Large Area X-ray Proportional Counter

Department of Astronomy & Astrophysics, TIFR

Launched on September 28, 2015

The LAXPC instrument subsystems; the system based time generator (STBG), processing electronics and low voltage detector electronics were switched on during 29 September- 1st October, 2015. The LAXPC payload was fully functional on 19 th October, 2015 when high voltage (HV) of all three LAXPC detectors was switched on. Onboard LAXPC detector purification was operated during 20-22 October, 2015 and 23-24 November, 2015. Energy resolution of LAXPC10 was degraded to around 18 % at 30 keV and after gas purification in orbit, energy resolution improved to around 14% (figure 1).

LAXPC in-Orbit Purification

Figure 1: LAXPC 10 detector energy resolution before and after purification of detector gas(upper and lower panels respectively) obtained from on- -board Am source located in a veto anode.

LAXPC observation of CAS A is shown in Figure 2 which suggest around 20% energy resolution at 6.4 keV Iron line. LAXPC observation of GX 301-2 also clearly resolved Iron line. A number of standard sources were observed to understand and fine tune the detector characteristics. Since ASTROSAT is in an equatorial orbit at an altitude of about 650 km, the charged particle flux is generally low, except during passage through the South Atlantic Anomaly (SAA) where the high voltage in the detector is switched off. To model the detector background, blank sky regions were observed over a period of 1 day on several occasions. The background counts vary by 10%-20% in orbit, with counts increasing close to SAA region. To model the background we assume that the count rate is a function of latitude and longitude. The resulting model matches the observed counts to within 4%. The average background counts are about 250 s-1 for LAXPC10 and 200 s-1 for other detectors. The higher count rate in LAXPC10 is because one of the veto anodes in this detector is not functioning. The observed background spectrum is close to what was obtained by GEANT4 simulations using diffuse cosmic X-ray background.

DT10 spectra

Figure 2: CAS-A (Supernova remnant) observation of LAXPC30. We have achieved ~20% energy Resolution at 6.4 keV.

A scan across the Crab X-ray source was used to estimate the field of view of the detector which is determined by the collimator geometry. The FWHM of the detector is found to about 55' which is close to what was estimated by GEANT4 simulations using collimator geometry. The observed spectrum of the Crab X-ray source was used to calibrate the response matrix. The energy to channel mapping was fine tuned to match the Crab spectrum and the results for LAXPC10 are shown in Figure 3.

DT10 spectra

Figure 3: Fit to observed spectrum of Crab X-ray source for LAXPC10.

The black line in upper panel shows the observed spectrum after subtracting the background and the red line shows the fitted spectrum using the response matrix for the detector assuming a power law. The lower panel shows the ratio. The fitted spectrum matches the observed spectrum to within 2% over most of the energy range of 3-80 keV. It gives power law index 2.08 with normalization 7.9 and reduced chi-square ~ 1.5 over 3-80 keV energy range. The response matrix for all three detectors is obtained by similar process. The final normalization to calculate the effective area needs to be fixed by cross-calibration with other instruments. Preliminary results of these calibration appear to suggest a total effective area of about 6000 cm 2 when the three detectors are combined. Detailed study of cross calibration data is in progress.

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