arXiv subjects

Ajay Ratheesh

Publications and source records attributed to Ajay Ratheesh.

81 records · Page 5Linked to original sources

arXiv2021

Exploring the accretion-ejection geometry of GRS 1915+105 in the obscured state with future X-ray spectro-polarimetry

GRS 1915+105 has been in a bright flux state for more than 2 decades, but in 2018 a significant drop in flux was observed, partly due to changes in the central engine along with increased X-ray absorption. The aim of this work is to explore how X-ray spectro-polarimetry can be used to derive the basic geometrical properties of the absorbing and reflecting matter. In particular, the expected polarisation of the radiation reflected off the disc and the putative outflow is calculated. We use \textit{NuSTAR} data collected after the flux drop to derive the parameters of the system from hard X-ray spectroscopy. The spectroscopic parameters are then used to derive the expected polarimetric signal, using results from a MonteCarlo radiative transfer code both in the case of neutral and fully ionised matter. From the spectral analysis, we find that the continuum emission becomes softer with increasing flux, and that in all flux levels the obscuring matter is highly ionised. This analysis, on the other hand, confirms that spectroscopy alone is unable to put constraints on the geometry of the reflectors. Simulations show that X-ray polarimetric observations, like those that will be provided soon by the Imaging X-ray Polarimetry Explorer (IXPE), will help to determine the geometrical parameters which are left unconstrained by the spectroscopic analysis.

astro-ph.HE↗

arXiv2021

The Instrument of the Imaging X-ray Polarimetry Explorer

While X-ray Spectroscopy, Timing and Imaging have improved verymuch since 1962, when the first astronomical non-solar source was discovered, especially with the launch of Newton/X-ray Multi-Mirror Mission, Rossi/X-ray Timing Explorer and Chandra/Advanced X-ray Astrophysics Facility, the progress of X-ray polarimetry has been meager. This is in part due to the lack of sensitive polarization detectors, in part due to the fate of approved missions and in part because the celestial X-ray sources appeared less polarized than expected. Only one positive measurement has been available until now. Indeed the eight Orbiting Solar Observatory measured the polarization of the Crab nebula in the 70s. The advent of techniques of microelectronics allowed for designing a detector based on the photoelectric effect in gas in an energy range where the optics are efficient in focusing X-rays. Herewe describe the Instrument, which is the major contribution of the Italian collaboration to the SmallExplorer mission called IXPE, the Imaging X-ray Polarimetry Explorer, which will be flown in late 2021. The instrument, is composed of three Detector Units, based on this technique, and a Detector Service Unit. Three Mirror Modules provided by Marshall Space Flight Center focus X-rays onto the detectors. In the following we will show the technological choices, their scientific motivation and the results from the calibration of the Instrument. IXPE will perform imaging, timing and energy resolved polarimetry in the 2-8 keV energy band opening this window of X-ray astronomy to tens of celestial sources of almost all classes.

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arXiv2021

Characterisation of Cosmic Ray Induced Noise Events in AstroSat-CZT Imager

The Cadmium Zinc Telluride (CZT) Imager onboard AstroSat, consists of pixelated CZT detectors, which are sensitive to hard X-rays above 20 keV. The individual pixels are triggered by ionising events occurring in them, and the detectors operate in a self-triggered mode, recording each event separately with information about its time of incidence, detector co-ordinates, and channel that scales with the amount of ionisation. The detectors are sensitive not only to photons from astrophysical sources of interest, but also prone to a number of other events like background X-rays, cosmic rays, and noise in detectors or the electronics. In this work a detailed analysis of the effect of cosmic rays on the detectors is made and it is found that cosmic rays can trigger multiple events which are closely packed in time (called 'bunches'). Higher energy cosmic rays, however, can also generate delayed emissions, a signature previously seen in the PICsIT detector on-board INTEGRAL. An algorithm to automatically detect them based on their spatial clustering properties is presented. Residual noise events are examined using examples of Gamma Ray Bursts as target sources.

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arXiv2021

A generalized event selection algorithm for AstroSat CZT Imager data

The Cadmium Zinc Telluride (CZT) Imager on board AstroSat is a hard X-ray imaging spectrometer operating in the energy range of 20 $-$ 100 keV. It also acts as an open hard X-ray monitor above 100 keV capable of detecting transient events like the Gamma-ray Bursts (GRBs). Additionally, the instrument has the sensitivity to measure hard X-ray polarization in the energy range of 100 $-$ 400 keV for bright on-axis sources like Crab and Cygnus X-1 and bright GRBs. As hard X-ray instruments like CZTI are sensitive to cosmic rays in addition to X-rays, it is required to identify and remove particle induced or other noise events and select events for scientific analysis of the data. The present CZTI data analysis pipeline includes algorithms for such event selection, but they have certain limitations. They were primarily designed for the analysis of data from persistent X-ray sources where the source flux is much less than the background and thus are not best suited for sources like GRBs. Here, we re-examine the characteristics of noise events in CZTI and present a generalized event selection method that caters to the analysis of data for all types of sources. The efficacy of the new method is reviewed by examining the Poissonian behavior of the selected events and the signal to noise ratio for GRBs.

astro-ph.HE↗

arXiv2021

Sub-MeV spectroscopy with AstroSat-CZT Imager for Gamma Ray Bursts

Cadmium Zinc Telluride Imager (CZTI) onboard AstroSat has been a prolific Gamma-Ray Burst (GRB) monitor. While the 2-pixel Compton scattered events (100 - 300 keV) are used to extract sensitive spectroscopic information, the inclusion of the low-gain pixels (around 20% of the detector plane) after careful calibration extends the energy range of Compton energy spectra to 600 keV. The new feature also allows single-pixel spectroscopy of the GRBs to the sub-MeV range which is otherwise limited to 150 keV. We also introduced a new noise rejection algorithm in the analysis ('Compton noise'). These new additions not only enhances the spectroscopic sensitivity of CZTI, but the sub-MeV spectroscopy will also allow proper characterization of the GRBs not detected by Fermi. This article describes the methodology of single, Compton event and veto spectroscopy in 100 - 600 keV for the GRBs detected in the first year of operation. CZTI in last five years has detected around 20 bright GRBs. The new methodologies, when applied on the spectral analysis for this large sample of GRBs, has the potential to improve the results significantly and help in better understanding the prompt emission mechanism.

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arXiv2020

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 $ϕ$ and hard $χ$ 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.

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arXiv2020

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.

astro-ph.IM↗

arXiv2020

A spectral study of the black hole X-ray binary MAXI J1820+070 with AstroSat and NuSTAR

MAXI J1820+070 is a newly discovered transient black hole X-ray binary, which showed several spectral and temporal features. In this work, we analyse the broadband X-ray spectra from all three simultaneously observing X-ray instruments onboard AstroSat, as well as contemporaneous X-ray spectra from NuSTAR, observed during the hard state of MAXI J1820+070 in March 2018. Implementing a combination of multi-colour disc model, relativistic blurred reflection model relxilllpCp and a distant reflection in the form of xillverCp, we achieve reasonable and consistent fits for AstroSat and NuSTAR spectra. The best-fit model suggests a low temperature disc ($kT_{\rm in} \sim 0.3$ keV), iron overabundance ($A_{\rm Fe} \sim 4-5$ solar), a short lamp-post corona height ($h \lesssim 8 R_{\rm g}$), and a high corona temperature ($kT_{\rm e} \sim 115-150$ keV). Addition of a second Comptonisation component leads to a significantly better fit, with the $kT_{\rm e}$ of the second Comptonisation component being $\sim 14-18$ keV. Our results from independent observations with two different satellites in a similar source state indicate an inhomogeneous corona, with decreasing temperature attributed to increasing height. Besides, utilising the broader energy coverage of AstroSat, we estimate the black hole mass to be $6.7-13.9 \ M_{\odot}$, consistent with independent measurements reported in the literature.

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arXiv2019

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 $\gtrapprox$3$σ$, and 1 GRB shows $\>$2$σ$ 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.

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