1,414 research outputs found
A Clumpy Stellar Wind and Luminosity-Dependent Cyclotron Line Revealed by The First Suzaku Observation of the High-Mass X-ray Binary 4U 1538-522
We present results from the first Suzaku observation of the high-mass X-ray
binary 4U 1538-522. The broad-band spectral coverage of Suzaku allows for a
detailed spectral analysis, characterizing the cyclotron resonance scattering
feature at keV and the iron K line at
keV, as well as placing limits on the strengths of the iron K line and
the iron K edge. We track the evolution of the spectral parameters both in time
and in luminosity, notably finding a significant positive correlation between
cyclotron line energy and luminosity. A dip and spike in the lightcurve is
shown to be associated with an order-of-magnitude increase in column density
along the line of sight, as well as significant variation in the underlying
continuum, implying the accretion of a overdense region of a clumpy stellar
wind. We also present a phase-resolved analysis, with most spectral parameters
of interest showing significant variation with phase. Notably, both the
cyclotron line energy and the iron K line intensity vary significantly
with phase, with the iron line intensity significantly out-of-phase with the
pulse profile. We discuss the implications of these findings in the context of
recent work in the areas of accretion column physics and cyclotron resonance
scattering feature formation.Comment: 15 pages, 8 figures. Accepted to ApJ on 2 July 201
X-ray vs. Optical Variations in the Seyfert 1 Nucleus NGC 3516: A Puzzling Disconnectedness
We present optical broadband (B and R) observations of the Seyfert 1 nucleus
NGC 3516, obtained at Wise Observatory from March 1997 to March 2002,
contemporaneously with X-ray 2-10 keV measurements with RXTE. With these data
we increase the temporal baseline of this dataset to 5 years, more than triple
to the coverage we have previously presented for this object. Analysis of the
new data does not confirm the 100-day lag of X-ray behind optical variations,
tentatively reported in our previous work. Indeed, excluding the first year's
data, which drive the previous result, there is no significant correlation at
any lag between the X-ray and optical bands. We also find no correlation at any
lag between optical flux and various X-ray hardness ratios. We conclude that
the close relation observed between the bands during the first year of our
program was either a fluke, or perhaps the result of the exceptionally bright
state of NGC 3516 in 1997, to which it has yet to return. Reviewing the results
of published joint X-ray and UV/optical Seyfert monitoring programs, we
speculate that there are at least two components or mechanisms contributing to
the X-ray continuum emission up to 10 keV: a soft component that is correlated
with UV/optical variations on timescales >1 day, and whose presence can be
detected when the source is observed at low enough energies (about 1 keV), is
unabsorbed, or is in a sufficiently bright phase; and a hard component whose
variations are uncorrelated with the UV/optical.Comment: 9 pages, AJ, in pres
High Temporal Resolution XMM Monitoring of PKS 2155-304
The bright, strongly variable BL Lac object PKS 2155-304 was observed by XMM
for two essentially uninterrupted periods of ~11 and 16 hr on 30-31 May 2000.
The strongest variations occurred in the highest energy bands. After scaling
for this effect, the three softest bands (0.1-1.7 keV) showed strong
correlation with no measurable lag to reliable limits of \tau \ls 0.3 hr.
However, the hardest band (~3 keV) was less well-correlated with the other
three, especially on short time scales, showing deviations of ~10-20% in ~1 hr
although, again, no significant interband lag was detected. This result and
examination of previous ASCA and BeppoSAX cross-correlation functions suggest
that previous claims of soft lags on time scales of 0.3-4 hr could well be an
artifact of periodic interruptions due to Earth-occultation every 1.6 hr.
Previous determinations of the magnetic field/bulk Lorentz factor were
therefore premature, as these data provide only a lower limit of B
\gamma^{1/3} \gs 2.5 G. The hardest band encompasses the spectral region above
the high-energy break; its enhanced variability could be indicating that the
break energy of the synchrotron spectrum, and therefore of the underlying
electron energy distribution, changes independently of the lower energies.Comment: 13 pages, 3 figures, accepted by Ap
Twelve and a Half Years of Observations of Centaurus A with RXTE
The Rossi X-ray Timing Explorer has observed the nearest radio galaxy,
Centaurus A, in 13 intervals from 1966 August to 2009 February over the 3--200
keV band. Spectra accumulated over the 13 intervals were well described with an
absorbed power law and iron line. Cut-off power laws and Compton reflection
from cold matter did not provide a better description. For the 2009 January
observation, we set a lower limit on the cut-off energy at over 2 MeV. The
power spectral density function was generated from RXTE/ASM and PCA data, as
well as an XMM-Newton long look, and clear evidence for a break at 18+10-7 days
(68% conf.) was seen. Given Cen A's high black hole mass and very low value of
Lx/LEdd, the break was a factor of 17+/-9 times higher than the break frequency
predicted by the McHardy and co-workers' relation, which was empirically
derived for a sample of objects, which are radio-quiet and accreting at
relatively high values of Lbol/LEdd. We have interpreted our observations in
the context of a clumpy molecular torus. The variability characteristics and
the broadband spectral energy distribution, when compared to Seyferts, imply
that the bright hard X-ray continuum emission may originate at the base of the
jet, yet from behind the absorbing line of sight material, in contrast to what
is commonly observed from blazars.Comment: 56 pages, 12 figures, 4 tables, revised manuscript submitted to The
Astrophysical Journa
Tracking the Complex Absorption in NGC 2110 with Two Suzaku Observations
We present spectral analysis of two Suzaku observations of the Seyfert 2
galaxy, NGC 2110. This source has been known to show complex, variable
absorption which we study in depth by analyzing these two observations set
seven years apart and by comparing to previously analyzed observations with the
XMM-Newton and Chandra observatories. We find that there is a relatively
stable, full-covering absorber with a column density of ~3
cm, with an additional patchy absorber that is likely variable in both
column density and covering fraction over timescales of years, consistent with
clouds in a patchy torus or in the broad line region. We model a soft emission
line complex, likely arising from ionized plasma and consistent with previous
studies. We find no evidence for reflection from an accretion disk in this
source with no contribution from relativistically broadened Fe Ka line emission
nor from a Compton reflection hump.Comment: Accepted to ApJ: March, 201
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