Scanning sky monitor (SSM)

1. Scanning sky monitor (SSM) Technical Physics Division, ISAC & Astrophysics Group, RRI

2. Role of a monitor • To detect, locate and monitor x-ray transients. - all x-ray sources are variable. factor of 100 increase in x-ray flux in few days is termed as a transient. - Nearly half of known x-ray binaries are transients • Monitor known bright sources - sampling time few minutes - several samples/day; monitor for many months. • To alert other instruments for detailed studies

3. Types of transients • Hard x-ray transients - Be binaries with very long orbital period • Soft x-ray transients or x-ray novae - episodic x-ray outbursts -spectra similar to LMXBs - no fixed period of recurrence (1year to 50 years)

4. Scientific objectives • Provides unique opportunity to study these objects over a large dynamic range. L - 1033 erg/s to 10 38 erg/s; dM/dt • Study of mass transfer in accretion discs and the processes causing instabilities. • Study of the compact object • Precursors; low level variabilities • optical counterparts- parameters of the system

5. Proposed sky monitor 1-D coded mask position-sensitive detector very similar to ASM on RXTE. • Proven design; minor modifications to improve performance - Detectors: proportional counters with resistive anodes; Ratio of signals on either ends of anode gives position. • Energy range 2-10 keV • Position resolution ~ 0.5 mm. • Field of view ~ 6° X 90° (FWHM) • Sensitivity ~10 mCrab (1 day integration) • Best time resolution 1ms

7. Coded mask • Coded mask casts a shadow on the detector plane • 63 element mask 6x 11cm2.made in 6 parts • Hadamard transform used for the design • Cross correlation technique used for image reconstruction. Satisfactory on simulated data. • Iterative removal of sources, to improve dynamic range implemented.

9. Scanning arrangement • Most other experiments on ASTROSAT are pointed to a specific object for relatively long periods of time (~hours to days). • Scanning mechanism necessary for monitors to scan the sky multiple times per day. • FOV of two monitors forms an ‘X’ in the sky. Third detector views the perpendicular direction. • Monitors to be mounted on a boom which can have scanning capability in discrete steps.

10. Scanning arrangement • Rotation of spacecraft not adopted because • SXT and UVIT images have to be deconvolved • Monitors to be mounted on a boom which can have scanning capability in discrete steps. • Integration time can be varied for studying transient (nominal 5 minutes) • Deconvolution of the image simpler • Onboard flagging of new transient possible

11. ssm2 ssm1 ssm3

12. Sky Coverage ssm2 ssm1 ssm3 Rot. axis

13. Status of Engg. Model (prev. meet) Detector (ISAC) • Fabricated and assembled • testing done on all wires with 16 preamps connected • position resolution 0.38mm Coded mask (RRI) Design and fabrication completed; basic software for image deconvolution completed;

15. Current status • Front end logic was tested; this includes: • Logic to veto simultaneous signals from any two wires, or which cross ULD; • Conversion of amplitude of each pulse to digital form and tag it with the respective wire ID, left/right identification; • Threshold levels for all the wires; • logic to take o/ps on either ends from same wire only; • O/p of the front end logic fed to PC to test the ADC o/p

16. Current status (contd) • Pulser readings with the ADC o/p checked. Found to be linear • The detector o/p fed to the front end logic; The o/ps measured for 2 wires; • Processing electronics: • FPGA based electronics designed and simulated using VHDL • Software simulation of different modes of operation tested.

17. Channel no of ADC o/P O/p voltage of CSPA

18. Total o/p(L+R) in channel no. Position in mm along the wire

19. Ratio in channel no. Position in mm along the wire

20. Plans for the next 3 months • Detector • The overall noise level to be estimated • Appropriate adjustments in the threshold to be made to cover the 2-10 keV range • Test of all wires to be done; characteristic curves to be established for each wire; • Thermovac test for 1 week and post thermovac monitoring.

21. Plans for the next 3 months (contd) • Processing electronics: • Design of hardware of FPGA electronics to start. • Boom • Requirements generated • Discussions regarding boom realisation undertaken • Length of boom • Fractional loss

22. Further Plans (3-6 months) • Opening the detector for checking problem on one wire • Mounting the coded mask on the detector • Tests of the detector with coded mask • Vibration test • Finalisation of design of all onboard electronics

23. R&D issues • Alternate gas mixtures; to improve detection efficiency; • Alternate wire testing under progress • Maintenance of purity of counter gas without onboard purification; coating options

24. Critical areas • Be foil for window material • Calibration set up for integrated tests with the coded mask ;

25. Summary • Overall test set up for the detector is in place. This can also be used to compare different types of wires. • The counter has been stable over the last year. The slope of the ratio graph in Feb 2001 at CSPA o/p was = 0.13; and the same in Jan 2002 at o/p of logic is 0.132 + 0.001 • The tests have to be completed with coded mask • The computer simulation of the deconvolution of images has also been completed.

26. Mission details • Payload Weight - 48 kg (excluding boom arrangement) • Onboard memory - 3X12 Mbyte • Power- 18W • Attitude - pointing : better than 3´ , preferred 1´ • Knowledge better than 1´ (at the end of boom, inclusive of tilt/resolver errors). • Proposed accuracy of position sensing ~ 5-10´ depending on intensity of the transient • Automation of ground software for preprocessing of the data