61,789 research outputs found
The Soft X-Ray Properties of a Complete Sample of Optically Selected Quasars II. Final Results
We present the final results of a ROSAT PSPC program to study the soft X-ray
emission properties of a complete sample of low quasars. The main results
are: 1. There is no evidence for significant soft excess emission or excess
foreground absorption by cold gas in 22 of the 23 quasars. 2. The mean 0.2-2
keV continuum of quasars agrees remarkably well with an extrapolation of the
mean 1050-350A continuum recently determined by Zheng et al. (1996), indicating
that there is no steep soft component below 0.2 keV. 3. The occurrence of warm
absorbers in quasars is rather rare, in sharp contrast to lower luminosity AGN.
4. The strongest correlation found is between the spectral slope, alpha_x, and
the Hb FWHM. This remarkably strong correlation may result from a dependence of
alpha_x on L/L_Edd, as seen in Galactic black hole candidates. 5. There appears
to exist a distinct class of ``X-ray weak'' quasars. These may be quasars where
the direct X-ray source is obscured, and only scattered X-rays are observed. 6.
Thin accretion disk models cannot reproduce the observed optical to soft X-ray
spectral shape. An as yet unknown physical mechanism maintains a strong
correlation between the optical and soft X-ray emission. 7. The well known
difference in alpha_x between radio-loud and radio-quiet quasars may be due
only to their different Hb FWHM. 8. The agreement of the 21 cm and X-ray
columns implies that He in the diffuse H II component of the Galactic ISM is
ionized to He II or He III (shortened abstract).Comment: 19 pages of text only, uses aas2pp4.sty file, to appear in ApJ vol.
447, 3/1/97, complete postscript version of 34 pages including 5 tables and 8
figures available at http://physics.technion.ac.il/~laor/rosat/paper.p
Missing hard states and regular outbursts: the puzzling case of the black hole candidate 4U 1630-472
4U 1630-472 is a recurrent X-ray transient classified as a black-hole
candidate from its spectral and timing properties. One of the peculiarities of
this source is the presence of regular outbursts with a recurrence period
between 600 and 730 d that has been observed since the discovery of the source
in 1969. We report on a comparative study on the spectral and timing behaviour
of three consecutive outbursts occurred in 2006, 2008 and 2010. We analysed all
the data collected by the INTErnational Gamma-Ray Astrophysics Laboratory
(INTEGRAL) and the Rossi X-ray timing Explorer (RXTE) during these three years
of activity. We show that, in spite of having a similar spectral and timing
behaviour in the energy range between 3 and 30 keV, these three outbursts show
pronounced differences above 30 keV. In fact, the 2010 outburst extends at high
energies without any detectable cut-off until 150-200 keV, while the two
previous outbursts that occurred in 2006 and 2008 are not detected at all above
30 keV. Thus, in spite of a very similar accretion disk evolution, these three
outbursts exhibit totally different characteristics of the Compton electron
corona, showing a softening in their evolution rarely observed before in a low
mass X-ray binary hosting a black hole. We argue the possibility that the
unknown perturbation that causes the outbursts to be equally spaced in time
could be at the origin of this particular behaviour. Finally we describe
several possible scenarios that could explain the regularity of the outbursts,
identifying the most plausible, such as a third body orbiting around the binary
system.Comment: April 2015: accepted for publication in MNRAS. May 2015: in pres
Disk-Jet Connection in the Radio Galaxy 3C 120
We present the results of extensive multi-frequency monitoring of the radio
galaxy 3C 120 between 2002 and 2007 at X-ray, optical, and radio wave bands, as
well as imaging with the Very Long Baseline Array (VLBA). Over the 5 yr of
observation, significant dips in the X-ray light curve are followed by
ejections of bright superluminal knots in the VLBA images. Consistent with
this, the X-ray flux and 37 GHz flux are anti-correlated with X-ray leading the
radio variations. This implies that, in this radio galaxy, the radiative state
of accretion disk plus corona system, where the X-rays are produced, has a
direct effect on the events in the jet, where the radio emission originates.
The X-ray power spectral density of 3C 120 shows a break, with steeper slope at
shorter timescale and the break timescale is commensurate with the mass of the
central black hole based on observations of Seyfert galaxies and black hole
X-ray binaries. These findings provide support for the paradigm that black hole
X-ray binaries and active galactic nuclei are fundamentally similar systems,
with characteristic time and size scales linearly proportional to the mass of
the central black hole. The X-ray and optical variations are strongly
correlated in 3C 120, which implies that the optical emission in this object
arises from the same general region as the X-rays, i.e., in the accretion
disk-corona system. We numerically model multi-wavelength light curves of 3C
120 from such a system with the optical-UV emission produced in the disk and
the X-rays generated by scattering of thermal photons by hot electrons in the
corona. From the comparison of the temporal properties of the model light
curves to that of the observed variability, we constrain the physical size of
the corona and the distances of the emitting regions from the central BH.Comment: Accepted for publication in the Astrophysical Journal. 28 pages, 21
figures, 2 table
The "universal" radio/X-ray flux correlation : the case study of the black hole GX 339-4
The existing radio and X-ray flux correlation for Galactic black holes in the
hard and quiescent states relies on a sample which is mostly dominated by two
sources (GX 339-4 and V404 Cyg) observed in a single outburst. In this paper,
we report on a series of radio and X-ray observations of the recurrent black
hole GX 339-4 with the Australia Telescope Compact Array, the Rossi X-ray
Timing Explorer and the Swift satellites. With our new long term campaign, we
now have a total of 88 quasi-simultaneous radio and X-ray observations of GX
339-4 during its hard state, covering a total of seven outbursts over a
15--year period. Our new measurements represent the largest sample for a
stellar mass black hole, without any bias from distance uncertainties, over the
largest flux variations and down to a level that could be close to quiescence,
making GX 339-4 the reference source for comparison with other accreting
sources (black holes, neutrons stars, white dwarfs and active galactic nuclei).
Our results demonstrate a very strong and stable coupling between radio and
X-ray emission, despite several outbursts of different nature and separated by
a period of quiescence. The radio and X-ray luminosity correlation of the form
L_X ~L_Rad^0.62 +/-0.01 confirms the non-linear coupling between the jet and
the inner accretion flow powers and better defines the standard correlation
track in the radio-X-ray diagram for stellar mass black holes. We further note
epochs of deviations from the fit that significantly exceed the measurement
uncertainties, especially during the formation and destruction of the compact
jets ...[abridged]. We incorporated our new data in a more global study of
black hole candidates strongly supporting a scale invariance in the
jet-accretion coupling of accreting black holes, and confirms the existence of
two populations of sources in the radio/X-ray diagram.Comment: Paper accepted in MNRAS. 18 pages, 9 figure
XMM-Newton monitoring of X-ray variability in the quasar PKS 0558-504
We present the temporal analysis of X-ray observations of the radio-loud
Narrow-Line Seyfert 1 galaxy (NLS1) PKS 0558-504 obtained during the XMM-Newton
Calibration and Performance Verification (Cal/PV) phase. The long term light
curve is characterized by persistent variability with a clear tendency for the
X-ray continuum to harden when the count rate increases. Another strong
correlation on long time scales has been found between the variability in the
hard band and the total flux. On shorter time scales the most relevant result
is the presence of smooth modulations, with characteristic time of ~ 2 hours
observed in each individual observation. The short term spectral variability
turns out to be rather complex but can be described by a well defined pattern
in the hardness ratio-count rate plane.Comment: 6 pages, 7 figures, accepted for publication in A&A special issue on
first results from XM
Soft X--Ray Properties of Seyfert Galaxies in the Rosat All--Sky Survey
We present the results of ROSAT All-Sky Survey observations of Seyfert and
IR-luminous galaxies from the Extended 12 Micron Galaxy Sample and the
optically-selected CfA Sample. Roughly half of the Seyferts (mostly Seyfert 1s)
have been fitted to an absorbed power-law model, yielding an average gamma of
2.26+-0.11 for 43 Seyfert 1s and 2.45+-0.18 for 10 Seyfert 2s, with both types
having a median value of 2.3.
The soft X-ray (SXR) luminosity correlates with the 12um luminosity, with
Seyfert 1s having relatively more SXR emission than Seyfert 2s of similar
mid-infrared luminosities, by a factor of 1.6+-0.3. Several physical
interpretations of these results are discussed, including the standard unified
model for Seyfert galaxies. Infrared-luminous non-Seyferts are shown to have
similar distributions of SXR luminosity and X-ray-to-IR slope as Seyfert 2s,
suggesting that some of them may harbor obscured active nuclei (as has already
been shown to be true for several objects) and/or that the soft X-rays from
some Seyferts 2s may be non-nuclear.
A SXR luminosity function (XLF) is calculated for the 12um sample, which is
well described by a single power-law with a slope of -1.75. The normalization
of this XLF agrees well with that of a HXR selected sample. Several of our
results, related to the XLF and the X-ray-to-IR relation are shown to be
consistent with the HXR observations of the 12um sample by Barcons et al.Comment: AASTeX, 40 pages. Text and Table 2 only. PostScript versions of this
file, figures, and Table 1, and a latex version of Table 1 are available by
ftp://ftp.astro.ucla.edu/pub/rush/papers, get rmfv*. Accepted by ApJ ~1996
May 10. Should be published in late 199
Chandra Observations of Radio-Loud Quasars at z > 4: X-rays from the Radio Beacons of the Early Universe
We present the results of Chandra observations of six radio-loud quasars
(RLQs) and one optically bright radio-quiet quasar (RQQ) at z = 4.1-4.4. These
observations cover a representative sample of RLQs with moderate radio-loudness
(R ~ 40-400), filling the X-ray observational gap between optically selected
RQQs and the five known blazars at z > 4 (R ~ 800-27000). We study the
relationship between X-ray luminosity and radio-loudness for quasars at high
redshift and constrain RLQ X-ray continuum emission and absorption. From a
joint spectral fit of nine moderate-R RLQs observed by Chandra, we find
tentative evidence for absorption above the Galactic N_H, with a best-fit
neutral intrinsic column density of N_H = 2.4^{+2.0}_{-1.8} x 10^{22} cm^{-2},
consistent with earlier claims of increased absorption toward high-redshift
RLQs. We also search for evidence of an enhanced jet-linked component in the
X-ray emission due to the increased energy density of the cosmic microwave
background (CMB) at high redshift, but we find neither spatial detections of
X-ray jets nor a significant enhancement in the X-ray emission relative to
comparable RLQs at low-to-moderate redshifts. Overall, the z ~ 4-5 RLQs have
basic X-ray properties consistent with comparable RLQs in the local universe,
suggesting that the accretion/jet mechanisms of these objects are similar as
well.Comment: 12 pages, The Astronomical Journal, in pres
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