LAXPC Instrument onboard AstroSat looks at various X-ray sources

The Large Area X-ray Proportional Counter (LAXPC) instrument became fully operational on October 19, 2015 for the first time in orbit. The detector calibration (purification of detector gas, gain adjustment and energy resolution) and the results of first light observations of Crab nebula, CAS-A, 4U 0115+63 (X-ray pulsar,) few black hole X-ray binaries like GRS 1915+105, Cyg X-1, and Cyg X-3 are present here. These results are based on preliminary analysis and detail analysis is under way. The cross scan across Crab suggests that all three LAXPC detectors are aligned within 8 arc minute.

Figure 1 :  All the three LAXPC flight detectors in Assembly, Integration and Testing lab (AIT) lab of ISAC after successfully completing all flight tests and final calibration (October 20, 2014). The purification pump is seen as black box in each detector. At the top is the protection cover which was removed before the launch.

AstroSat is India's first dedicated astronomy space mission which was launched on September 28, 2015. The Large Area X-ray Proportional Counter (LAXPC) is one of the major payloads on AstroSat with total eight flight packages. The System Time Based Generator (STBG), Processing Electronics and low voltage detector electronics were switched ON during  September 29, to  October 1, 2015.  The LAXPC payload was fully functional on October 19, 2015 when high voltage (HV) of all three LAXPC detectors was switched ON. The purification pump was operated during October 20-22, 2015 and it has responded well. Background is very stable and all the planned X-ray sources are detected quickly and have provided very high quality data. LAXPC payload consists of three co-aligned identical LAXPC detectors each with a multi-wire multi-layer configuration and a Field of View   0.8o X 0.8o. It provides large collecting area for X-ray timing and low-resolution spectral studies over a broad energy band of 3-80keV. Each detector has its own independent front-end electronics, HV supply, gas purification system and signal processing electronics. The Time mark generating electronic unit  (STBG) is common for all the three LAXPCs. The data from all the LAXPCs are independently acquired preserving the identity of each unit.

1. Detector operation and detection of first source Crab Nebula:

High voltage (HV) of LAXPC detectors were switched ON  in steps at 10 am (IST) on October 19, 2015, while satellite was pointing to blank sky. The background was very stable. After the switching ON sequence, the satellite was pointed to Crab Nebula; a bright X-ray source in the sky often used as an X-ray standard source for instrument calibration, on the same day. Crab pulsar is at distance of about 2 kpc in the Perseus Arm of our galaxy and spin rate of the pulsar is 30.2 times per second. The source was detected within seconds as it appears in live telemetry data. The Crab flux on October 19, 2015 (Figure 2 left panel)) was initially stable for few hundred seconds and then starts dropping. This source is very stable and not expected to show such decline. Various possible explanations like any detector issue, partly Earth occultation and stability of satellite platform were looked into. It was observed that Crab on   October 22, 2015 (right panel of Figure) which shows clear linear decline. It was discussed with mission control that satellite platform is not stable.



Figure 2 :  Light curve of Crab Nebula with  LAXPC detector  taken on  October 19, 2015 ; first day of observation (left panel) and on  October 22, 2015 (right panel).

Indeed satellite was drifting with 0.250 per hour. This has happened as Moon came in the Field of View (FOV) of one of the star sensors. Thereafter, it was corrected and Crab flux was stable. This data is taken before purification and gain adjustment. Power spectrum calculated shows pulse peak at  30.2Hz  and its harmonics (many harmonics are visible) which are shown in Figure 3.

Crab spectrum for LAXPC-30 is presented in figure 4 along with the background spectrum. Detailed analysis is underway.

  Figure 4: Crab spectrum for LAXPC-30 (blue) and the background spectrum (Red). Sharp drop at 600 channel (around 80keV) is due to upper threshold cutoff.

2.  First time Purification of LAXPC detectors in orbit:

Great care was taken by keeping collimator evacuated of all the three detectors with the top cover for most of the time, when detectors are not being tested (top covers are visible in Figure 1). However LAXPC-10 detector gas was last purified in November 2013 and was likely to absorb impurities. Also all the LAXPC detector covers had to be opened at SHAR prior to the launch. The purification pumps were operated during October 20-22, 2015 and the data after purification was analysed at each stage.  The purification of detector gas of LAXPC-10, LAXPC-20 and LAXPC-30 were carried out for total 2.30 hrs, 3.30 hrs and 1.00 hr respectively. The energy resolution for LAXPC-10 was improved from 21% to about 14 %.  LAXPC-20 and LAXPC-30 detectors have achieved energy resolution around 10-12% for onboard calibration source (embedded in the detector) data in A8 anode (at 30keV and 60keV). In the main anodes, resolution will degrade by 2-3 %.

3. Detection of GRS 1915+105; comparison with NASA’s RXTE/PCA:

It is a black hole X-ray binary in our galaxy at a distance of about 11 kpc with 10-18 solar mass black hole at its center. This X-ray source has been most interesting source in our galaxy as it shows fourteen or more different X-ray classes as well as transient radio jets more often than any other X-ray source in our galaxy. GRS 1915+105 was observed for hardly 20 minutes with LAXPC instrument on October 24, 2015 to test its capabilities. During this observation, GRS 1915+105 was in delta X-ray class; a sub class of the high soft state, where thermal emission dominates. Figure 5 shows the light curve of GRS 1915+104 observed with the LAXPC on   October 24, 2015 (bottom panel) while top panel of figure 5 shows this source in the same X-ray class observed by NASA's  RXTE/PCA on September 28,  2011 (for 40 minutes; double of LAXPC time).

Figure 5:  GRS 1915+105 in  the delta X-ray class  observed  by LAXPC on October 24, 2015 for about 20 minutes (bottom panel) and by NASA's RXTE/PCA on  September 28, 2011 for about 40 minutes (top panel).

  Figure 6:  Energy spectra of two observations shown (top panel) while power spectra are shown in bottom panel.

   Figure 6 shows the energy spectra (top) and time spectra (bottom) for both data sets plotted in figure 5. The counts drop fast above 10keV (top panel) while power is negligible above 5Hz (bottom panel). Both these observations are consistent with delta class (a high soft X-ray state where thermal emission dominates). All the data is from single LAXPC detector which clearly establishes its data quality.  

4.  4U 0115+63;  a pulsar with cyclotron line caught in outburst:

 4U 0115+63 is a pulsar with cyclotron line feature which shows outbursts in 3-6 years.  LAXPC observed this source during outburst on October 24, 2015. It provided great opportunity to study cyclotron line features.


Figure 7:  4U 0115+63 caught in outburst on October 24, 2015 by three LAXPC detectors. Identical light curve is seen by each LAXPC detector over few orbits.

The observation continued for over four orbits and light curves are shown in Figure 7.  The pulse profile  is shown in Figure 8 . Further analysis is under way.

Figure 8:  The pulse profile of 4U 0115+63 (calculated from single LAXPC data.

5. Detection of CasA; a supernova remnant to study LAXPC detector resolution at 6.4keV:

CasA is at a distance of 3.4 kpc in our galaxy. NASA’s RXTE/PCA used this as iron line source to calibrate detector at 6.4keV in orbit. LAXPC detector observed this source on October 24, 2015 and light curve is shown in Figure 9. After fitting a continuum with a blackbody and power law, the intensity of Iron line was estimated and shown in the bottom penal of Figure 9. It clearly shows an energy resolution of about 20% at 6.4keV.


Figure 9: Light curve of CasA observed with LAXPC on October 24, 2015 (top panel). Bottom panel shows plot of Iron line after subtracting a blackbody and power law fit. Iron line clearly shows about 20% energy resolution at 6.4keV.

6. Detection of Cyg X-1; a black hole X-ray binary:

Cyg X-1 is a high mass X-ray binary with about 14.8 solar mass black hole at the center.  LAXPC detector observed Cyg X-1 on 23rd   and 25th October 2015 and the light curve is shown in Figure 10 along with background. It shows large variability.



Figure10: LAXPC observed Cyg X-1 on 23rd and 25thOctober, 2015 along with background.

7. Detection of Cyg X-3; a black hole source with frequent radio jets:

It is a persistent X-ray binary and also a micro quasar as it shows frequent transient radio jets. It was observed by LAXPC instrument on October 23, 2015 and light curve for all three LAXPC detectors is shown in Figure 11.



Figure 11: Light curve of Cyg X-3 for all the three LAXPC detectors observed on October 23, 2015.

8. Alignment of LAXPC detectors and Cross scan:

It is important to know how the LAXPC detectors are aligned to each other as well as how much is the offset for individual detectors when satellite is pointing to a source. It is important for calculating the exact effective area of individual detectors. Scans along RA and DEC was tried on October 22, 2015 which were however not possible. Then eight grid points in a circle of 0.30 was tired which took almost seven hour of observation but did not achieve much. Finally, cross scans on October 25, 2015 during 2.23 to 14.44UT (total about 13 hours) was tried, which provided useful data of about 1600 sec. Duty cycle is so low as most of the time goes to manage SAA region, earth occultation and following conditions put forward by different payloads.

Figure 12: Top panel shows plot of RA (red), DEC (blue) and net angle (black). Bottom panel shows the count rate observed (black), count rate corrected for dead time (red) and count rate expected from simulation of FOV and corrected for dead time (blue).

Scan was done with a speed of 0.010 per second from +3 RA to -3 RA. It was repeated for  +3 Dec to -3 Dec. This data has been analysed and results are presented in Figure 12 for RA scan. Similar results are obtained for DEC scan which are not shown here.  The RA and DEC scans were repeated for all three LAXPC detectors. This has provided pointing accuracy for Crab:

                        RA (deg)         DEC (deg)      offset (deg)     offset from average (deg)

LAXPC10       83.406             21.903             0.252               0.069

LAXPC20       83.538             21.810             0.225               0.093

LAXPC30       83.458             21.896             0.211               0.027

Average           83.458             21.870             0.220

Crab coordinates are   RA= 83.6330 and   DEC= 22.0140. All the LAXPC detectors are aligned well;  the spread  from each other in RA within 8 arc min and it is in  DEC within 6 arc min. These will be further refined. 

LAXPC instrument is designed and developed indigenously at Tata Institute of Fundamental Research (TIFR), Mumbai. ISRO Satellite Centre (ISAC), Bengaluru, Raman Research Institute (RRI), Bengaluru, ISRO Inertial Systems Unit (IISU), Thiruvananthapuram and Space Application Centre (SAC), Ahmedabad were involved at various stages of its development and have made important contributions. A number of industries in Mumbai and Bengaluru were also participated in the fabrication of the payload.