Early In-orbit Performance of Scanning Sky Monitor Onboard AstroSat

  • M. C. Ramadevi
  • B. T. Ravishankar
  • N. Sitaramamurthy
  • G. Meena
  • Brajpal Singh
  • Anand Jain
  • Reena Yadav
  • Anil Agarwal
  • V. Chandra Babu
  • Kumar
  • Ankur Kushwaha
  • S. Vaishali
  • Nirmal Kumar Iyer
  • Anuj Nandi
  • Girish V.
  • Vivek Kumar Agarwal
  • S. Seetha
  • Dipankar Bhattacharya
  • K. Balaji
  • Manoj Kumar
  • Prashanth Kulshresta
Review

Abstract

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.

Keywords

X-ray sky monitor 1D position sensitive proportional counter coded mask AstroSat X-ray transients 

1 Introduction

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.

Many sky monitors have been flown till date and a large number of transient sources have been discovered in which most of them are X-ray binary systems. In addition to earlier flown sky monitors, the All Sky Monitor (ASM) on RXTE (Levine et al.1996) has done long-term observations of the X-ray sky adding a number of new X-ray sources to the already existing source catalog. At present, the Monitor of All sky X-ray Image (MAXI) (Matsuoka et al.2009) on-board the International Space Station (ISS) is doing an all sky survey for X-ray sources.
Fig. 1

Photo of SSM flight payload (Ramadevi et al.2017).

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 consists of three nearly identical one-dimensional position sensitive gas-filled proportional counters with a coded-mask and associated electronics. The whole assembly is mounted on a platform capable of rotation. Figure 1 shows the picture of the three detectors of SSM mounted on a single platform. The platform can be rotated with the help of a rotating mechanism (Balaji et al.2005, 2008) and its respective electronics, so that the instrument can scan the sky. SSM rotates about the Yaw axis of the spacecraft as it is mounted on the +Yaw side of the spacecraft as shown in Fig. 2, while all the other instruments point to the +Roll axis of the spacecraft.
Fig. 2

Schematic of AstroSat spacecraft with SSM on the +Yaw axis.

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.

The specifications of SSM is mentioned in Table 1.
Table 1

SSM specifications table.

Parameters

Specifications

Detector

Position sensitive

 

gas-filled proportional

 

counter

Gas mixture

25% Xe + 75% P-10

Gas pressure

800 torr

Anode diameter

25 microns

Cathode diameter

75 microns

Anode active length

60 mm

Anode wires

Carbon-coated quartz

Cathode wires

Gold-coated tungsten

Cell size

12 sq-mm

Entrance window

50 microns

 

thick alumnized Mylar

Energy range

2.5–10 keV

Energy resolution

\(\sim \)25% at 6 keV

Position resolution

\(\sim \)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\(^\circ \)

Angular resolution

 12’ in coding direction

 

2.5\(^\circ \) across

Effective area

\(\sim \)11 cm\(^2\) at 2.5 keV

 

\(\sim \)51 cm\(^2\) at 5 keV

Sensitivity

 

(10 min, 3 sigma)

\(\sim \)28 mCrab for Detector-1 and Detector-2

 

\(\sim \)27 mCrab for Detector-3

2.1 Detectors for SSM

Detectors for SSM are position-sensitive proportional counters, with resistive anode wires providing position sensing along the anode wires and the anode-wire-ID across the anode wires. Each unit has eight anodes that are powered with 1500 V high voltage, while the cathodes surrounding the anodes are at zero potential. Figure 3 shows the schematic of the wire module for SSM, where it can be seen that the anode wire is surrounded by wire-walled cathode.
Fig. 3

Schematic of the wire module with anode and cathode wires forming the cells inside SSM detectors.

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

Coded mask imaging (Zand 1992) is one of the possible ways of performing wide-field imaging. Several sources are observed simultaneously through the coded mask. Therefore, the shadow cast on the detector plane is a summation of the shadows due to each of the sources in the FoV as shown in Fig. 4. There is no ’direct’ one-to-one correspondence between the sky plane and the detector plane. The superposed shadow in the detector has to be deconvolved to reconstruct the sky image. This is the principle of the coded mask imaging and is used in SSM. The coded-mask pattern for SSM is shown in Fig. 5 (Ramadevi et al.2017). Details of coded-mask imaging for SSM are discussed in Bhattacharya & Ravishankar (2002) and Ravishankar & Bhattacharya (2003).
Fig. 4

The principle of coded-mask imaging.

Fig. 5

Coded-mask for SSM (Ramadevi et al.2017).

Fig. 6

Image constructed with one source at the centre of FoV of SSM during on-ground calibration tests; surface plot.

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.

Imaging experiments were carried out on-ground on SSM detectors. Figures 6 and 7 show the surface and contour plots respectively, of one of the SSM detectors with one source in the FoV of SSM that are obtained from experiments done on-ground.
Fig. 7

Contour plot of the image shown in Fig. 6 (on-ground calibration tests).

Experiments were carried out to estimate the detection efficiency of SSM at 6 keV and were found to match the theoretically predicted value within the limits as shown in Fig. 8. The average sensitivity of SSM is estimated to be about 27 milliCrab at 3 sigma in the energy range 2.5 to 10 keV for 10 min integration time. Figure 9 shows the sensitivity of SSM instrument in the energy range of interest.
Fig. 8

Detection efficiency of SSM (theoretically calculated and experimentally measured).

Fig. 9

Estimated sensitivity of individual SSM detector.

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.

The low-Earth orbit for AstroSat at an altitude of 650 km will require the spacecraft to graze through the SAA regions as shown in Fig. 10. The Charge Particle Monitor (CPM) onboard AstroSat was powered ON since day one after launch and the SAA region entry and exit were studied. Accordingly, the pre-computed SAA models were updated for AstroSat and the payloads on-board AstroSat were powered OFF while inside SAA regions and so is the SSM instrument.
Fig. 10

SAA regions seen by AstroSat spacecraft in its orbit.

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).

The fields-of-view of the three SSM detectors when projected on the sky would overlap as shown in Fig. 11. The rotation axis is about +Yaw which is the centre of FoV of the central detector (and the common central of edge detectors traces a circle about +Yaw direction as the assembly rotates).
Fig. 11

The fields-of-view of the three SSM detectors projected onto the sky, indicated with the spacecraft principle axes. The three detectors rotate about the +Yaw axis.

In order to determine the sky coordinates of any vector within the FoV of any given SSM detector, one has to undertake a series of three transformations: (a) first from the given detector coordinates to the SSM-platform reference where the relative mounting angles of the three SSM detectors are considered, (b) from the platform reference to the body coordinates of the spacecraft, wherein the rotation due to platform angle is taken into account, and finally, (c) from the spacecraft coordinates to the inertial reference frame sky coordinates.
Fig. 12

Count rate recorded in SSM1 during the first orbit operations; X-axis is SSM time in seconds. Here the blue regions are the X-ray sky and the pink shade shows the counts recorded at the ingress of SAA region. Also about 1000 s HV ramp down and up were done as a check for tele-commands, which shows counts going to zero as the detector goes under-biased.

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).

After about two weeks of launch, the detectors on SSM were powered ON. In addition to the required positioning of the attitude of the spacecraft, the platform of SSM was rotated to a position in such a way that the Crab, standard X-ray source, was at about the centre of SSM1, one of the edge-detectors in SSM. The detectors were powered ON one after the other, with SSM1 being powered ON first. The high voltage for operating SSM detectors (1500 V) was fed in HV steps. The light curve obtained from the data of SSM1 during the first orbit is shown in Fig. 12. The combined spectra of all the sources in the FoV of all the eight anodes in SSM1 is shown in Fig. 13.
Fig. 13

Combined spectra of the sources in the FoV of all the eight anodes in SSM1 in the first orbit of initial operations.

Fig. 14

Surface plot of the SSM1 sky with Crab near FoV-centre in orbit 00210 – reconstruction undertaken using Richardson–Lucy deconvolution method on the unfiltered raw data.

Fig. 15

Contour plot of Fig. 14 SSM1 field with Crab near FoV-centre.

Fig. 16

Contour plot near FoV-centre of SSM1 with the Crab source.

Fig. 17

SSM1 Crab observations; Crab at the centre of the FoV; First Sky Observations of SSM (with filtered data) (Ramadevi et al.2017).

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.

The first orbit data after filtering it for events from SAA regions and Earth-occult regions, sky image observed by SSM with Crab at the centre of its FoV is shown in Fig. 17 (Ramadevi et al.2017).

Following this, the spacecraft was maneuvered such that SSM’s field contains the well-known enigmatic galactic black hole source, GRS 1915+105 on October 14th, 2015. The profile of the light curve of GRS 1915+05 as observed by SSM (Ramadevi et al.2017) shown in Fig. 18 indicated that the source is in its beta-class variability, which is a rare catch. The variability profile of thelight curve matches well with one of the earlier observations of the source with NASA’s RXTE satellite as shown in the inset of Fig. 18. This result was immediately published in the form of Astronomer’s Telegram (Atel #8185, Ramadevi et al.2015) informing the international astronomical community that this source GRS 1915+105 is in its beta-class variability, encouraging further follow-up observations of this source with other observatories in different wave-bands. The SSM produced an ATel with its observations just the 3rd day since it was made operational, which is remarkable.
Fig. 18

Light curve of GRS 1915+105 black hole binary system as observed by SSM (Ramadevi et al.2015).

Fig. 19

Hardness Ratio plot of GRS 1915+105 Black Hole Binary system as observed by SSM.

Fig. 20

Scattered X-rays from Earth during a solar flare observed by SSM.

Fig. 21

Light curve of 4U 0115+634 as observed by SSM.

Fig. 22

Pulse period of Be-X-ray pulsar 4U 0115+634 observed by SSM.

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.

SSM operates largely in its default mode of step and stare in which it stares for 10 min at a given FoV and steps by \(10^{\circ }\) to another FoV. It continues to monitor the sky with its platform rotation continuously ON in this mode. In this way, SSM has scanned a large part of the sky as shown in Fig. 23, which shows the sky coverage by SSM till Dec 2015. The points marked are the centre of the FoV of each SSM detector. Each point would mean about a 100 \(\times \) 22 sq-deg in the sky. The yellow gives the position of the Sun for the duration of October to December.
Fig. 23

SSM sky coverage till Dec. 2015 by SSM, since operational. The points marked are the centre of the FoV of each SSM detector.

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.

7 Conclusion

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.

Notes

Acknowledgements

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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Copyright information

© Indian Academy of Sciences 2017

Authors and Affiliations

  • M. C. Ramadevi
    • 1
  • B. T. Ravishankar
    • 1
  • N. Sitaramamurthy
    • 1
  • G. Meena
    • 1
  • Brajpal Singh
    • 1
  • Anand Jain
    • 1
  • Reena Yadav
    • 1
  • Anil Agarwal
    • 1
  • V. Chandra Babu
    • 1
  • Kumar
    • 1
  • Ankur Kushwaha
    • 1
  • S. Vaishali
    • 1
  • Nirmal Kumar Iyer
    • 1
  • Anuj Nandi
    • 1
  • Girish V.
    • 1
  • Vivek Kumar Agarwal
    • 1
  • S. Seetha
    • 2
  • Dipankar Bhattacharya
    • 3
  • K. Balaji
    • 1
  • Manoj Kumar
    • 1
  • Prashanth Kulshresta
    • 1
  1. 1.Space Astronomy GroupISRO Satellite CentreBangaloreIndia
  2. 2.ISRO HeadquartersBangaloreIndia
  3. 3.Inter University Centre for Astronomy and AstrophysicsPuneIndia

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