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We present Daksha, a proposed high energy transients mission for the study of electromagnetic counterparts of gravitational wave sources, and gamma ray bursts. Daksha will comprise of two satellites in low earth equatorial orbits, on opposite sides of earth. Each satellite will carry three types of detectors to cover the entire sky in an energy range from 1 keV to >1 MeV. Any transients detected on-board will be announced publicly within minutes of discovery. All photon data will be downloaded in ground station passes to obtain source positions, spectra, and light curves. In addition, Daksha will address a wide range of science cases including monitoring X-ray pulsars, studies of magnetars, solar flares, searches for fast radio burst counterparts, routine monitoring of bright persistent high energy sources, terrestrial gamma-ray flashes, and probing primordial black hole abundances through lensing. In this paper, we discuss the technical capabilities of Daksha, while the detailed science case is discussed in a separate paper., Comment: 9 pages, 3 figures, 1 table. Accepted in Experimental Astronomy. Additional information about the mission is available at https://www.dakshasat.in/

We present the science case for the proposed Daksha high energy transients mission. Daksha will comprise of two satellites covering the entire sky from 1~keV to $>1$~MeV. The primary objectives of the mission are to discover and characterize electromagnetic counterparts to gravitational wave source; and to study Gamma Ray Bursts (GRBs). Daksha is a versatile all-sky monitor that can address a wide variety of science cases. With its broadband spectral response, high sensitivity, and continuous all-sky coverage, it will discover fainter and rarer sources than any other existing or proposed mission. Daksha can make key strides in GRB research with polarization studies, prompt soft spectroscopy, and fine time-resolved spectral studies. Daksha will provide continuous monitoring of X-ray pulsars. It will detect magnetar outbursts and high energy counterparts to Fast Radio Bursts. Using Earth occultation to measure source fluxes, the two satellites together will obtain daily flux measurements of bright hard X-ray sources including active galactic nuclei, X-ray binaries, and slow transients like Novae. Correlation studies between the two satellites can be used to probe primordial black holes through lensing. Daksha will have a set of detectors continuously pointing towards the Sun, providing excellent hard X-ray monitoring data. Closer to home, the high sensitivity and time resolution of Daksha can be leveraged for the characterization of Terrestrial Gamma-ray Flashes., Comment: 19 pages, 7 figures. Accepted in Experimental Astronomy. More details about the mission at https://www.dakshasat.in/

Elements with low First Ionization Potential (FIP) are known to be three to four times more abundant in active region loops of the solar corona than in the photosphere. There have been observations suggesting that this observed "FIP bias" may be different in other parts of the solar corona and such observations are thus important in understanding the underlying mechanism. The Solar X-ray Monitor (XSM) on board the Chandrayaan-2 mission carried out spectroscopic observations of the Sun in soft X-rays during the 2019-20 solar minimum, considered to be the quietest solar minimum of the past century. These observations provided a unique opportunity to study soft X-ray spectra of the quiescent solar corona in the absence of any active regions. By modelling high resolution broadband X-ray spectra from XSM, we estimate the temperature and emission measure during periods of possibly the lowest solar X-ray intensity. We find that the derived parameters remain nearly constant over time with a temperature around 2 MK, suggesting the emission is dominated by X-ray Bright Points (XBPs). We also obtain the abundances of Mg, Al, and Si relative to H, and find that the FIP bias is ~2, lower than the values observed in active regions., Comment: Accepted for publication in ApJL; 10 pages, 5 figures; All figures of figure set 5 available in the tar file

Solar flares, with energies ranging over several orders of magnitude, result from impulsive release of energy due to magnetic reconnection in the corona. Barring a handful, almost all microflares observed in X-rays are associated with the solar active regions. Here we present, for the first time, a comprehensive analysis of a large sample of quiet Sun microflares observed in soft X-rays by the Solar X-ray Monitor (XSM) on board the Chandrayaan-2 mission during the 2019-20 solar minimum. A total of 98 microflares having peak flux below GOES A-level were observed by the XSM during observations spanning 76 days. By using the derived plasma temperature and emission measure of these events obtained by fitting the XSM spectra along with volume estimates from concurrent imaging observations in EUV with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA), we estimated their thermal energies to be ranging from 3e26 to 6e27 erg. We present the frequency distribution of the quiet Sun microflares with energy and discuss the implications of these observations of small-scale magnetic reconnection events outside active regions on coronal heating., Comment: Accepted for publication in ApJL; 13 pages, 5 figures; All figures of figure set 3 available in the tar file

The Solar X-ray Monitor (abbreviated as XSM) on board India's Chandrayaan-2 mission is designed to carry out broadband spectroscopy of the Sun from lunar orbit. It observes the Sun as a star and measures the spectrum every second in the soft X-ray band of 1 - 15 keV with an energy resolution better than 180 eV at 5.9 keV. The primary objective of the XSM is to provide the incident solar spectrum for the X-ray fluorescence spectroscopy experiment on the Chandrayaan-2 orbiter, which aims to generate elemental abundance maps of the lunar surface. However, observations with the XSM can independently be used to study the Sun as well. The Chandrayaan-2 mission was launched on 22 July 2019, and the XSM began nominal operations, in lunar orbit, from September 2019. The in-flight observations, so far, have shown that its spectral performance has been identical to that on the ground. Measurements of the effective area from ground calibration were found to require some refinement, which has been carried out using solar observations at different incident angles. It also has been shown that the XSM is sensitive enough to detect solar activity well below A-class. This makes the investigations of microflares and the quiet solar corona feasible in addition to the study of the evolution of physical parameters during intense flares. This article presents the in-flight performance and calibration of the XSM instrument and discusses some specific science cases that can be addressed using observations with the XSM., Comment: 31 pages, 11 figures; accepted for publication in Solar Physics

Solar X-ray Monitor (XSM) instrument of India's Chandrayaan-2 lunar mission carries out broadband spectroscopy of the Sun in soft X-rays. XSM, with its unique features such as low background, high time cadence, and high spectral resolution, provides the opportunity to characterize transient and quiescent X-ray emission from the Sun even during low activity periods. It records the X-ray spectrum at one-second cadence, and the data recorded on-board are downloaded at regular intervals along with that of other payloads. During ground pre-processing, the XSM data is segregated, and the level-0 data is made available for higher levels of processing at the Payload Operations Center (POC). XSM Data Analysis Software (XSMDAS) is developed to carry out the processing of the level-0 data to higher levels and to generate calibrated light curves and spectra for user-defined binning parameters such that it is suitable for further scientific analysis. A front-end for the XSMDAS named XSM Quick Look Display (XSMQLD) is also developed to facilitate a first look at the data without applying calibration. XSM Data Management-Monitoring System (XSMDMS) is designed to carry out automated data processing at the POC and to maintain an SQLite database with relevant information on the data sets and an internal web application for monitoring data quality and instrument health. All XSM raw and calibrated data products are in FITS format, organized into day-wise files, and the data archive follows Planetary Data System-4 (PDS4) standards. The XSM data will be made available after a lock-in period along with the XSM Data Analysis Software from ISRO Science Data Archive (ISDA) at Indian Space Science Data Center(ISSDC). Here we discuss the design and implementation of all components of the software for the XSM data processing and the contents of the XSM data archive., Comment: Submitted to Astronomy and Computing, 13 pages, 9 figures

Chandrayaan-2, the second Indian mission to the Moon, carries a spectrometer called the Solar X-ray Monitor (XSM) to perform soft X-ray spectral measurements of the Sun while a companion payload measures the fluorescence emission from the Moon. Together these two payloads will provide quantitative estimates of elemental abundances on the lunar surface. XSM is also expected to provide significant contributions to the solar X-ray studies with its highest time cadence and energy resolution spectral measurements. For this purpose, the XSM employs a Silicon Drift Detector and carries out energy measurements of incident photons in the 1 -- 15 keV range with a resolution of less than 180 eV at 5.9 keV, over a wide range of solar X-ray intensities. Extensive ground calibration experiments have been carried out with the XSM using laboratory X-ray sources as well as X-ray beam-line facilities to determine the instrument response matrix parameters required for quantitative spectral analysis. This includes measurements of gain, spectral redistribution function, and effective area, under various observing conditions. The capability of the XSM to maintain its spectral performance at high incident flux as well as the dead-time and pile-up characteristics have also been investigated. The results of these ground calibration experiments of the XSM payload are presented in this article., Comment: Submitted to Experimental Astronomy

Alpha Particle X-ray Spectrometer (APXS) is one of the two scientific experiments on Chandrayaan-2 rover named as Pragyan. The primary scientific objective of APXS is to determine the elemental composition of the lunar surface in the surrounding regions of the landing site. This will be achieved by employing the technique of X-ray fluorescence spectroscopy using in-situ excitation source Cm-244 emitting both X-rays and alpha particles. These radiations excite characteristic X-rays of the elements by the processes of particle induced X-ray emission (PIXE) and X-ray fluorescence (XRF). The characteristic X-rays are detected by the state-of-the-art X-ray detector known as Silicon Drift Detector (SDD), which provides high energy resolution as well as high efficiency in the energy range of 1 to 25 keV. This enables APXS to detect all major rock forming elements such as, Na, Mg, Al, Si, Ca, Ti and Fe. The Flight Model (FM) of the APXS payload has been completed and tested for various instrument parameters. The APXS provides energy resolution of 135 eV at 5.9 keV for the detector operating temperature of about -35 deg C. The design details and the performance measurement of APXS are presented in this paper.

Solar X-ray Monitor (XSM) is one of the scientific instruments on-board Chandrayaan-2 orbiter. The XSM along with instrument CLASS (Chandras Large Area Soft x-ray Spectrometer) comprise the remote X-ray fluorescence spectroscopy experiment of Chandrayaan-2 mission with an objective to determine the elemental composition of the lunar surface on a global scale. XSM instrument will measure the solar X-rays in the energy range of 1-15 keV using state-of-the-art Silicon Drift Detector (SDD). The Flight Model (FM) of the XSM payload has been designed, realized and characterized for various operating parameters. XSM provides energy resolution of 180 eV at 5.9 keV with high time cadence of one second. The X-ray spectra of the Sun observed with XSM will also contribute to the study of solar corona. The detailed description and the performance characteristics of the XSM instrument are presented in this paper.

There is growing interest in high-energy astrophysics community for the development of sensitive instruments in the hard X-ray energy extending to few hundred keV. This requires position sensitive detector modules with high efficiency in the hard X-ray energy range. Here, we present development of a detector module, which consists of 25 mm x 25 mm CeBr3 scintillation detector, read out by a custom designed two dimensional array of Silicon Photo-Multipliers (SiPM). Readout of common cathode of SiPMs provides the spectral measurement whereas the readout of individual SiPM anodes provides measurement of interaction position in the crystal. Preliminary results for spectral and position measurements with the detector module are presented here., Comment: Published in SPIE Proceedings Vol. 10392

X-ray polarization measurement of cosmic sources provides two unique parameters namely degree and angle of polarization which can probe the emission mechanism and geometry at close vicinity of the compact objects. Specifically, the hard X-ray polarimetry is more rewarding because the sources are expected to be intrinsically highly polarized at higher energies. With the successful implementation of Hard X-ray optics in NuSTAR, it is now feasible to conceive Compton polarimeters as focal plane detectors. Such a configuration is likely to provide sensitive polarization measurements in hard X-rays with a broad energy band. We are developing a focal plane hard X-ray Compton polarimeter consisting of a plastic scintillator as active scatterer surrounded by a cylindrical array of CsI(Tl) scintillators. The scatterer is 5 mm diameter and 100 mm long plastic scintillator (BC404) viewed by normal PMT. The photons scattered by the plastic scatterer are collected by a cylindrical array of 16 CsI(Tl) scintillators (5 mm x 5 mm x 150 mm) which are read by Si Photomultiplier (SiPM). Use of the new generation SiPMs ensures the compactness of the instrument which is essential for the design of focal plane detectors. The expected sensitivity of such polarimetric configuration and complete characterization of the plastic scatterer, specially at lower energies have been discussed in Chattopadhyay et al. (Exp. Astron. 35, 391-412, 2013; Astrophys. J. Suppl. 212, 12, 2014). In this paper, we characterize the CsI(Tl) absorbers coupled to SiPM. We also present the experimental results from the fully assembled configuration of the Compton polarimeter.