98 research outputs found
Universality of coronal properties in accreting black holes across mass and accretion rate
Through their radio loudness, lack of thermal UV emission from the accretion
disk and power-law dominated spectra, Low Luminosity AGN (LLAGN) display
similarity with the hard state of stellar-mass black hole X-Ray Binaries
(BHBs). In this work we perform a systematic hard X-ray spectral study of a
carefully selected sample of unobscured LLAGN using archival data, to
understand the central engine properties in the lower accretion regime. We
analyze the spectra of a sample of 16 LLAGN. We model the continuum
emission with detailed Comptonization models. We find a strong anti-correlation
between the optical depth and the electron temperature of the corona,
previously also observed in the brighter Seyferts. This anti-correlation is
present irrespective of the shape of the corona, and the slope of this
anti-correlation in the log space for LLAGN (0.68-1.06) closely matches that of
the higher accretion rate Seyferts (0.55-1.11) and hard state of BHBs
(0.87). This anti-correlation may indicate a departure from a fixed
disk-corona configuration in radiative balance. Our result, therefore,
demonstrates a possible universality in Comptonization processes of black hole
X-ray sources across multiple orders of magnitude in mass and accretion rate.Comment: 9 pages, 3 figures. Shortened abstract. Accepted for publication in
A&A Letter
A variable magnetic disc wind in the black hole X-ray binary GRS 1915+105?
GRS 1915+105 being one of the brightest transient black hole binary (BHB) in
the X-rays, offers a unique test-bed for the study of the connection between
accretion and ejection mechanisms in BHBs. In particular, this source can be
used to study the accretion disc wind and how it depends on the state changes
in BHBs. Our aim is to investigate the origin and geometry of the accretion
disc wind in GRS 1915+105. We analysed the spectra of GRS 1915+105 in the soft
and hard classes, using the high resolution spectroscopy offered
by Chandra HETGS. In the soft state, we find a series of wind absorption lines
that follow a non linear dependence of velocity width, velocity shift and
equivalent width with respect to ionisation, indicating a multiple component or
stratified outflow. In the hard state we find only a faint Fe XXVI absorption
line. We model the absorption lines in both the states using a dedicated MHD
wind model to investigate a magnetic origin of the wind and to probe the cause
of variability in the observed lines flux between the two states. The MHD disc
wind model provides a good fit for both states, indicating the possibility of a
magnetic origin of the wind. The multiple ionisation components of the wind are
well characterised as a stratification of the same magnetic outflow. We find
that the observed variability in the lines flux between soft and hard states
cannot be explained by photo-ionisation alone but it is most likely due to a
large (three orders of magnitude) increase in the wind density. We find the
mass outflow rate of the wind to be comparable to the accretion rate,
suggesting a intimate link between accretion and ejection processes that lead
to state changes in BHBs.Comment: 11 pages, 9 figures. Shortened abstract. Accepted for publication in
A&
Prompt emission polarimetry of Gamma Ray Bursts with ASTROSAT CZT-Imager
X-ray and Gamma-ray polarization measurements of the prompt emission of
Gamma-ray bursts (GRBs) are believed to be extremely important for testing
various models of GRBs. So far, the available measurements of hard X-ray
polarization of GRB prompt emission have not significantly constrained the GRB
models, particularly because of the difficulty of measuring polarization in
these bands. The CZT Imager (CZTI) onboard {\em AstroSat} is primarily an X-ray
spectroscopic instrument that also works as a wide angle GRB monitor due to the
transparency of its support structure above 100 keV. It also has experimentally
verified polarization measurement capability in the 100 300 keV energy
range and thus provides a unique opportunity to attempt spectro-polarimetric
studies of GRBs. Here we present the polarization data for the brightest 11
GRBs detected by CZTI during its first year of operation. Among these, 5 GRBs
show polarization signatures with 3, and 1 GRB shows
2 detection significance. We place upper limits for the remaining 5
GRBs. We provide details of the various tests performed to validate our
polarization measurements. While it is difficult yet to discriminate between
various emission models with the current sample alone, the large number of
polarization measurements CZTI expects to gather in its minimum lifetime of
five years should help to significantly improve our understanding of the prompt
emission.Comment: Accepted for Publication in ApJ ; a figure has been update
Tracking the X-Ray Polarization of the Black Hole Transient Swift J1727.8–1613 during a State Transition
We report on an observational campaign on the bright black hole (BH) X-ray binary Swift J1727.8–1613 centered around five observations by the Imaging X-ray Polarimetry Explorer. These observations track for the first time the evolution of the X-ray polarization of a BH X-ray binary across a hard to soft state transition. The 2–8 keV polarization degree decreased from ∼4% to ∼3% across the five observations, but the polarization angle remained oriented in the north–south direction throughout. Based on observations with the Australia Telescope Compact Array, we find that the intrinsic 7.25 GHz radio polarization aligns with the X-ray polarization. Assuming the radio polarization aligns with the jet direction (which can be tested in the future with higher-spatial-resolution images of the jet), our results imply that the X-ray corona is extended in the disk plane, rather than along the jet axis, for the entire hard intermediate state. This in turn implies that the long (≳10 ms) soft lags that we measure with the Neutron star Interior Composition ExploreR are dominated by processes other than pure light-crossing delays. Moreover, we find that the evolution of the soft lag amplitude with spectral state does not follow the trend seen for other sources, implying that Swift J1727.8–1613 is a member of a hitherto undersampled subpopulation
In-flight calibration system of imaging x-ray polarimetry explorer
The NASA/ASI Imaging X-ray Polarimetry Explorer, which will be launched in 2021, will be the first instrument to perform spatially resolved X-ray polarimetry on several astronomical sources in the 2-8 keV energy band. These measurements are made possible owing to the use of a gas pixel detector (GPD) at the focus of three X-ray telescopes. The GPD allows simultaneous measurements of the interaction point, energy, arrival time, and polarization angle of detected X-ray photons. The increase in sensitivity, achieved 40 years ago, for imaging and spectroscopy with the Einstein satellite will thus be extended to X-ray polarimetry for the first time. The characteristics of gas multiplication detectors are subject to changes over time. Because the GPD is a novel instrument, it is particularly important to verify its performance and stability during its mission lifetime. For this purpose, the spacecraft hosts a filter and calibration set (FCS), which includes both polarized and unpolarized calibration sources for performing in-flight calibration of the instruments. In this study, we present the design of the flight models of the FCS and the first measurements obtained using silicon drift detectors and CCD cameras, as well as those obtained in thermal vacuum with the flight units of the GPD. We show that the calibration sources successfully assess and verify the functionality of the GPD and validate its scientific results in orbit; this improves our knowledge of the behavior of these detectors in X-ray polarimetry
IXPE instrument integration, testing and verification
The Imaging X-ray Polarimetry Explorer (IXPE) is a scientific observatory with the purpose of expand observation space adding polarization property to the X-ray source's currently measured characteristics. The mission selected in the context of NASA Small Explorer (SMEX) is a collaboration between NASA and ASI that will provide to observatory the instrumentation of focal plane. IXPE instrument is composed by three photoelectric polarimeters based on the Gas Pixel Detector (GPD) design, integrated by INFN inside the detector unit (DU) that comprises of the electrical interfaces required to control and communicate with the GPD. The three DUs are interfaced with spacecraft through a detector service unit (DSU) that collect scientific and ancillary data and provides a basically data handling and interfaces to manage the three DUs. AIV has been planned to combine calibration of DUs and Instrument integration and verification activities. Due the tight schedule and the scientific and functional requirements to be verified, in IAPS/INAF have been assembled two equipment's that work in parallel. The flight model of each DU after the environmental tests campaign was calibrated on-ground using the Instrument Calibration Equipment (ICE) and subsequently integrated in the instrument in the AIV-T process on a AIV and Calibration Equipment (ACE), both the facilities managed by Electrical Ground Support Equipment (EGSE) that emulate the spacecraft interfaces of power supply, functional and thermal control and scientific data collection. AIV activities test functionalities and nominal/off-nominal orbits activities of IXPE instrument each time a calibrated DU is connected to DSU flight model completing step by step the full instrument. Here we describe the details of instrumentation and procedures adopted to make possible the full integration and test activities compatibly with calibration of IXPE Instrument
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