Celestial X-ray Sources Research Articles

Study of Secondary Cosmic Rays and Astronomical X-Ray Sources using Small Stratospheric Balloons

The X-ray sources of the universe are extraterrestrial in nature which emit X-ray photons. The closest strong X-ray source is the Sun, which is followed by various compact sources such as neutron stars, black holes, the Crab pulsar, etc. In this paper, we analyze the data received from several low-cost lightweight meteorological balloon-borne missions launched by the Indian Centre for Space Physics. Our main interest is to study the variation of the vertical intensity of secondary cosmic rays, the detection of strong X-ray sources, and their spectra in the energy band of ∼10–80 keV during the complete flights. Due to the lack of an onboard pointing system, low exposure time, achieving a maximum altitude of only ∼42 km, and freely rotating the payload about its axis, we modeled the background radiation flux for the X-ray detector using physical assumptions. We also present the source detection method, observation of the pulsation of the Crab (∼33 Hz), and spectra of some sources such as the quiet Sun and the Crab pulsar.

Unsupervised machine learning for the classification of astrophysical X-ray sources

ABSTRACT The automatic classification of X-ray detections is a necessary step in extracting astrophysical information from compiled catalogues of astrophysical sources. Classification is useful for the study of individual objects, statistics for population studies, as well as for anomaly detection, that is, the identification of new unexplored phenomena, including transients and spectrally extreme sources. Despite the importance of this task, classification remains challenging in X-ray astronomy due to the lack of optical counterparts and representative training sets. We develop an alternative methodology that employs an unsupervised machine learning approach to provide probabilistic classes to Chandra Source Catalog sources with a limited number of labelled sources, and without ancillary information from optical and infrared catalogues. We provide a catalogue of probabilistic classes for 8756 sources, comprising a total of 14 507 detections, and demonstrate the success of the method at identifying emission from young stellar objects, as well as distinguishing between small- and large-scale compact accretors with a significant level of confidence. We investigate the consistency between the distribution of features among classified objects and well-established astrophysical hypotheses such as the unified active galactic nucleus model. This provides interpretability to the probabilistic classifier. Code and tables are available publicly through GitHub. We provide a web playground for readers to explore our final classification at https://umlcaxs-playground.streamlit.app.

Incorporation of realistic intrinsic profiles for the Fe Kα and Fe Kβ emission lines in X-ray reprocessor models

ABSTRACT The Fe Kα fluorescent emission line from neutral matter is widely used as a diagnostic tool in X-ray spectroscopy for many astrophysical X-ray sources. The line properties can potentially be used to constrain the geometry, column density, and kinematics of the material in which the line is formed. The intrinsic width and the shape of the intrinsic line profile has been neglected thus far, because it is much smaller than the instrumental broadening in all space-based X-ray detectors before Hitomi. However, the instrumental broadening of X-ray microcalorimeters is comparable to the intrinsic line width. We have performed new calculations based on Monte Carlo simulations of the MYTORUS X-ray reprocessor spectral-fitting model that utilize parametrizations of historical high-precision laboratory measurements of the intrinsic profiles of the Fe Kα and Fe Kβ lines. A publicly available table is provided that can seamlessly replace the existing Fe Kα and Fe Kβ line spectrum in the MYTORUS model. The new table can also be used in a standalone mode (without the MYTORUS model) in order to derive empirical velocity widths, if the line-emitting matter is Compton thin. Neglecting to account for the intrinsic profiles of the Fe Kα and Fe Kβ lines can result in a significant overestimate of the velocity broadening, if the true velocity broadening is less than ${\sim} 2000 \ \rm km \ s^{-1}$. Residual artefacts may also appear in observed line profiles in data with a high signal-to-noise ratio.

Digging a little deeper: characterizing three new extreme ULX candidates

ABSTRACT A prime motivation for compiling catalogues of any celestial X-ray source is to increase our numbers of rare subclasses. In this work, we take a recent multimission catalogue of ultraluminous X-ray sources (ULXs) and look for hitherto poorly-studied ULX candidates that are luminous ($L_{\rm X} \ge 10^{40}\,{\rm erg\,s}^{-1}$), bright ($f_{\rm X} \ge 5 \times 10^{-13}\,{\rm erg\,cm}^{-2}\,{\rm s}^{-1}$), and have archival XMM–Newton data. We speculate that this luminosity regime may be ideal for identifying new pulsating ULXs (PULXs), given that the majority of known PULXs reach similar high luminosities. We find three sources that match our criteria and study them using archival data. We find 4XMM J165251.5−591503 to possess a bright and variable Galactic optical/IR counterpart, and so conclude it is very likely to be a foreground interloper. 4XMM J091948.8−121429 does appear to be an excellent ULX candidate associated with the dwarf irregular galaxy PGC 26378, but has only one detection to date with low data quality. The best data set belongs to 4XMM J112054.3+531040 which we find to be a moderately variable, spectrally hard (Γ ≈ 1.4) X-ray source located in a spiral arm of NGC 3631. Its spectral hardness is similar to known PULXs, but no pulsations are detected by accelerated pulsation searches in the available data. We discuss whether other missions provide objects for similar studies and compare this method to others suggested for identifying good PULX candidates.

Establishing the X-ray source detection strategy for eROSITA with simulations

Context. The eROSITA X-ray telescope on board the Spectrum-Roentgen-Gamma satellite has started to detect new X-ray sources over the full sky at an unprecedented rate. Understanding the performance and selection function of the source detection is important for the subsequent scientific analysis of the eROSITA catalogs. Aims. Through simulations, we test and optimize the eROSITA source detection procedures, and we characterize the detected catalog quantitatively. Methods. Taking the eROSITA Final Equatorial-Depth Survey (eFEDS) as an example, we ran extensive photon-event simulations based on our best knowledge of the instrument characteristics, the background spectrum, and the population of astronomical X-ray sources. We introduce a method of analyzing source detection completeness, purity, and efficiency based on the origin of each photon. Results. According to the source detection efficiency measured in the simulation, we chose a two-pronged strategy to build eROSITA X-ray catalogs, creating a main catalog using only the most sensitive band (0.2–2.3 keV) and an independent hard-band-selected catalog using multiband detection in a range up to 5 keV. Because our mock data are highly representative of the real eFEDS data, we used the mock catalogs to measure the completeness and purity of the eFEDS catalogs as a function of multiple parameters, such as detection likelihood, flux, and luminosity. These measurements provide a basis for choosing the eFEDS catalog selection thresholds. The mock catalogs (available with this paper) can be used to construct the selection function of active galactic nuclei and galaxy clusters. A direct comparison of the output and input mock catalogs also gives rise to a correction curve that converts the raw point-source flux distribution into the intrinsic number counts distribution.

21 cm forest constraints on primordial black holes

Abstract Primordial black holes (PBHs) as part of the dark matter (DM) would modify the evolution of large-scale structures and the thermal history of the universe. Future 21 cm forest observations, sensitive to small scales and the thermal state of the intergalactic medium (IGM), could probe the existence of such PBHs. In this article, we show that the shot noise isocurvature mode on small scales induced by the presence of PBHs can enhance the amount of low-mass halos, or minihalos, and thus, the number of 21 cm absorption lines. However, if the mass of PBHs is as large as MPBH ≳ 10 $M_\odot$, with an abundant enough fraction of PBHs as DM, fPBH, the IGM heating due to accretion on to the PBHs counteracts the enhancement due to the isocurvature mode, reducing the number of absorption lines instead. The concurrence of both effects imprints distinctive signatures on the number of absorbers, allowing the abundance of PBHs to be bound. We compute the prospects for constraining PBHs with future 21 cm forest observations, finding achievable competitive upper limits on the abundance as low as fPBH ∼ 10−3 at MPBH = 100 $M_\odot$, or even lower at larger masses, in regions of the parameter space unexplored by current probes. The impact of astrophysical X-ray sources on the IGM temperature is also studied, which could potentially weaken the bounds.

Calibration and Performance of the REgolith X-Ray Imaging Spectrometer (REXIS) Aboard NASA’s OSIRIS-REx Mission to Bennu

The REgolith X-ray Imaging Spectrometer (REXIS) instrument on board NASA’s OSIRIS-REx mission to the asteroid Bennu is a Class-D student collaboration experiment designed to detect fluoresced X-rays from the asteroid’s surface to measure elemental abundances. In July and November 2019 REXIS collected ∼615 hours of integrated exposure time of Bennu’s sun-illuminated surface from terminator orbits. As reported in Hoak et al. (Results from the REgolith X-ray Imaging Spectrometer (REXIS) at Bennu, 2021) the REXIS data do not contain a clear signal of X-ray fluorescence from the asteroid, in part due to the low incident solar X-ray flux during periods of observation. To support the evaluation of the upper limits on the detectable X-ray signal that may provide insights for the properties of Bennu’s regolith, we present an overview of the REXIS instrument, its operation, and details of its in-flight calibration on astrophysical X-ray sources. This calibration includes the serendipitous detection of the transient X-ray binary MAXI J0637-430 during Bennu observations, demonstrating the operational success of REXIS at the asteroid. We convey some lessons learned for future X-ray spectroscopy imaging investigations of asteroid surfaces.

The Instrument of the Imaging X-Ray Polarimetry Explorer

Abstract While X-ray spectroscopy, timing, and imaging have improved much since 1962 when the first astronomical nonsolar source was discovered, especially wi the launch of the 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, which in turn is a result of the fate of approved missions and because celestial X-ray sources appear less polarized than expected. Only one positive measurement has been available until now: the Orbiting Solar Observatory measured the polarization of the Crab Nebula in the 1970s. The advent of microelectronics techniques has allowed for designing a detector based on the photoelectric effect of gas in an energy range where the optics are efficient at focusing in X-rays. Here we describe the instrument, which is the major contribution of the Italian collaboration to the Small Explorer mission called IXPE, the Imaging X-ray Polarimetry Explorer, which will launch 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. We show the technological choices, their scientific motivation, and 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.

Fabrication and testing of a 1024-pixel SiPM camera

We have fabricated a 1024-pixel SiPM sensor and the associated electronics. We integrated the SiPM sensor and the electronics to build a SiPM camera. In this paper, we present the fabrication and assembly procedure of the SiPM sensor and the readout electronics, and the preliminary result of testing the camera. This camera can be readily used as an X-ray detector with an array of the scintillator pixels placed in front of the SiPM sensor. The application of such an X-ray detector includes the X-ray or gamma-ray imaging in the medical field and the detection of astronomical or astrophysical X-ray sources in space. This camera also can be used as a detector that counts photons in low light environment.

Practical, corrected Kirkpatrick-Baez telescope for x-ray astronomy.

Kirkpatrick-Baez (K-B) optics offer a means of imaging soft x-rays with modest resolution and a multi-arc-minute field of view at a cost far below the conventional Wolter design. Such a low-cost system could be useful for dedicated, long-time-line observation of astronomical x-ray sources from orbit. A K-B telescope consists of crossed arrays of parabolic mirrors at grazing incidence. The classic K-B telescope is subject to significant aberration arising from the interplay between the focusing of the fore and aft mirror arrays. We demonstrate here a corrected K-B prescription with aberrations reduced by an order of magnitude. We show, furthermore, that it is possible to construct such a system by constraining flat "slats" of commercially available glass in precision-milled grooves. The slats deform into shapes that adequately approximate the optimal figures, thereby yielding focusing better than the best version of the classic K-B telescope. The result is a new approach that greatly simplifies the task of achieving both useful resolution and high effective area for x-ray astronomy applications.

The Imaging X-ray Polarimetry Explorer (IXPE)

Recently selected by NASA as the next SMall EXplorer (SMEX) mission for a launch in 2021, the Imaging X-ray Polarimetry Explorer (IXPE) marks the renaissance of X-ray polarimetry, with the prospect of measuring linear polarization from tens of astronomical X-ray sources. IXPE will expand the X-ray view of the Universe by providing new and unique insight into our understanding of some of the most violent celestial objects, such as stellar and supermassive black holes and neutron stars . In this contribution we shall discuss the scientific prospects of the mission, as well as the status of the program, with particular emphasis on the development of the focal plane detectors sensitive to polarization. • The Imaging X-ray Polarimetry Explorer (IXPE) will be the next NASA SMall EXplorer. • IXPE will be devoted to X-ray polarimetry in the 2–8 keV energy band. • The Gas Pixel Detector (GPD) made possible polarimetry at a few keV energy range. • The GPD exploit photoelectric effect in gas, with an ASIC as collecting anode.

Shedding new light on the Crab with polarized X-rays

Strong magnetic fields, synchrotron emission, and Compton scattering are omnipresent in compact celestial X-ray sources. Emissions in the X-ray energy band are consequently expected to be linearly polarized. X-ray polarimetry provides a unique diagnostic to study the location and fundamental mechanisms behind emission processes. The polarization of emissions from a bright celestial X-ray source, the Crab, is reported here for the first time in the hard X-ray band (~20–160 keV). The Crab is a complex system consisting of a central pulsar, a diffuse pulsar wind nebula, as well as structures in the inner nebula including a jet and torus. Measurements are made by a purpose-built and calibrated polarimeter, PoGO+. The polarization vector is found to be aligned with the spin axis of the pulsar for a polarization fraction, PF = (20.9 ± 5.0)%. This is higher than that of the optical diffuse nebula, implying a more compact emission site, though not as compact as, e.g., the synchrotron knot. Contrary to measurements at higher energies, no significant temporal evolution of phase-integrated polarisation parameters is observed. The polarization parameters for the pulsar itself are measured for the first time in the X-ray energy band and are consistent with observations at optical wavelengths.

Study of high energy phenomena from near space using low-cost meteorological balloons

Indian Centre for Space Physics has taken a novel strategy to study low energy cosmic rays and astrophysical X-ray sources which involve very light weight payloads up to about five kilograms on board a single or multiple balloons which are usually used for meteorological purposes. The mission duration could be anywhere from 3-12 hours. Our strategy provides extreme flexibility in mission preparation and its operation using a very economical budget. There are several limitations but our innovative approach has been able to extract significant amount of scientific data out of these missions. So far, over one hundred missions have been completed by us to near space and a wealth of data has been collected. The payloads are recovered and are used again. Scientific data is stored on board computer and the atmospheric data or payload location is sent to ground in real time. Since each mission is different, we present here the general strategy for a typical payload and provide some results we obtained in some of these missions.

Performance of the PRAXyS X-ray polarimeter

The performance of the Time Projection Chamber (TPC) polarimeter for the Polarimeter for Relativistic Astrophysical X-ray Sources (PRAXyS) Small Explorer was evaluated using polarized and unpolarized X-ray sources. The PRAXyS mission will enable exploration of the universe through X-ray polarimetry in the 2–10keV energy band. We carried out performance tests of the polarimeter at the Brookhaven National Laboratory, National Synchrotron Light Source (BNL-NSLS) and at NASA's Goddard Space Flight Center. The polarimeter was tested with linearly polarized, monochromatic X-rays at 11 different energies between 2.5 and 8.0keV. At maximum sensitivity, the measured modulation factors at 2.7, 4.5 and 8.0keV are 27%, 43% and 59%, respectively and the measured angle of polarization is consistent with the expected value at all energies. Measurements with a broadband, unpolarized X-ray source placed a limit of less than 1% on false polarization in the PRAXyS polarimeter.

Autonomous navigation for lunar satellite using X‐ray pulsars with measurement faults

X-ray pulsar-based navigation (XNAV) is a navigation method using celestial X-ray source observations for spacecraft orbit determination. However, if the measurements are not reliable due to any kind of malfunction, the performance of XNAV may result in considerable errors and even divergence. In this study, a new algorithm called robust extended Kalman filter (REKF) is proposed for the lunar satellite autonomous navigation system, which is robust against measurement malfunctions. First, the satellite dynamic model applied perturbations is derived under the J2 perturbation of the Moon. The pulse time-of-arrival (TOA) is used to build the observation model. Second, the performance of XNAV system is discussed by analysing the transformation error of TOA, system observability and controllability. Then, an adaptive measurement noise scale factor (AMNSF) is designed by using the innovation sequence in REKF. Meanwhile, the gain matrix is modified by adding the AMNSF to reduce the influence of malfunction and enhance the robustness of XNAV system. Finally, the simulation results show the proposed navigation scheme is valid and feasible, and it is appropriate for lunar satellite autonomous navigation.

Modeling X-ray emission line profiles from massive star winds – A review

The Chandra and XMM-Newton X-ray telescopes have led to numerous advances in the study and understanding of astrophysical X-ray sources. Particularly important has been the much increased spectral resolution of modern X-ray instrumentation. Wind-broadened emission lines have been spectroscopically resolved for many massive stars. This contribution reviews approaches to the modeling of X-ray emission line profile shapes from single stars, including smooth winds, winds with clumping, optically thin versus thick lines, and the effect of a radius-dependent photoabsorption coefficient.

Kalman Filter Analysis for Orbit Estimation using Pulsars for Interplanetary Missions

Abstract Autonomous Navigation in Interplanetary Missions is the main challenge due to lack of dense ground tracking network measurements. This paper presents a novel technique for determining spacecraft position and velocity using celestial X-ray sources, such as pulsars. It portrays the formulation of the pulsar measurements and describes the development of a noise model for X-ray Navigation and brings out the cumulative efforts in this developing field. In addition to that, it also covers the blending of the pulsar-derived measurements with a Kalman filter for continuous determination of position and velocity of an Earth orbiting spacecraft. Several sampling analysis are presented to establish the expected performance using measurements obtained from models of pulsed X-ray signals. This is the first step and future efforts shall make the orbit estimation more sophisticated.

The imprint of the cosmic supermassive black hole growth history on the 21 cm background radiation

The redshifted 21 cm transition line of hydrogen tracks the thermal evolution of the neutral intergalactic medium (IGM) at "cosmic dawn," during the emergence of the first luminous astrophysical objects (~100 Myr after the Big Bang) but before these objects ionized the IGM (~400-800 Myr after the Big Bang). Because X-rays, in particular, are likely to be the chief energy courier for heating the IGM, measurements of the 21 cm signature can be used to infer knowledge about the first astrophysical X-ray sources. Using analytic arguments and a numerical population synthesis algorithm, we argue that the progenitors of supermassive black holes (SMBHs) should be the dominant source of hard astrophysical X-rays---and thus the primary driver of IGM heating and the 21 cm signature---at redshifts $z < 20$, if (i) they grow readily from the remnants of Population III stars and (ii) produce X-rays in quantities comparable to what is observed from active galactic nuclei and high-mass X-ray binaries. We show that models satisfying these assumptions dominate over contributions to IGM heating from stellar populations, and cause the 21 cm brightness temperature to rise at $z > 20$. An absence of such a signature in the forthcoming observational data would imply that SMBH formation occurred later (e.g. via so-called direct collapse scenarios), that it was not a common occurrence in early galaxies and protogalaxies, or that it produced far fewer X-rays than empirical trends at lower redshifts, either due to intrinsic dimness (radiative inefficiency) or Compton-thick obscuration close to the source.

X-ray echoes used as a cosmic yardstick

Astrophysics Determining accurate distances to astronomical x-ray sources is notoriously difficult, but Heinz et al. have developed a new technique for doing so. They exploited “light echoes” from the object Cir X-1, in which x-rays emitted during a large flare bounced off foreground dust and

A Preliminary Research on the Development of the Hard X-ray Imaging Telescope

The hard X-ray imaging telescope based on the Fourier transform imaging technique is introduced. The double-layer parallel gratings are used to make the modulation and coding on the light emerging from a celestial X-ray source, the modulated light is acquired, to make the optoelectronic conversion by scintillation crystal detectors, and finally read out by the electronic system. The modulation collimator X-ray telescopes can be divided into two types: the spatial modulation and temporal modulation. The temporal modulation system requires the scanning motion of the detector system, but the spatial modulation system requires no motion. The technology of grating fabrication is investigated, and the basic structure design of the collimators is given. The principal compo- nents of the prototype hard X-ray imaging telescope of spatial modulation type are successfully developed, including the 8 CsI crystal detector modules (contain- ing photomultipliers or PMTs), 8-channel shaping amplifiers (two of them are prepared for experiments), and the data acquisition system. And the preliminary test results of the electronic system are also given.