13,834 research outputs found
ASCA and ROSAT observations of NGC5548: discrepant spectral indices
We report on simultaneous ASCA and ROSAT observations of the Seyfert galaxy
NGC5548 made during the ASCA Performance Verification phase. Spectral features
due to a warm absorber and reflection are clearly seen in the X-ray spectra. We
find that the continuum spectral shape differs between the ASCA and ROSAT
datasets. The photon-index obtained from the ROSAT PSPC exceeds that from the
ASCA SIS about 0.4. The discrepancy is clear even in the 0.5-2 keV energy band
over which both detectors are sensitive. The spectra cannot be made consistent
by choosing a more complex model. The problem likely lies in the response curve
(estimated effective area) of one, or both, detectors. There may be important
consequences for a wide range of published results.Comment: 9 pages, accepted for publication in MNRA
The Orbit and Position of the X-ray Pulsar XTE J1855-026 - an Eclipsing Supergiant System
A pulse timing orbit has been obtained for the X-ray binary XTE J1855-026
using observations made with the Proportional Counter Array on board the Rossi
X-ray Timing Explorer. The mass function obtained of ~16Mo together with the
detection of an extended near-total eclipse confirm that the primary star is a
supergiant as predicted. The orbital eccentricity is found to be very low with
a best fit value of 0.04 +/- 0.02. The orbital period is also refined to be
6.0724 +/- 0.0009 days using an improved and extended light curve obtained with
RXTE's All Sky Monitor. Observations with the ASCA satellite provide an
improved source location of R.A. = 18h 55m 31.3s}, decl. = -02o 36' 24.0"
(2000) with an estimated systematic uncertainty of less than 12". A
serendipitous new source, AX J1855.4-0232, was also discovered during the ASCA
observations.Comment: Accepted for publication in the Astrophysical Journa
Nature of the Soft Spectral Component in the X-ray Pulsars SMC X-1 and LMC X-4
We present here the results of an investigation of the pulse averaged and
pulse phase resolved energy spectra of two high luminosity accretion powered
X-ray pulsars SMC X-1 and LMC X-4 made with ASCA. The phase averaged energy
spectra definitely show the presence of a soft excess in both the sources. If
the soft excess is modeled as a separate black-body or thermal bremsstrahlung
type component, pulse phase resolved spectroscopy of SMC X-1 shows that the
soft component also has a pulsating nature. Same may be true for LMC X-4,
though a very small pulse fraction limits the statistical significance. The
pulsating soft component is found to have a nearly sinusoidal profile,
dissimilar to the complex profile seen at higher energies, which can be an
effect of smearing. Due to very high luminosity of these sources, the size of
the emission zone required for the soft component is large (radius ~300-400
km). We show that the pulsating nature of the soft component is difficult to
explain if a thermal origin is assumed for it. We further investigated with
alternate models, like inversely broken power-law or two different power-law
components and found that these models can also be used to explain the excess
at low energy. A soft power-law component may be a common feature of the
accreting X-ray pulsars, which is difficult to detect because most of the HMXB
pulsars are in the Galactic plane and experience large interstellar absorption.
In LMC X-4, we have also measured two additonal mid-eclipse times, which
confirm the known orbital decay.Comment: 28 pages (AAS preprint), 10 Postscript figures, ApJ accepte
X-ray absorption and rapid variability of the dwarf Seyfert nucleus of NGC4395
We report the detection of an absorbed central X-ray source and its strong,
rapid variability in NGC4395, the least luminous Seyfert nucleus known. The
X-ray source exhibits a number of flares with factors of 3-4 flux changes
during a half day ASCA observation. Such X-ray variability is in constrast to
the behaviour of other low luminosity active galaxies. It provides further
support for an accreting black hole model rather than an extreme stellar
process in accounting for the nuclear activity of NGC4395. The soft X-ray
emission below 3 keV is strongly attenuated by absorption. The energy spectrum
in this absorption band shows a dramatic change in response to the variation in
continuum luminosity. A variable warm absorber appears to be an explanation for
the spectral change. The absorption-corrected 2-10 keV luminosity is 4e39 erg/s
for a source distance of 2.6 Mpc, and at 1 keV is one order of magnitude above
previous ROSAT estimates. Our X-ray results infer the nuclear source of NGC4395
to be a scaled-down version of higher luminosity Seyfert nuclei, with an
intermediate mass (10^4-10^5 Msun) black hole, unlike the nearby low luminosity
active galaxies in which underfed massive black holes are suspected to reside.Comment: 12 pages, accepted for publication in MNRA
Origin of the "Disk-Line" Feature in the X-Ray Energy Spectrum of a Seyfert Galaxy, NGC4151
We have studied the origin of the broad and skewed feature at 4.5-7.5 keV in
the energy spectra of NGC4151 using the ASCA and RXTE data. The feature
consists of a narrow peak at 6.4 keV and a broad wing extended between 4.5-7.5
keV. An analysis of the long-term variations revealed that the feature became
variable only on a time scale longer than 1.5x10^6 s. Through a comparison with
the continuum variabilities, we found that the emission region of the excess
flux at 4.5-7.5 keV has an extent of 10^17 cm. The broad and skewed feature at
4.5-7.5 keV may be explained by the so-called ``disk-line'' model. If so, the
size of the line-emitting region, 10^17 cm, should be equal to several or
ten-times the Schwarzschild radius of the central black hole. This results in a
black hole mass of 10^11 solar mass, which may be too large for NGC4151. We
propose an alternative explanation for the broad and skewed feature, i.e. a
``reflection'' model, which can also reproduce the overall energy spectra very
well. In this model, cold matter with a sufficiently large column density is
irradiated by X-rays to produce a reflected continuum, which constitutes the
broad wing of the feature, and narrow fluorescent lines. The equivalent width
of the iron fluorescent line (~2 keV) and the upper limit of its width (sigma <
92 eV) are also consistent with this model. From these results and
considerations, we conclude that the ``disk-line'' model has difficulty to
explain the spectral variations of NGC4151, and the reflection model is more
plausible.Comment: 19 pages, 16 figures, PASJ accepte
The X-ray spectra and spectral variability of intermediate type Seyfert galaxies: ASCA observations of NGC 4388 and ESO 103-G35
The X-ray spectra of two intermediate type Seyfert galaxies are investigated
using ASCA observations separated by more than a year. Both NGC 4388 and ESO
103-G35 exhibit strong, narrow Fe K alpha line emission and absorption by cold
neutral gas with a column density ~ 10^23 cm^-2, characteristic of the X-ray
spectra of type 2 Seyfert galaxies. The power law continuum flux has changed by
a factor of 2 over a time-scale of ~ 2 years for both objects, declining in the
case of NGC 4388 and rising in ESO 103-G35. No variation was observed in the
equivalent width of the Fe K alpha line in the spectra of NGC 4388, implying
that the line flux declined with the continuum. We find that the strength of
the line cannot be accounted for by fluorescence in line-of-sight material with
the measured column density unless a `leaky-absorber' model of the type favored
for IRAS 04575-7537 is employed. The equivalent width of the Fe K alpha
emission line is seen to decrease between the observations of ESO 103-G35 while
the continuum flux increased. The 1996 observation of ESO 103-G35 can also be
fitted with an absorption edge at 7.4 0.2 keV due to partially ionized
iron, and when an ionized absorber model is fitted to the data it is found that
the equivalent column of neutral hydrogen rises to 3.5 x 10^23 cm^-2. The Fe K
alpha line flux can be accounted by fluorescence in this material alone and
this model is also a good representation of the 1988 and 1991 Ginga
observations. There is then no requirement for a reflection component in the
ASCA spectra of ESO 103-G35 or NGC 4388.Comment: 45 pages, 5 tables, 11 figures. Accepted for publication in the
Astrophysical Journa
X-ray and optical monitoring of the peculiar source 4U 1700+24/V934 Her
(Abridged) We report on ASCA and BeppoSAX observations of the X-ray source 4U
1700+24 and on (quasi-)simultaneous spectroscopy of its optical counterpart,
V934 Her, from the Loiano 1.5-meter telescope. Archival ROSAT and RXTE data as
well as the RXTE ASM light curve of 4U 1700+24 are also analyzed along with a
1985 EXOSAT pointing. The optical spectra are typical of a M2 III star; a
revised distance to the object of ~400 pc is inferred. While these spectra do
not show either any change between the two epochs or any peculiar feature, the
X-ray spectra reveal a complex and long-term variable shape, with a clear soft
excess. The X-ray spectral properties of the source are best described by a
thermal Comptonization spectrum plus a soft energy(<1 keV) excess, which can be
modeled with a blackbody emission with kT_BB ~ 1 keV; the latter component is
not detected at the lowest source flux levels. The ratio between the two
components varies substantially with the source flux. The X-ray emission from
the object appears to become harder as its luminosity increases, and the RXTE
data acquired during an outburst occurred during Fall 1997 display a hard tail
detected up to 100 keV. Apart from erratic shot-noise variability on timescales
of tens to thousands of seconds, no significant pulsations or QPOs are found
from the timing analysis of the X-ray light curves. With the new distance
determination, the 2-10 keV X-ray luminosity range spanned in the considered
observations lies between ~2x10^32 and ~1x10^34 erg/s. All this allows us to
suggest a scenario consisting of a wide binary system in which a neutron star
accretes matter from the wind of an M giant.Comment: 14 pages, 5 figures, to be published on Astronomy & Astrophysics,
Main Journa
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