sec is present. The part of the light curve during which the QPO is present is very close to the off states and just following a high intensity state, thus showing that all these phenomena are related
Orbital Parameters and Spectroscopy of the Transient X-Ray Pulsar 4U 0115+63
We report on an outburst of the high mass X-ray binary 4U 0115+63 with a pulse period of 3.6s in spring 2008 as observed with INTEGRAL and RXTE. By analyzing the lightcurves we derive an updated orbital- and pulse period ephemeris of the neutron star. We also study the pulse profile variations as a function of time and energy as well as the variability of the spectral parameters. We find clear evidence for at least three cyclotron line features. In agreement with previous observations of 4U 0115+63, we detect an anti-correlation between the luminosity and the fundamental cyclotron line energy
The smooth cyclotron line in Her X-1 as seen with NuSTAR
Her X-1, one of the brightest and best studied X-ray binaries, shows a
cyclotron resonant scattering feature (CRSF) near 37 keV. This makes it an
ideal target for detailed study with the Nuclear Spectroscopic Telescope Array
(NuSTAR), taking advantage of its excellent hard X-ray spectral resolution. We
observed Her X-1 three times, coordinated with Suzaku, during one of the high
flux intervals of its 35d super-orbital period. This paper focuses on the shape
and evolution of the hard X-ray spectrum. The broad-band spectra can be fitted
with a powerlaw with a high-energy cutoff, an iron line, and a CRSF. We find
that the CRSF has a very smooth and symmetric shape, in all observations and at
all pulse-phases. We compare the residuals of a line with a Gaussian optical
depth profile to a Lorentzian optical depth profile and find no significant
differences, strongly constraining the very smooth shape of the line. Even
though the line energy changes dramatically with pulse phase, we find that its
smooth shape does not. Additionally, our data show that the continuum is only
changing marginally between the three observations. These changes can be
explained with varying amounts of Thomson scattering in the hot corona of the
accretion disk. The average, luminosity-corrected CRSF energy is lower than in
past observations and follows a secular decline. The excellent data quality of
NuSTAR provides the best constraint on the CRSF energy to date.Comment: 13 pages, 13 figures, accepted for publication in Ap
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INTEGRAL and RXTE Observations of Centaurus A
INTEGRAL and RXTE performed three simultaneous observations of the nearby radio galaxy Centaurus A in 2003 March, 2004 January, and 2004 February with the goals of investigating the geometry and emission processes via the spectral/temporal variability of the X-ray/low energy gamma ray flux, and intercalibration of the INTEGRAL instruments with respect to those on RXTE. Cen A was detected by both sets of instruments from 3-240 keV. When combined with earlier archival RXTE results, we find the power law continuum flux and the line-of-sight column depth varied independently by 60% between 2000 January and 2003 March. Including the three archival RXTE observations, the iron line flux was essentially unchanging, and from this we conclude that the iron line emitting material is distant from the site of the continuum emission, and that the origin of the iron line flux is still an open question. Taking X-ray spectral measurements from satellite missions since 1970 into account, we discover a variability in the column depth between 1.0 x 10{sup 23} cm{sup -2} and 1.5 x 10{sup 23} cm{sup -2} separated by approximately 20 years, and suggest that variations in the edge of a warped accretion disk viewed nearly edge-on might be the cause. The INTEGRAL OSA 4.2 calibration of JEM-X, ISGRI, and SPI yields power law indices consistent with the RXTE PCA and HEXTE values, but the indices derived from ISGRI alone are about 0.2 greater. Significant systematics are the limiting factor for INTEGRAL spectral parameter determination
Angling for x-ray pulsar geometry with polarimetry
Using observations of X-ray pulsar Her X-1 by the Imaging X-ray Polarimetry
Explorer, we report on a highly significant detection of the polarization
signal from an accreting neutron star. The observed degree of the polarization
of % is found to be far below theoretical expectations for this object,
and stays low throughout the spin cycle of the pulsar. Both the polarization
degree and the angle exhibit variability with pulse phase, which allowed us to
measure the pulsar spin position angle and magnetic obliquity of the neutron
star, which is an essential step towards detailed modeling of intrinsic
emission of X-ray pulsars. Combining our results with the optical polarimetric
data, we find that the spin axis of the neutron star and the angular momentum
of the binary orbit are misaligned by at least 20 deg, which is a strong
argument in support of the models explaining stability of the observed
super-orbital variability with the precession of the neutron star.Comment: Submitte