18 research outputs found
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
Drop in the hard pulsed fraction and a candidate cyclotron line in IGR J16320-4751 seen by NuSTAR
We report on a timing and spectral analysis of a 50-ks NuSTAR observation of
IGR J16320-4751 (= AX J1631.9-4752); a high-mass X-ray binary hosting a
slowly-rotating neutron star. In this observation from 2015, the spin period
was 1,308.8+/-0.4 s giving a period derivative dP/dt ~ 2E-8 s s-1 when compared
with the period measured in 2004. In addition, the pulsed fraction decreased as
a function of energy, as opposed to the constant trend that was seen
previously. This suggests a change in the accretion geometry of the system
during the intervening 11 years. The phase-averaged spectra were fit with the
typical model for accreting pulsars: a power law with an exponential cutoff.
This left positive residuals at 6.4 keV attributable to the known iron K-alpha
line, as well as negative residuals around 14 keV from a candidate cyclotron
line detected at a significance of 5-sigma. We found no significant differences
in the spectral parameters across the spin period, other than the expected
changes in flux and component normalizations. A flare lasting around 5 ks was
captured during the first half of the observation where the X-ray emission
hardened and the local column density decreased. Finally, the binary orbital
period was refined to 8.9912+/-0.0078 d thanks to Swift/BAT monitoring data
from 2005-2022.Comment: 17 pages, 11 figures, Referee-revised version accepted for
publication in the Astrophysical Journa
Absorption lines from magnetically driven winds in X-ray binaries II: high resolution observational signatures expected from future X-ray observatories
In our self-similar, analytical, magneto-hydrodynamic (MHD)
accretion-ejection solution, the density at the base of the outflow is
explicitly dependent on the disk accretion rate - a unique property of this
class of solutions. We had earlier found that the ejection index is a key MHD parameter that decides if the flow
can cause absorption lines in the high resolution X-ray spectra of black hole
binaries. Here we choose 3 dense warm solutions with and
carefully develop a methodology to generate spectra which are convolved with
the Athena and XRISM response functions to predict what they will observe
seeing through such MHD outflows. In this paper two other external parameters
were varied - extent of the disk, , and the angle of the line of sight, .
Resultant absorption lines (H and He-like Fe, Ca, Ar) change in strength and
their profiles manifest varying degrees of asymmetry. We checked if a) the
lines and ii) the line asymmetries are detected, in our suit of synthetic
Athena and XRISM spectra. Our analysis shows that Athena should detect the
lines and their asymmetries for a standard 100 ksec observation of a 100 mCrab
source - lines with equivalent width as low as a few eV should be detected if
the 6-8 keV counts are larger than even for the least favourable
simulated cases.Comment: 18 pages, 13 figures in the main body and 3 figures in the appendix.
Accepted for publication in MNRA
Chandra, NuSTAR, and Optical Observations of the Cataclysmic Variables IGR J17528-2022 and IGR J20063+3641
International audienceWe report on Chandra, NuSTAR, and MDM observations of two International Gamma-ray Astrophysics Laboratory (INTEGRAL) sources, namely IGR J17528−2022 and IGR J20063+3641. IGR J17528−2022 is an unidentified INTEGRAL source, while IGR J20063+3641 was recently identified as a magnetic cataclysmic variable (mCV) by Halpern et al. The Chandra observation of IGR J17528−2022 has allowed us to locate the optical counterpart to the source and to obtain its optical spectrum, which shows a strong Hα emission line. The optical spectrum and flickering observed in the optical time-series photometry in combination with the X-ray spectrum, which is well fit by an absorbed partially covered thermal bremsstrahlung model, suggest that this source is a strong mCV candidate. The X-ray observations of IGR J20063+3641 reveal a clear modulation with a period of 172.46 ± 0.01 s, which we attribute to the white dwarf spin period. Additional MDM spectroscopy of the source has also allowed for a clear determination of the orbital period at 0.731 ± 0.015 days. The X-ray spectrum of this source is also well fit by an absorbed partially covered thermal bremsstrahlung model. The X-ray spectrum, spin periodicity, and orbital periodicity allow this source to be further classified as an intermediate polar
Are Low-Frequency Quasi-Periodic Oscillations seen in accretion flows the disk response to a jet instability?
Low Frequency Quasi-Periodic Oscillations or LF QPOs are ubiquitous in BH
X-ray binaries and provide strong constraints on the accretion-ejection
processes. Although several models have been proposed so far, none has been
proven to reproduce all observational constraints and no consensus has emerged
yet. We make the conjecture that disks are threaded by a large scale vertical
magnetic field that splits it into two radial zones. In the inner Jet Emitting
Disk (JED), a near equipartition field allows to drive powerful self-collimated
jets, while beyond a transition radius, the disk magnetization is too low and a
Standard Accretion Disk (SAD) is settled. In a series of papers, this hybrid
JED-SAD disk configuration has been shown to successfully reproduce most
multi-wavelength (radio and X-rays) observations, as well as the concurrence
with the LFQPOs for the archetypal source GX 339-4. We first analyze the main
QPO scenarios provided in the literature: 1) a specific process occurring at
the transition radius, 2) the accretion-ejection instability and 3) the
solid-body Lense-Thirring disk precession. We recall their main assumptions and
shed light on some severe theoretical issues that question the capability to
reproduce LF QPOs. We then argue that none of these models could be operating
under the JED-SAD physical conditions. We finally propose an alternative
scenario where LF QPOs would be the disk response to an instability triggered
in the jets, near a magnetic recollimation zone. Such a situation could account
for most Type-C QPO phenomenology and is consistent with the global behavior of
black hole binaries. The calculation of this non-destructive jet instability
remains however to be done. If the existence of this instability is numerically
confirmed, then it could also naturally account for the jet wobbling
phenomenology seen in various accreting sources.Comment: 11 pages, 1 figure, accepted for publication in Astronomy &
Astrophysic