Scanning sky monitor (SSM)

Technical Physics Division, ISAC

&

Astrophysics Group, RRI

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

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)

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

Proposed sky monitor1-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

Coded mask

• Cross correlation technique used for image reconstruction. Satisfactory on simulated data.

• Iterative removal of sources, to improve dynamic range

implemented.

• Coded mask casts a shadow on the detector plane

• 63 element mask 6x 11cm2.made in 6 parts

• Hadamard transform used for the design

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.

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

ssm1

ssm3

ssm2

Rot. axis

ssm1 ssm2 ssm

3

Sky Coverage

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;

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

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.

O/p voltage of CSPA

Cha

nnel

no

of A

DC

o/P

Position in mm along the wire

Tot

al o

/p(L

+R

) in

cha

nnel

no.

Position in mm along the wire

Rat

io in

cha

nnel

no.

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.

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

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

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

Critical areas

• Be foil for window material

• Calibration set up for integrated tests with the coded mask ;

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.

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