168 research outputs found
Probing the Interstellar Dust towards the Galactic Centre: Dust Scattering Halo around AX J1745.6-2901
AX J1745.6-2901 is an X-ray binary located at only 1.45 arcmin from Sgr A*,
showcasing a strong X-ray dust scattering halo. We combine Chandra and
XMM-Newton observations to study the halo around this X-ray binary. Our study
shows two major thick dust layers along the line of sight (LOS) towards AX
J1745.6-2901. The LOS position and of these two layers depend on the
dust grain models with different grain size distribution and abundances. But
for all the 19 dust grain models considered, dust Layer-1 is consistently found
to be within a fractional distance of 0.11 (mean value: 0.05) to AX
J1745.6-2901 and contains only (19-34)% (mean value: 26%) of the total LOS
dust. The remaining dust is contained in Layer-2, which is distributed from the
Earth up to a mean fractional distance of 0.64. A significant separation
between the two layers is found for all the dust grain models, with a mean
fractional distance of 0.31. Besides, an extended wing component is discovered
in the halo, which implies a higher fraction of dust grains with typical sizes
590 \AA\ than considered in current dust grain models. Assuming AX
J1745.6-2901 is 8 kpc away, dust Layer-2 would be located in the Galactic disk
several kpc away from the Galactic Centre (GC). The dust scattering halo biases
the observed spectrum of AX J1745.6-2901 severely in both spectral shape and
flux, and also introduces a strong dependence on the size of the instrumental
point spread function and the source extraction region. We build Xspec models
to account for this spectral bias, which allow us to recover the intrinsic
spectrum of AX J1745.6-2901 free from dust scattering opacity. If dust Layer-2
also intervenes along the LOS to Sgr A* and other nearby GC sources, a
significant spectral correction for the dust scattering opacity would be
necessary for all these GC sources.Comment: 20 pages, published by MNRAS; revised values in Table-1 and Table-B
Effects of Interstellar Dust Scattering on the X-ray Eclipses of the LMXB AX J1745.6-2901 in the Galactic Center
AX J1745.6-2901 is an eclipsing low mass X-ray binary (LMXB) in the Galactic
Centre (GC). It shows significant X-ray excess emission during the eclipse
phase, and its eclipse light curve shows an asymmetric shape. We use archival
XMM-Newton and Chandra observations to study the origin of these peculiar X-ray
eclipsing phenomena. We find that the shape of the observed X-ray eclipse light
curves depends on both photon energy and the shape of the source extraction
region, and also shows differences between the two instruments. By performing
detailed simulations for the time-dependent X-ray dust scattering halo, as well
as directly modelling the observed eclipse and non-eclipse halo profiles of AX
J1745.6-2901, we obtained solid evidence that its peculiar eclipse phenomena
are indeed caused by the X-ray dust scattering in multiple foreground dust
layers along the line-of-sight (LOS). The apparent dependence on the
instruments is caused by different instrumental point-spread-functions. Our
results can be used to assess the influence of dust scattering in other
eclipsing X-ray sources, and raise the importance of considering the timing
effects of dust scattering halo when studying the variability of other X-ray
sources in the GC, such as Sgr A*. Moreover, our study of halo eclipse
reinforces the existence of a dust layer local to AX J1745.6-2901 as reported
by Jin et al. (2017), as well as identifying another dust layer within a few
hundred parsecs to Earth, containing up to several tens of percent LOS dust,
which is likely to be associated with the molecular clouds in the Solar
neighbourhood. The remaining LOS dust is likely to be associated with the
molecular clouds located in the Galactic disk in-between.Comment: 25 pages, 18 figures, 5 tables, accepted by MNRA
The XMM-Newton view of GRS1915+105 during a "plateau"
Two XMM-Newton observations of the black-hole binary GRS1915+105 were
triggered in 2004 (April 17 and 21), during a long "plateau" state of the
source. We analyzed the data collected with EPIC-pn in Timing and Burst modes,
respectively. No thermal disc emission is required by the data; the spectrum is
well fitted by four components: a primary component (either a simple power law
or thermal Comptonization) absorbed by cold matter with abundances different
than those of standard ISM; reprocessing from an ionized disc; emission and
absorption lines; and a soft X-ray excess around 1 keV. The latter is not
confirmed by RGS (which were used in the second observation only); if real, the
excess could be due to reflection from the optically thin, photoionized plasma
of a disc wind, in which case it may provide a way to disentangle intrinsic
from interstellar absorption. Indeed, the former is best traced by the higher
abundances of heavier elements, while an independent estimate of the latter may
be given by the value we get for the disc wind component only, which roughly
coincides with what is found for lower-Z species.Comment: 5 pages, 3 figures, 1 table. Submitte
Relativistic iron K alpha line detection in the Suzaku spectra of IC 4329A
We present an analysis of five Suzaku observations of the bright Seyfert1
galaxy IC 4329A. The broad energy band and high signal-to-noise ratio of the
data give new constraints on the iron K alpha line profile and its relationship
with the Compton hump at higher energies. The Fe K bandpass is dominated by a
narrow core (EW=57 eV) at 6.4 keV consistent with neutral material. Using a
physically-motivated model, our analysis also reveals the presence of a broad
Iron K alpha line (EW=124 eV), most likely produced in the inner part of the
accretion disk and blurred by general relativistic effects. This component is
not immediately evident from the individual spectra, but is clearly present in
the stacked residuals of all five observations, and has high statistical
significance. This highlights the difficulty in identifying broad iron lines in
AGN, even in data with very high signal-to-noise ratio, as they are difficult
to disentangle from the continuum. The data are consistent with the narrow and
broad iron line components tracking the Compton Hump, but do not provide clear
evidence that this is the case. An additional narrow FeXXVI emission line at
6.94 keV is also seen, suggesting the presence of ionized material relatively
distant from the central region. There is also a hint of variability, so the
precise origin of this line remains unclear.Comment: Accepted for publication on MNRAS Letters, 6 pages, 3 figure
Exploring the Interstellar Medium Using an Asymmetric X-ray Dust Scattering Halo
SWIFT J1658.2-4242 is an X-ray transient discovered recently in the Galactic
plane, with severe X-ray absorption corresponding to an equivalent hydrogen
column density of cm. Using new
Chandra and XMM-Newton data, we discover a strong X-ray dust scattering halo
around it. The halo profile can be well fitted by the scattering from at least
three separated dust layers. During the persistent emission phase of SWIFT
J1658.2-4242, the best-fit dust scattering based on the
COMP-AC-S dust grain model is consistent with . The best-fit
halo models show that 85-90 percent of the intervening gas and dust along the
line of sight of SWIFT J1658.2-4242 are located in the foreground ISM in the
Galactic disk. The dust scattering halo also shows significant azimuthal
asymmetry, which appears consistent with the inhomogeneous distribution of
foreground molecular clouds. By matching the different dust layers to the
distribution of molecular clouds along the line of sight, we estimate the
source distance to be 10 kpc, which is also consistent with the results
given by several other independent methods of distance estimation. The dust
scattering opacity and the existence of a halo can introduce a significant
spectral bias, the level of which depends on the shape of the instrumental
point spread function and the source extraction region. We create the Xspec
dscor model to correct for this spectral bias for different X-ray instruments.
Our study reenforces the importance of considering the spectral effects of dust
scattering in other absorbed X-ray sources.Comment: 21 pages, 13 figures, 6 tables, accepted for publication in Ap
Testing the disk-corona interplay in radiatively-efficient broad-line AGN
The correlation observed between monochromatic X-ray and UV luminosities in
radiatively-efficient active galactic nuclei (AGN) lacks a clear theoretical
explanation despite being used for many applications. Such a correlation, with
its small intrinsic scatter and its slope that is smaller than unity in log
space, represents the compelling evidence that a mechanism regulating the
energetic interaction between the accretion disk and the X-ray corona must be
in place. This ensures that going from fainter to brighter sources the coronal
emission increases less than the disk emission. We discuss here a
self-consistently coupled disk-corona model that can identify this regulating
mechanism in terms of modified viscosity prescriptions in the accretion disk.
The model predicts a lower fraction of accretion power dissipated in the corona
for higher accretion states. We then present a quantitative observational test
of the model using a reference sample of broad-line AGN and modeling the
disk-corona emission for each source in the plane. We used the
slope, normalization, and scatter of the observed relation to constrain the
parameters of the theoretical model. For non-spinning black holes and static
coronae, we find that the accretion prescriptions that match the observed slope
of the relation produce X-rays that are too weak with respect to
the normalization of the observed relation. Instead, considering
moderately-outflowing Comptonizing coronae and/or a more realistic
high-spinning black hole population significantly relax the tension between the
strength of the observed and modeled X-ray emission, while also predicting very
low intrinsic scatter in the relation. In particular, this latter
scenario traces a known selection effect of flux-limited samples that
preferentially select high-spinning, hence brighter, sources.Comment: 17 pages, 15 figures. Accepted in A&
NuSTAR and XMM-Newton observations of the Arches cluster in 2015: fading hard X-ray emission from the molecular cloud
We present results of long NuSTAR (200 ks) and XMM-Newton (100 ks)
observations of the Arches stellar cluster, a source of bright thermal (kT~2
keV) X-rays with prominent Fe XXV K_alpha 6.7 keV line emission and a nearby
molecular cloud, characterized by an extended non-thermal hard X-ray continuum
and fluorescent Fe K_alpha 6.4 keV line of a neutral or low ionization state
material around the cluster. Our analysis demonstrates that the non-thermal
emission of the Arches cloud underwent a dramatic change, with its homogeneous
morphology, traced by fluorescent Fe K_alpha line emission, vanishing after
2012, revealing three bright clumps. The declining trend of the cloud emission,
if linearly fitted, is consistent with half-life decay time of ~8 years. Such
strong variations have been observed in several other molecular clouds in the
Galactic Centre, including the giant molecular cloud Sgr B2, and point toward a
similar propagation of illuminating fronts, presumably induced by the past
flaring activity of Sgr A*.Comment: 15 pages, 14 figures, 7 tables, submitted to MNRAS; comments welcom
The XMM-Newton View of GRS1915+105
(abridged) Two XMM-Newton observations of the black-hole binary GRS1915+105
were triggered in 2004, during a long "plateau" state of the source. (...)
While the light curves show just small amplitude variations (a few percent) at
timescales longer than a few seconds, a QPO is seen at about 0.6 Hz (...). The
pn spectrum is well fitted without invoking thermal disk emission, on the base
of four main components: a primary one (...), absorbed by cold matter with
abundances different than those of standard ISM; reprocessing from an ionized
disk; emission and absorption lines; and a soft X-ray excess around 1 keV.
However, the latter is not confirmed by the RGS spectra, whose difference from
the EPIC-pn ones actually lacks of a fully satisfactory explanation. If real,
the soft X-ray excess may be due to reflection from an optically thin,
photoionized disk wind; in this case it may yield a way to disentangle
intrinsic from interstellar absorption.Comment: 11 pages, 7 figures. Accepted for publication on Astronomy and
Astrophysic
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