Early In-orbit Performance of Scanning Sky Monitor Onboard AstroSat
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We report the in-orbit performance of Scanning Sky Monitor (SSM) onboard AstroSat. The SSM operates in the energy range 2.5 to 10 keV and scans the sky to detect and locate transient X-ray sources. This information of any interesting phenomenon in the X-ray sky as observed by SSM is provided to the astronomical community for follow-up observations. Following the launch of AstroSat on 28th September, 2015, SSM was commissioned on October 12th, 2015. The first power ON of the instrument was with the standard X-ray source, Crab in the field-of-view. The first orbit data revealed the basic expected performance of one of the detectors of SSM, SSM1. Following this in the subsequent orbits, the other detectors were also powered ON to find them perform in good health. Quick checks of the data from the first few orbits revealed that the instrument performed with the expected angular resolution of 12’ \(\times \) 2.5\(^\circ \) and effective area in the energy range of interest. This paper discusses the instrument aspects along with few on-board results immediately after power ON.
KeywordsX-ray sky monitor 1D position sensitive proportional counter coded mask AstroSat X-ray transients
Scanning Sky Monitor (SSM) is one of the payloads on AstroSat (Agrawal 2006; Singh et al.2014), a multi-wavelength satellite to observe the Universe in the broad energy band spanning from optical, Near-UV (NUV), Far-UV (FUV), soft X-rays to hard X-rays. AstroSat was launched on 28th September 2015 by the Indian Space Research Organisation. There are five payloads on-board AstroSat which are Ultra Violet Imaging Telescope UVIT, Large Area X-ray Proportional Counter LAXPC, Soft X-ray Telescope SXT, Cadmium-Zinc Telluride Imager CZTI and Scanning Sky Monitor SSM.
X-ray sky is highly variable with a number of transient X-ray sources, most of which remain below detection threshold and brighten up once a while. It is necessary to keep looking for any such transient phenomenon of already known X-ray sources in addition to having a finite possibility of detecting new transient sources.
Scanning Sky Monitor (SSM) (Seetha et al.2006) on-board AstroSat is an X-ray Sky Monitor to detect and locate transient X-ray sources in the energy range 2.5 to 10 keV. With its large Field-of-View (FoV) and with a good angular resolution, SSM can detect and locate an X-ray transient source within a few arcmin in the sky. Once a transient is detected after processing the data from SSM on-ground, this information is provided to the astronomical community so that co-ordinated observations of the source can be carried out with other observatories including AstroSat. In addition to this, SSM will observe known transient X-ray sources in its subsequent scans which will be used to generate long term light curves to study behavior of the sources over a period of time.
SSM instrument is a soft X-ray wide field imager, the first of its kind in India, with every element of it made indigenously.
At present, SSM along with MAXI (Matsuoka et al.2009) observe the X-ray sky in the soft X-ray range for detecting transient X-ray sources. While SSM has a sensitivity of about 27 mCrab for 10 min integration at 3\(\sigma \) level, MAXI (Matsuoka et al.2009) has a sensitivity of 20 mCrab for 90 min integration at 5\(\sigma \) level.
2 Instrument details
SSM detectors are position sensitive detectors and details of design optimization of the detectors for SSM are discussed in Ramadevi et al. (2006). The FoV of detector-1 (SSM-1) and detector-2 (SSM-2) is larger when compared to that of the detector-3 (SSM-3) at the centre. The sensitivity of these two units is estimated to be 28 mCrab for 10 min integration. The SSM-3 detector had lost one of its anode wires during Thermo-Vac tests prior to launch and also its FoV is larger, and the sensitivity of this detector is estimated to be 27 mCrab for 10 min integration in the energy range 2.5 to 10 keV.
SSM specifications table.
25% Xe + 75% P-10
Anode active length
thick alumnized Mylar
\(\sim \)25% at 6 keV
\(\sim \)1.5 mm (FWHM) at 6 keV
Field of View
Central detector: 22.1\(^\circ \times \) 100\(^\circ \);
Edge detectors: 26.8\(^\circ \times \) 100\(^\circ \)
12’ in coding direction
2.5\(^\circ \) across
\(\sim \)11 cm\(^2\) at 2.5 keV
\(\sim \)51 cm\(^2\) at 5 keV
(10 min, 3 sigma)
\(\sim \)28 mCrab for Detector-1 and Detector-2
\(\sim \)27 mCrab for Detector-3
2.1 Detectors for SSM
The eight anode cells make the geometric area of the detector to detect X-ray photons from the source that are incident on the surface of the detector through the coded-mask on top. The wire module is placed inside the gas-filled chamber made of aluminium having an entrance window made of aluminized Mylar of thickness 50 microns through which the X-ray photons enter the detector. Ramadevi et al. (2006) describe the design optimization of detectors for SSM.
Ramadevi et al. (2015) discuss the end effects with design details of the wire modules of SSM. An X-ray photon entering the detector through the window ionizes the gas and the charge cloud produced is collected at the anode. Anode wires for SSM being resistive wires, charge division happens and the charge is collected on both sides of the anodes. The amplitudes of the pulses on either ends of the anode wires for every event incident on the detector is used to estimate the position of incidence of the photon.
2.2 Imaging system in SSM
3 SSM testing and calibration
SSM has been tested on ground for various parameters. For every photon incident on SSM detectors, we get the time of incidence, energy and position of incidence of the photon. Calibration of SSM includes both position and spectral calibration aspects. Details of spectral calibration can be found in Ramadevi and Seetha (2011) and that of position calibration can be found in Ramadevi et al. (2011). Details of on-board calibration will be discussed in a another paper.
4 Initial operations
Immediately after the launch of AstroSat, the first operation was the deployment of the rotation platform of SSM. The platform which was stowed on to the deck of the spacecraft by the Hold Down and Release Mechanism (HRDM) unit was deployed by tele-command. After the release of the platform, the platform was free to be rotated to the required angles for observations. On day 2 and day 3 (September 29 and September 30, 2015) after launch, operations related to SSM platform rotation were carried out successfully, as planned. All the modes of rotation were checked and the parameters were verified.
4.1 Spacecraft pointing axes for SSM observations
As can be seen in Fig. 2, the SSM assembly is mounted on the deck perpendicular to the boresight axis of the spacecraft about +Yaw direction. Also, the SSM assembly is capable of rotating about +Yaw axis between 5\(^\circ \) to 355\(^\circ \) clockwise and counter-clockwise in stare-and-step mode of operation, details of which are mentioned in Ramadevi et al. (2017).
For the observations with SSM detectors during the Performance Verification phase, we developed an algorithm which uses the above series of coordinate transformations (and inverse transformations) to determine the possible solutions to the inertial coordinates of the spacecraft Yaw-Roll-Pitch principle axes such that the desired source appears at the specified detector coordinate within the FoV. The algorithm also takes into account the mission constraints of the Sun vector being close to normal to the spacecraft pitch-axis, the angle between +Roll and Sun greater than the specified threshold, as well the limit on the RAM angle (angle between +Roll and velocity vector).
5 Few results from initial operations of SSM
The very first source observed with the SSM1 detector was the Crab pulsar. The satellite pointing was determined such that the Crab would appear near the centre of the FoV, almost normal incidence in one of the SSM edge detectors, SSM1. The entire orbit raw data was processed using the Richardson–Lucy deconvolution technique (Bhattacharya & Ravishankar 2002; Ravishankar & Bhattacharya 2003) and the Crab appeared at the expected location. The Crab was detected at the centre of the FoV with the expected angular resolution within a pixel of \(12' \times 2.5^\circ \). This image is produced without applying any filter on the data, which could contain events collected during ‘Earth’ in SSM’s FoV, events recorded during ingress and egress of SAA regions, etc. No other strong source is expected in this field within the detector’s FoV and all other peaks seen in the image are spurious either due to charged particles registered during SAA-entry/exit and/or due to scattered X-rays from the Earth. However, the data has to be filtered for events from the Earth which will be modelled as background and removed before image processing. Figure 14 shows the surface plot of the reconstructed full-FoV sky image, while Fig. 15 is a contour plot of the same and Fig. 16 is a section of the image near FoV-centre with the SVDFIT localization overplotted on top of the Richardson-Lucy contours.
A more detailed analysis of this data clearly showed the spectral information of the source during the beta-class variability in the form of hardness-ratio which is the ratio of the flux in the hard and soft energy bands as shown in Fig. 19.
During its initial operations in the first week of power ON, SSM recorded an increased intensity which appeared to be like a transient or a Gamma Ray Burst (GRB), but on a detailed look was figured out to be the scattered X-rays reflected off the Earth as a result of an intense solar flare. Figure 20 shows the light curve recorded in one of the SSM detectors during the solar flare as an inset plot and matched with the timings of the solar flare from GOES data plotted in the same figure.
Amongst its various observations, SSM was pointed to an X-ray pulsar 4U 0115+634 in its bursting phase and the pulse period of the pulsar was confirmed to be 3.62 s, which indicates that the neutron star is rotating every 3.62 s. Figure 21 shows the light curve of the Be X-ray pulsar as recorded in SSM. The light curve contains events from the full FoV of SSM in which the pulsar is the brightest source and hence the contribution from other very faint sources are considered negligible. Figure 22 shows the pulse-period detection of the source as 3.62 s. Pulse-periods of two other binary pulsars, Centaurus X3 and Vela X-1 have also been seen by SSM.
6 SSM deliverables
Data products from SSM are the light curves of the sources observed by SSM and also the respective hardness ratio derived from the fluxes in different energy bands within the energy range of interest of SSM. Light curves of different sources in the FoV generated will be made available to the public in the SSM website hosted at Indian Space Science Data Centre (ISSDC). The light curve in 4 different bands (2.5–10, 2.5–4, 4–6 and 6–10 keV) and the hardness ratios will be made available in the SSM website. The data will be available in fits as well as in ASCII format.
SSM in its early in-orbit operations has given interesting results. The theoretical sensitivity limits for SSM is estimated to be about 27 milliCrab for 10 min integration. However, detailed study of sensitivity limits along with X-ray background from different regions in the sky using onboard observations of SSM is being done and will be discussed in an upcoming paper. Data pipeline processing to automate data analysis and also to generate alerts from SSM are underway. Initial results from the instrument are very encouraging and we look forward to interesting discoveries and dedicated X-ray sky survey with SSM.
The SSM team would like to acknowledges Director, ISAC for the constant support for delivery of this payload followed by the successful launch of AstroSat. The team also acknowledges various entities including SMG, CSG and others at ISAC and also CMSE, VSSC, who have contributed in making of the payload and the successful completion of various processes and procedures towards delivery of the payload. The SSM team also acknowledges all the members of SAG and other groups at ISAC who have contributed to the development of this successful payload. They also thank all the members of the review committees at various stages for the support in making this payload a success.
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