791 research outputs found
The X-ray transient XTE J1118+480: Multiwavelength observations of a low-state mini-outburst
We present multiwavelength observations of the newly discovered X-ray
transient XTE J1118+480 obtained in the rising phase of the 2000 April
outburst. This source is located at unusually high Galactic latitude and in a
very low absorption line of sight. This made the first EUVE spectroscopy of an
X-ray transient outburst possible. Together with our HST, RXTE, and UKIRT data
this gives unprecedented spectral coverage. We find the source in the low hard
state. The flat IR-UV spectrum appears to be a combination of optically thick
disk emission and flat spectrum emission, possibly synchrotron, while at higher
energies, including EUV, a typical low hard state power-law is seen. EUVE
observations reveal no periodic modulation, suggesting an inclination low
enough that no obscuration by the disk rim occurs. We discuss the nature of the
source and this outburst and conclude that it may be more akin to
mini-outbursts seen in GRO J0422+32 than to a normal X-ray transient outburst.Comment: 4 pages, 3 figures, replaced with accepted version. Uses
emulateapj5.st
XTE J1550-564: INTEGRAL Observations of a Failed Outburst
The well known black-hole X-ray binary transient XTE J1550-564 underwent an
outburst during the spring of 2003 which was substantially underluminous in
comparison to previous periods of peak activity in that source. In addition,
our analysis shows that it apparently remained in the hard spectral state over
the duration of that outburst. This is again in sharp contrast to major
outbursts of that source in 1998/1999 during which it exhibited an irregular
light curve, multiple state changes and collimated outflows. This leads us to
classify it as a "failed outburst." We present the results of our study of the
spring 2003 event including light curves based on observations from both
INTEGRAL and RXTE. In addition, we studied the evolution of the high-energy
3-300 keV continuum spectrum using data obtained with three main instruments on
INTEGRAL. These spectra are consistent with typical low-hard-state thermal
Comptonization emission. We also consider the 2003 event in the context of a
multi-source, multi-event period-peak luminosity diagram in which it is a clear
outlyer. We then consider the possibility that the 2003 event was due to a
discrete accretion event rather than a limit-cycle instablility. In that
context, we apply model fitting to derive the timescale for viscous propagation
in the disk, and infer some physical characteristics.Comment: 22 pages, 8 figures, to be published in The Astrophysical Journa
Long-term variability of AGN at hard X-rays
Variability at all observed wavelengths is a distinctive property of AGN.
Hard X-rays provide us with a view of the innermost regions of AGN, mostly
unbiased by absorption along the line of sight. Swift/BAT offers the unique
opportunity to follow, on time scales of days to years and with a regular
sampling, the 14-195 keV emission of the largest AGN sample available up to
date for this kind of investigation. We study the amplitude of the variations,
and their dependence on sub-class and on energy, for a sample of 110 radio
quiet and radio loud AGN selected from the BAT 58-month survey. About 80% of
the AGN in the sample are found to exhibit significant variability on months to
years time scales, radio loud sources being the most variable. The amplitude of
the variations and their energy dependence are incompatible with variability
being driven at hard X-rays by changes of the absorption column density. In
general, the variations in the 14-24 and 35-100 keV bands are well correlated,
suggesting a common origin of the variability across the BAT energy band.
However, radio quiet AGN display on average 10% larger variations at 14-24 keV
than at 35-100 keV and a softer-when-brighter behavior for most of the Seyfert
galaxies with detectable spectral variability on month time scale. In addition,
sources with harder spectra are found to be more variable than softer ones.
These properties are generally consistent with a variable power law continuum,
in flux and shape, pivoting at energies >~ 50 keV, to which a constant
reflection component is superposed. When the same time scales are considered,
the timing properties of AGN at hard X-rays are comparable to those at lower
energies, with at least some of the differences possibly ascribable to
components contributing differently in the two energy domains (e.g.,
reflection, absorption).Comment: 17 pages, 11 figures, accepted for publication in A&
The First INTEGRAL AGN Catalog
We present the first INTEGRAL AGN catalog, based on observations performed
from launch of the mission in October 2002 until January 2004. The catalog
includes 42 AGN, of which 10 are Seyfert 1, 17 are Seyfert 2, and 9 are
intermediate Seyfert 1.5. The fraction of blazars is rather small with 5
detected objects, and only one galaxy cluster and no star-burst galaxies have
been detected so far. A complete subset consists of 32 AGN with a significance
limit of 7 sigma in the INTEGRAL/ISGRI 20-40 keV data. Although the sample is
not flux limited, the distribution of sources shows a ratio of obscured to
unobscured AGN of 1.5 - 2.0, consistent with luminosity dependent unified
models for AGN. Only four Compton-thick AGN are found in the sample. Based on
the INTEGRAL data presented here, the Seyfert 2 spectra are slightly harder
(Gamma = 1.95 +- 0.01) than Seyfert 1.5 (Gamma = 2.10 +- 0.02) and Seyfert 1
(Gamma = 2.11 +- 0.05).Comment: 17 pages, 12 figures, accepted for publication in Ap
The Evolving Accretion Disc in the Black Hole X-ray Transient XTE J1859+226
We present HST, RXTE, and UKIRT observations of the broad band spectra of the
black hole X-ray transient XTE J1859+226 during the decline from its 1999-2000
outburst. Our UV spectra define the 2175A interstellar absorption feature very
well and based on its strength we estimate E(B-V)=0.58+/-0.12. Hence we
deredden our spectra and follow the evolution of the spectral energy
distribution on the decline from outburst. We find that the UV and optical
data, and the X-ray thermal component when detectable, can be fit with a simple
blackbody model of an accretion disc heated by internal viscosity and X-ray
irradiation, and extending to close to the last stable orbit around the black
hole, although the actual inner radius cannot be well constrained. During the
decline we see the disc apparently evolving from a model with the edge
dominated by irradiative heating towards one where viscous heating is dominant
everywhere. The outer disc radius also appears to decrease during the decline;
we interpret this as evidence of a cooling wave moving inwards and discuss its
implications for the disc instability model. Based on the normalisation of our
spectral fits we estimate a likely distance range of 4.6-8.0kpc, although a
value outside of this range cannot securely be ruled out.Comment: 10 pages including figures. Accepted for publication in MNRA
Integral results on GRB030320: a long gamma-ray burst detected at the edge of the field of view
GRB030320 is the 5th Gamma-ray burst (GRB) detected by INTEGRAL in the field
of view (FoV). It is so far the GRB with the largest off-axis angle with
respect to the INTEGRAL pointing direction, near to the edge of the FoV of both
main instruments, IBIS and SPI. Nevertheless, it was possible to determine its
position and to extract spectra and fluxes. The GRB nature of the event was
confirmed by an IPN triangulation. It is a ~ 60 s long GRB with two prominent
peaks separated by ~ 35 s. The spectral shape of the GRB is best represented by
a single power law with a photon index Gamma ~ 1.7. The peak flux in the 20 -
200 keV band is determined to ~ 5.7 photons cm-2 s-1 and the GRB fluence to 1.1
x 10-5 erg cm-2. Analysing the spectral evolution of the GRB, a
``hard-to-soft'' behaviour emerges. A search for an optical counterpart has
been carried out, but none was found.Comment: 5 pages, 3 figures. Accepted for publication in A&AL (INTEGRAL issue
The High Energy Spectrum of NGC 4151
We present the first INTEGRAL observations of the type 1.5 Seyfert galaxy NGC
4151. Combining several INTEGRAL observations performed during 2003, totaling
about 400 ks of exposure time, allows us to study the spectrum in the 2 - 300
keV range. The measurements presented here reveal an overall spectrum from
X-rays up to soft gamma-rays that can be described by an absorbed (NH = 6.9e22
1/cm**2) model based on a Compton continuum from a hot electron population (kT
= 94 keV) from an optically thick (tau = 1.3) corona, reflected on cold
material (R=0.7), consistent with earlier claims. The time resolved analysis
shows little variation of the spectral parameters over the duration of the
INTEGRAL observations. The comparison with CGRO/OSSE data shows that the same
spectral model can be applied over a time span of 15 years, with flux
variations of the order of a factor of 2 and changes in the underlying
continuum reflected by the temperature of the electron population (kT = 50 -
100 keV). When modeled with an exponentially cut-off power law plus Compton
reflection this results in photon indices ranging from Gamma = 1.5 to Gamma =
1.9 and a cut-off energy in the range 100 - 500 keV.Comment: 13 pages, 7 figures, accepted for publication in Ap
The Concordance Cosmic Star Formation Rate: Implications from and for the Supernova Neutrino and Gamma Ray Backgrounds
We constrain the Cosmic Star Formation Rate (CSFR) by requiring that massive
stars produce the observed UV, optical, and IR light while at the same time not
overproduce the Diffuse Supernova Neutrino Background as bounded by
Super-Kamiokande. With the massive star component so constrained we then show
that a reasonable choice of stellar Initial Mass Function and other parameters
results in SNIa rates and iron yields in good agreement with data. In this way
we define a `concordance' CSFR that predicts the optical SNII rate and the SNIa
contribution to the MeV Cosmic Gamma-Ray Background. The CSFR constrained to
reproduce these and other proxies of intermediate and massive star formation is
more clearly delineated than if it were measured by any one technique and has
the following testable consequences: (1) SNIa contribute only a small fraction
of the MeV Cosmic Gamma-Ray Background, (2) massive star core-collapse is
nearly always accompanied by a successful optical SNII, and (3) the Diffuse
Supernova Neutrino Background is tantalizingly close to detectability.Comment: Improved discussion. Version accepted for publication in JCA
The High-Energy Spectra of Accreting Black Holes: Observational Evidence for Bulk-Motion Infall
We discuss the emergent spectra from accreting black holes, considering in
particular the case where the accretion is characterized by relativistic bulk
motion. We suggest that such accretion is likely to occur in a wide variety of
black hole enviroments, where the strong gravitational field is expected to
dominate the pressure forces, and this likely to lead to a characteristic
high-energy spectriscopic signature; an extended power tail. It is in the high
(soft) state that matter impinging upon the event horizon can be viewed
directly, and intrinsic power-law seen. A test of the model is presented using
observational data from the Compton Gamma Ray Observatory and the Rossi X-ray
Timing Explorer, covering 2- 200 keV for recent galactic black hole X-ray nova
outbursts.Comment: latex, 18 pages, 2 figures. Accepted by ApJLetter
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