68 research outputs found
Correlation among QPO frequencies and Quiescence-state Duration in Black Hole Candidate GRS 1915+105
We discover a definite correlation between the frequency of the
quasi-periodic oscillations (QPO) in quiescence states and the duration of the
quiescence state of the transient X-ray source GRS 1915+105. We find that while
the QPO frequency can be explained with the oscillation of shocks in accretion
flows, the switching of burst to quiescence states (and vice versa) and their
duration can be explained by assuming an outflow from the post-shock region.
The duration of the quiescence state is inversely related to the QPO-frequency.
We derive this relation. We also find the correlation between the observed low
(Hz) and the intermediate (1-10Hz) QPO frequencies. Our
analytical solutions are verified by analyzing several days of public-domain
data from RXTE.Comment: Latex, 13 pages with 3 figures; Accepted for Publication in
Astrophysical Journal Letter
Identification of four RXTE Slew Survey sources with nearby luminous active galactic nuclei
Based on RXTE scans and observations with the SWIFT/XRT telescope and
INTEGRAL observatory, we report the identification of four X-ray sources
discovered during the RXTE Slew Survey of the |b|>10deg sky with nearby (z ~
0.017-0.098) luminous (log L_2-10keV ~ 42.7-44 erg/s) active galactic nuclei.
Two of the objects exhibit heavily intrinsically absorbed X-ray spectra
(NHL~10^23 cm^-2).Comment: 4 pages, 2 figures. Sibmitted to Astronomy and Astrophysics Letter
Reflection and noise in the low spectral state of GX339-4
We analyze RXTE/PCA observations of GX339-4 in the low spectral state from
1996--1997 and show that the pattern of its spectral and temporal variability
is nearly identical to that of Cyg X-1. In particular, a tight correlation
exists between the QPO centroid frequency and the spectral parameters. An
increase of the QPO centroid frequency is accompanied with an increase of the
amplitude of the reflected component and a steepening the slope of the
underlying power law. Fourier frequency resolved spectral analysis showed, that
the variability of the reflected component at frequencies higher than ~1-10 Hz
is suppressed in comparison with that of the primary emission.Comment: 4 pages, 4 figures. Accepted in Astronomy and Astrophysics Main
Journa
XMM-Newton discovery of 217 s pulsations in the brightest persistent supersoft X-ray source in M31
We report on the discovery of a periodic modulation in the bright supersoft
X-ray source XMMU J004252.5+411540 detected in the 2000-2004 XMM-Newton
observations of M31. The source exhibits X-ray pulsations with a period P~217.7
s and a quasi-sinusoidal pulse shape and pulsed fraction ~7-11%. We did not
detect statistically significant changes in the pulsation period on the time
scale of 4 years. The X-ray spectra of XMMU J004252.5+411540 are extremely soft
and can be approximated with an absorbed blackbody of temperature 62-77 eV and
a weak power law tail of photon index ~1.7-3.1 in the 0.2-3.0 keV energy band.
The X-ray properties of the source and the absence of an optical/UV counterpart
brighter than 19 mag suggest that it belongs to M31. The estimated bolometric
luminosity of the source varies between ~2e38 and ~8e38 ergs/s at 760 kpc,
depending on the choice of spectral model. The X-ray pulsations and supersoft
spectrum of XMMU J004252.5+411540 imply that it is almost certainly an
accreting white dwarf, steadily burning hydrogen-rich material on its surface.
We interpret X-ray pulsations as a signature of the strong magnetic field of
the rotating white dwarf. Assuming that the X-ray source is powered by disk
accretion, we estimate its surface field strength to be in the range 4e5 G
<B_{0}<8e6 G. XMMU J004252.5+411540 is the second supersoft X-ray source in M31
showing coherent pulsations, after the transient supersoft source XMMU
J004319.4+411758 with 865.5 s pulsation period.Comment: 11 pages, 4 figures, uses emulateapj style. Submitted to Ap
XMM-Newton discovery of transient X-ray pulsar in NGC 1313
We report on the discovery and analysis of the transient X-ray pulsar XMMU
J031747.5-663010 detected in the 2004 November 23 XMM-Newton observation of the
spiral galaxy NGC 1313. The X-ray source exhibits pulsations with a period
P~765.6 s and a nearly sinusoidal pulse shape and pulsed fraction ~38% in the
0.3-7 keV energy range. The X-ray spectrum of XMMU J031747.5-663010 is hard and
is well fitted with an absorbed simple power law of photon index ~1.5 in the
0.3-7 keV energy band. The X-ray properties of the source and the absence of an
optical/UV counterpart brighter than 20 mag allow us to identify XMMU
J031747.5-663010 as an accreting X-ray pulsar located in NGC 1313. The
estimated absorbed 0.3-7 keV luminosity of the source L~1.6\times 10^{39}
ergs/s, makes it one of the brightest X-ray pulsars known. Based on the
relatively long pulse period and transient behaviour of the source, we classify
it as a Be binary X-ray pulsar candidate. XMMU J031747.5-663010 is the second
X-ray pulsar detected outside the Local Group, after transient 18 s pulsating
source CXOU J073709.1+653544 discovered in the nearby spiral galaxy NGC 2403.Comment: 6 pages, 4 figures. Accepted for publication in MNRAS. Updated to
match the accepted versio
A Cluster of Galaxies hiding behind M31: XMM-Newton observations of RX J0046.4+4204
We report on our serendipitous discovery with the XMM-Newton Observatory of a
luminous X-ray emitting cluster of galaxies that is located behind the
Andromeda galaxy (M31). X-ray emission from the cluster was detected previously
by ROSAT, and cataloged as RX J0046.4+4204, but it was not recognized as a
galaxy cluster. The much greater sensitivity of our XMM-Newton observations
revealed diffuse x-ray emission that extends at least 5 arcmin and has a
surface brightness profile that is well fit by the alpha-beta model with beta =
0.70 +/- 0.08, a core radius r_c = 56 arcsec +/- 16, and alpha = 1.54 +/- 0.25.
A joint global spectral fit of the EPIC/MOS1, MOS2, and PN observations with
Mewe-Kaastra-Liedahl plasma emission model gives a cluster temperature of 5.5
+/- 0.5 keV. The observed spectra also show high significance iron emission
lines that yield a measured cluster redshift of z = 0.290 with a 2% accuracy.
For a cosmological model with H_0 = 71 km s^{-1} Mpc^{-1}, Omega_M = 0.3 and
Omega_{Lambda} = 0.7 we derive a bolometric luminosity of L_x=(8.4 +/-
0.5)*10^{44}$ erg/s. This discovery of a cluster behind M31 demonstrates the
utility of x-ray surveys for finding rich clusters of galaxies, even in
directions of heavy optical extinction.Comment: ApJ in press, updated to match the accepted versio
Supersoft X-ray sources in M31: II. ROSAT-detected supersoft sources in the ROSAT, Chandra and XMM eras
We have performed Chandra observations during the past 3 years of 5 of the
M31 supersoft X-ray sources (SSS) discovered with ROSAT. Surprisingly, only one
of these sources has been detected, despite a predicted detection of about
20-80 counts. This has motivated a thorough check of the ROSAT M31 survey I
data, including a relaxation of the hardness ratio requirement used to select
SSS. This increases the number of SSS identified in survey I by 7. We then
carried out a comparison with the ROSAT M31 survey II dataset which had
hitherto not been explicitly investigated for SSS. We find that most of the
ROSAT survey I sources are not detected, and only two new SSS are identified.
The low detection rate in the ROSAT survey II and our Chandra observations
implies that the variability time scale of SSS is a few months. If the majority
of these sources are close-binary SSS with shell hydrogen burning, it further
implies that half of these sources predominantly experience large mass transfer
rates.Comment: accepted for publ. in ApJ; 2 ps-figures; high-quality figures
available at http://www.mpe.mpg.de/~jcg/publis.htm
On the harmonics of the low-frequency quasi-periodic oscillation in GRS 1915+105
GRS 1915+105 is a widely studied black hole binary, well known because of its
extremely fast and complex variability. Flaring periods of high variability
alternate with "stable" phases (the plateaux) when the flux is low, the spectra
are hard and the timing properties of the source are similar to those of a
number of black hole candidates in hard spectral state. In the plateaux the
power density spectra are dominated by a low frequency quasi periodic
oscillation (LFQPO) superposed onto a band limited noise continuum and
accompanied by at least one harmonic. In this paper we focus on three plateaux,
presenting the analysis of the power density spectra and in particular of the
LFQPO and its harmonics. While plotting the LFQPO and all the harmonics
together on a frequency-width plane, we found the presence of a positive trend
of broadening when the frequency increases. This trend can shed light in the
nature of the harmonic content of the LFQPO and challenges the usual
interpretation of these timing features.Comment: 10 pages, 8 figures. Accepted for publication in MNRA
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