Absolute Time Calibration of LAXPC aboard AstroSat
JJ. Astrophys. Astr. (0000) :
Absolute Time Calibration of LAXPC aboard AstroSat
Avishek Basu , Dipankar Bhattacharya and Bhal Chandra Joshi Jodrell Bank Centre for Astrophysics, University of Manchester, Oxford Road, Manchester, M13 9PL, UK Inter University Centre for Astronomy and Astrophysics, Post Bag 4, Pune 411007, India National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Ganeshkhind, Pune 411007,India * Corresponding author. E-mail: [email protected]
Abstract.
The AstroSat mission carries several high-energy detectors meant for fast timing studies of cosmicsources. In order to carry out high precision multi-wavelength timing studies, it is essential to calibrate the absolutetime stamps of these instruments to the best possible accuracy. We present here the absolute time calibration ofthe AstroSat LAXPC instrument, utilising the broad-band electromagnetic emission from the Crab Pulsar to crosscalibrate against Fermi-LAT and ground based radio observatories Giant Metrewave Radio Telescope (GMRT) andthe Ooty Radio Telescope (ORT). Using the techniques of pulsar timing, we determine the fixed timing o ff sets ofLAXPC with respect to these di ff erent instruments and also compare the o ff sets with those of another AstroSatinstrument, CZTI. Keywords.
Pulsars—Instrumentation—Multi-wavelength astronomy
1. Introduction
Multiwavelength observations are key to unravellingthe physical processes ongoing in a variety of astro-physical sources. Such observations commonly involvemultiple instruments situated at di ff erent locations. Thetarget sources being studied often have time variableintensity and spectrum, so to characterise their prop-erties it is essential to synchronise the intrinsic timestamps at various observatories being used. Even ifthe reference time information is obtained from a com-mon source like the Global Positioning System, therecould be internal delays in data processing electronicswhich need to be measured to achieve the necessarysynchronisation. In this paper we report the result ofour attempt to calibrate the timing o ff set of the AstroSatLAXPC instrument with respect to ground based In-dian radio observatories, namely the Ooty Radio Tele-scope (ORT) and the Giant Metrewave Radio Telescope(GMRT), the Large Area Telescope (LAT) instrumentaboard the Fermi gamma-ray observatory. Results of asimilar timing o ff set calibration experiment for anotherAstroSat instrument, the Cadmium Zinc Telluride Im-ager (CZTI), have been reported in Basu et al . (2018),and are also included here for comparison.This work is motivated by our ongoing e ff ort tocharacterise the multi-wavelength properties of theGiant Radio Pulses (GRP) emitted by radio pulsarsfrom time to time. In order to phase align thesepulses between radio and X-ray bands an accurate alignment of the time stamps across the instrumentsis required. We therefore begin by measuring thetiming o ff sets between the instruments involved in ourexperiment, the results of which we report here. Weaim to obtain the o ff sets precise enough to track everypulse in the radio and X-ray bands unambiguously.The GRP arrives randomly within the ∼ et al ., 1970; Manchester et al ., 2005). Our desirableuncertainty on the o ff set measurement is one-tenth ofthe pulse width, i.e. less than 300 µ s.Our aim can be achieved by timing and monitoringpulsars over a long span of time. Since our experimentinvolves telescopes operational at low-frequency radiowavelengths to γ − rays, we have chosen the Crabpulsar (PSR J0534 + γ -rays. At L-band (around 1.4 GHz) the Crabpulsar has two distinct components in its profile: themain-pulse (MP) and a relatively weaker inter-pulse(IP). The components are highly aligned across thespectrum except for some intrinsic emission delay. TheMP at high energies leads the radio main pulse by 241 ± µ s ( >
30 MeV ; Kuiper et al ., 2003), 344 ± µ s(2-30 keV ; Rots et al ., 2004), (280 ± µ s) (Kuiper et al ., 2003), 235 ± µ s (10-600 keV; Terada et al .,2008) and 275 ± µ s (20-100 keV; Molkov et al .,2009). It is essential to account for these intrinsic © Indian Academy of Sciences 1 a r X i v : . [ a s t r o - ph . I M ] F e b J. Astrophys. Astr. (0000) : delays while computing the time of arrival (TOA) ofthe pulses (discussed in Section 3.5).The results obtained from our experiment presentedin this paper would allow us to time-align the Radio andX-ray time series data, enabling us to search for X-rayphoton count enhancements coincident with the GRP.In Section 2. we discuss the instruments used for theexperiment, in Section 3. we discuss the methodologyadopted to measure the timing o ff sets and finally con-clude the paper by presenting the results in Section 4.
2. Instruments and Observations
We performed multi-epoch, multi-frequency observa-tions using Indian facilities like the Giant MetrewaveRadio Telescope (GMRT), the Ooty Radio Telescope(ORT) operational at radio wavelength, and two pay-loads aboard AstroSat, the Cadmium Zinc Telluride Im-ager (CZTI), and the Large Area Proportional Counter(LAXPC). We have also made use of the publicly avail-able data from
Fermi-
LAT operational at high-energy γ -rays ( ∼
20 MeV −
300 GeV).2.1
AstroSat
AstroSat, India’s first space based observatory waslaunched in October 2015 with five payloads on board(Singh et al ., 2014). These are the Cadmium ZincTelluride Imager (
CZTI ; Bhalerao et al ., 2017), theLarge Area X-ray Proportional Counter (
LAXPC ; Ya-dav et al ., 2016), the Soft X-ray Telescope (
SXT ;Singh et al ., 2016), the Ultra Violet Imaging Telescope(
UVIT ; Hutchings, 2014) and the Scanning Sky Mon-itor (
SSM ) allowing observations covering a wide fre-quency range from 1300 Å to 380 keV.2.1.1
LAXPC
The LAXPC is the prime X-ray de-tector on AstroSat, operating in the energy range 3–80 keV. LAXPC consists of three proportional counterunits filled with primarily Xenon gas at a pressure ofabout 2 atmosphere, presenting a combined e ff ectivearea of ∼ at energies below 20 keV, declin-ing to ∼ at 80 keV (Antia et al ., 2017). Thislarge e ff ective area, along with high timing resolution(10 µ s) makes this an excellent instrument for X-raytiming studies, including those of pulsars. A collima-tor restricts the Field of View (FOV) of LAXPC to ap-proximately 1 ◦ × ◦ . LAXPC detectors record eventmode data, with each event tagged with an instrumenttime stamp derived from a System Time Base Gener-ator (STBG) driven by a temperature controlled crys-tal oscillator. Once every 16 seconds, a synchronisingpulse is sent to all AstroSat instruments including the on-board Spacecraft Positioning System (SPS), whichprovides time in UTC based on the Global PositioningSystem. All the instruments record their current timestamp at the arrival of the synchronising pulse and thesevalues are collected in a Time Correlation Table whichis used in o ffl ine analysis to convert, by interpolation,the instrument time stamps assigned to the recordedevents to UTC time stamps (Bhattacharya, 2017).2.1.2 CZTI
The Cadmium Zinc Telluride Imager ex-tends the high-energy coverage of AstroSat to ∼
380 keV, starting from ∼
20 keV. It consists of a solidstate, pixellated CZT detector array of total geometricarea ∼
976 cm , with a collimator and a Coded Aper-ture Mask situated above it. The Coded Mask and thecollimator provide a 4 . ◦ × . ◦ imaging Field of Viewat energies below ∼
100 keV and gradually becometransparent at higher energies. The CZTI records pho-ton events time stamped at 20 µ s resolution by its in-ternal clock. These time stamps are converted to UTCtime stamps during o ffl ine analysis in the same manneras for the LAXPC instrument. The CZTI has carriedout extensive studies of the Crab pulsar at high ener-gies, including that of its polarization (Vadawale et al .,2018).2.1.3 Orbit determination
Comparing time stampsacross observatories requires the arrival times to be re-ferred to a common reference system, for which weadopt the Solar System Barycentre. The event timestamps recorded by AstroSat are referred to the cor-responding Barycentric arrival times, using the knowl-edge of the orbit of the satellite. The orbital positionand velocity of AstroSat are measured by an on-board10-channel Spacecraft Positioning System (SPS) unitthat operates on signals received from the Global Posi-tioning System (GPS) satellites. These measurementsare regularly calibrated against those obtained by rang-ing from the AstroSat ground station. The housekeep-ing data stream of AstroSat provides the orbital positionvalues sampled every 128 milliseconds, with an accu-racy of better than 5 metres. The error budget in thebarycentric correction arising from the uncertainties inthe orbital position is thus limited to less than 0.017 µ s.We have ignored this contribution in the reported un-certainties in our final results, which are much larger.2.2 Fermi-LAT
The
Fermi-LAT is a high-energy γ − ray telescope sensi-tive to the photons with the energy from below 20 MeVto more than 300 GeV (Atwood et al ., 2009a). It mon-itors γ − ray pulsars with a cadence of one-sixth of itsduty-cycle. Individual photon events are recorded witha time resolution better than 1 µ s (Smith et al ., 2008). . Astrophys. Astr. (0000) : We use data of the Crab pulsar retrieved from the publicarchive of the Fermi mission.2.3 The Giant Metrewave Radio Telescope (GMRT)
The GMRT (Swarup et al ., 1991) is an “Y”-shaped in-terferometer with thirty, 45-m steerable dishes opera-tional at low-frequency radio-wavelengths. Fourteenantennas are arranged in a compact array within a ra-dius of 1 km, the remaining antennas are arranged inthree arms. The observations were carried out by com-bining all 14 antennas and the first arm antennas in atied array with an overall gain of 3.5K / Jy. The Crabpulsar was observed using the GMRT at seven di ff er-ent epochs (shown with green markers in Figure 2).The typical observation duration was 1-2 hours. Thetime-series raw voltage data acquired at 1390 MHz with16 MHz bandwidth from every antenna were FourierTransformed to obtain 256 channels voltage data. Theinstrumental phase lags among the antennas were de-termined by observing the point source 3C147, whichwere then compensated in the Fourier domain andadded coherently. Further analysis was done o ffl ine de-scribed in Section 3.2.4 The Ooty Radio Telescope (ORT)
The ORT is a 30 m wide o ff set parabolic cylindricalantenna in the east-west direction. It is 530 m longin the north-south direction sensitive to a single polar-isation and operational at 334.5 MHz (Swarup et al .,1971). The gain of the telescope is 3.3 K / Jy and thesystem temperature is 150 K. The pulsar observationsback-end at ORT is called as PONDER (Naidu et al .,2015), which starts recording the data on arrival of therising edge of the minute pulse obtained from the GPSsystem. PONDER performs coherent de-dispersionin real-time and produce time-stamped folded pulse-profiles in ASCII format. PSR J0534 + et al ., 2018) andthe high cadence pulsar glitch monitoring program atthe ORT (Basu et al ., 2019). In this paper we have usedthe data from September 01, 2015 (MJD 57226) to Jan-uary 14, 2017 (MJD 57767)
3. Methodology
The method for the absolute time calibration relies onthe technique of pulsar timing. The technique of pulsartiming (Edwards et al ., 2006) compares the observedTOAs with the predicted TOAs obtained from a simple https: // fermi.gsfc.nasa.gov / cgibin / ssc / LAT / LATDataQuery.cgi rotation model of the pulsars. We perform the analysisin multiple steps in an iterative manner until the bestsolution is achieved.3.1
ORT-Analysis
As mentioned earlier in Section 2.4, coherently de-dispersed time-stamped profiles are obtained from theORT. The TOA of a pulse was computed using the soft-ware package PSRCHIVE (Hotan et al ., 2004) from ev-ery profile by cross-correlating with a noise-free tem-plate of the pulse profile in the frequency domain de-scribed in Taylor (1992). The typical timing accuracy at ORT is 318 µ s. The noise-free template was cre-ated in PSRCHIVE by fitting an optimal number ofGaussian waveform to a high S / N observed pulse pro-file. The TOAs obtained from our high cadence ob-servations were then used to create a phase connectedsolution. Such a high cadence is especially importantfor the Crab pulsar to obtain a reliable phase connectedsolution because of the strong timing noise. The tim-ing noise is observed as systematic wandering in theTOA residuals after removal of the standard spin-downmodel (Cordes & Helfand, 1980). The TOAs were fur-ther segmented with a time span of a month to pro-duce the monthly ephemeris from our data by fittingthe spin-down model in the high precision pulsar timingpackage TEMPO2 (Hobbs et al ., 2006). The monthlyephemeris with precise rotation parameters was used tore-fold the time-series data to obtain the precise TOAs.The Crab nebula provides a strong scattering screento the radio waves emitted from the pulsar. The ef-fect of scatter broadening is pronounced at 334.5 MHzand can contribute to the timing residuals as a system-atic. Hence, in case of the Crab pulsar, it is di ffi cultto de-couple the e ff ect of timing noise from the scatterbroadening, which in our analysis has been taken careby using the publicly available data from Fermi -LAT.3.2
Fermi-LAT Analysis
The γ -ray pulse profiles are free from the propaga-tion e ff ects, therefore the Fermi -LAT (Atwood et al .,2009b) archival data were used to model the timingnoise which is a frequency-independent phenomenon.We use all the events in the energy range 0.1 to 300GeV within a radius of 3 ◦ around PSR J0534 + We refer the median of the TOA errors as the “typical timing ac-curacy” http: // psrchive.sourceforge.net / https: // bitbucket.org / psrsoft / tempo2 / src / master / https: // fermi.gsfc.nasa.gov / cgibin / ssc / LAT / LATDataQuery.cgi https: // fermi.gsfc.nasa.gov / ssc / data / analysis / scitools / overview.html J. Astrophys. Astr. (0000) : into smaller event data each of 7 days duration. Thesetime stamps were referred to the solar system barycen-tre (SSB) adopting JPL planetary ephemeris DE200and folded using the Fermi-plugin (Ray et al ., 2011)of TEMPO2 using the ephemeris obtained from theORT timing solutions. The standard template was con-structed in a similar manner as explained in Section 3.1However, to account for the intrinsic delay between theradio and the γ − ray pulse profile the templates werealigned with an appropriate shift mentioned in the Sec-tion 1. The TOAs were computed from every pulseprofile by cross-correlating the standard template. Thetiming accuracy was 309 µ s. The timing analysis wasperformed using TEMPO2. The timing noise at thisband was modelled with the combination of eight sinewaves to obtain the white timing residuals using theFITWAVES tool in TEMPO2.3.3 Re-analysis of the ORT data
The timing solution with modelled timing noise fromthe
Fermi -LAT TOAs was applied on the ORT TOAs.At this stage, the TOAs a ff ected from the scatter broad-ening were removed from our analysis and monthlyephemeris were re-generated. Hence, the rotation pa-rameters were obtained which are free from timingnoise and the scatter broadening e ff ects. We refer tothis timing solution as the “iteration-2” solution. Theiteration-2 solutions were used to re-fold the ORT time-series data and produce TOAs following similar stepsas explained in Section 3.1.3.4 GMRT Analysis
The raw voltage data obtained from the GMRT werealso coherently de-dispersed o ffl ine with our pipelinediscussed in Naidu et al . (2015). The values of thedispersion measure (DM) were taken from the JodrellBank monthly ephemeris (Lyne et al ., 1993) nearestto the epoch of observations. The de-dispersed time-series were further folded using the iteration-2 monthlyephemeris obtained from the ORT data. The GMRT of-fline analysis also supplies with de-dispersed 64 chan-nel sub-banded data with 32 sub-integrations. The stan-dard template was constructed from the highest S / N ob-served profile following the same method explained inSection 3.1. The template of the pulse profile from theGMRT data was aligned with the ORT templates andthen the TOAs were computed following the methoddiscussed in Section 3.1. The timing accuracy obtainedat GMRT is 162 µ s. The dynamic pulsar wind withinnebular filaments leads to the variation in the electrondensity along the line of sight, which results in time http: // / pulsar / crab.html variation of the DM. The typical variation in DM is ofthe order of 0.01 pc cm − , which incorporates a changein time of arrival by 21 µ s and 370 µ s at 1390 MHzand 334.5 MHz respectively. Therefore it is essentialto correct for DM variations to obtain reliable and pre-cise estimates of the o ff sets. The TOAs computed fromthe ORT data in Section 3.3 and from the GMRT datawere used to measure the DM at di ff erent epochs. Thefixed o ff set between the data acquisition pipelines at theORT and GMRT were known from previous measure-ments (Surnis et al ., 2018). Hence, the DM was esti-mated after accounting for this delay between GMRTand ORT using the JUMP parameter in TEMPO2. Itmay be noted that only 8 observations were nearly si-multaneous between ORT and GMRT. Therefore, 8 dif-ferent estimates of DM at 8 di ff erent epochs were ob-tained (Figure 4 of Basu et al ., 2018). The DM was fur-ther used to perform the o ffl ine coherent de-dispersionto obtain the de-dispersed time series data, which werefolded using the iteration-2 timing solutions. The TOAsfrom the GMRT data were finally produced by follow-ing the methods described above.3.5 AstroSat-CZTI and LAXPC Analysis
The CZTI has four detectors arranged in four quadrants.There are no relative o ff sets between individual quad-rants. Hence data from all the four quadrants were com-bined and the time tags of the photons were convertedto SSB adopting the JPL planetary ephemeris DE200and using the satellite position in the code as1bary .The barycentre recorded events were folded to con-struct the pulse profile using iteration-2 timing solutionobtained in Section 3.3 using our own codes. Further,the standard template was created following the samemethod as described in Section 3.1. In case of LAXPC,the event files were created by combining the data fromthree consecutive orbits, which were then barycentredusing the as1bary and folded using the iteration-2 tim-ing solution obtained in the Section 3.3. The standardtemplates were created following a similar method asmentioned above. The templates obtained from theCZTI and the LAXPC were appropriately shifted withrespect to those obtained from the GMRT, ORT and Fermi-
LAT to take into account the intrinsic energy-dependent emission delays. Finally, the TOAs werecomputed for the CZTI profiles and the LAXPC pro-files using the templates thus constructed. This methodof incorporating the known energy-dependent intrinsicemission delays in the template construction allows usto find the true clock o ff sets between two instrumentsdirectly from the TOA di ff erences between them. http: // astrosat-ssc.iucaa.in / ?q = data and analysis . Astrophys. Astr. (0000) :
01 ORT0 . . N o r m a li ze d I n t e n s i t y / C o un t s LAXPC01 CZTI0 . . . . . . Figure 1 . The multi-wavelength pulse profiles of the Crabpulsar. The pulse profiles were aligned after correcting theclock-o ff sets between the telescopes. O ff set Measurements The TOAs from all the telescopes were collated to-gether to compute the o ff sets between the di ff erent tele-scopes. These TOAs were analysed using a timingmodel obtained by merging the pulsar rotation model,which gave the phase connected solution, with a con-strained DMMODEL in TEMPO2. The DMMODELis obtained from the DM time series, fitted from simul-taneous observations at the ORT and the GMRT usingthe procedure described in Section 3.4. Inclusion ofDMMODEL terms in the timing model accounts for theDM o ff sets from the chosen reference DM. The tim-ing residuals obtained by applying this timing modelto TOAs from all the telescopes have been shown in theupper panel of Figure 2. The systematic trend as a func-tion of time in these residuals for all telescopes is due tothe timing noise of the pulsar. The residuals representthe di ff erence between the predicted and the observedTOAs. As the TOA from each telescope additionallyconsists of a clock o ff set which is fixed with respect tothe observation epoch, the residuals of a pair of tele-scopes are seen as parallel tracks in the diagram. Thus,fitting a constant di ff erence to residuals of a pair of tele-scope in Figure 2 determines the timing o ff sets betweenthe telescopes. The timing model, described above, wasmerged with the timing noise model obtained in Section3.2. This reference timing solution after accounting forthe timing noise and the DM variation is given in Ta-ble 1. Finally, we use the JUMP feature of TEMPO2 to measure the o ff sets between the telescopes discussed inSection 4.Pulsar parameter ValueRAJ (hh:mm:ss) 05:34:31.973DECJ (dd:mm:ss) + − ) − − − ) 1.1905(3) E − − ) 56.7957PMRA (mas / year) − / year) 2WAVE OM (year − ) 0.0054325986245627Solar system planetaryephemeris DE200WAVEEPOCH (MJD) 57311.000000136START (MJD) 57278FINISH (MJD) 58026 Table 1 . Table presents the reference timing solution of theCrab pulsar after considering the e ff ect of DM variation andthe timing noise.
4. Results
TEMPO2 allows one to measure the o ff sets betweendi ff erent telescopes using the JUMP feature. Utilis-ing this, we estimate the o ff set between the GMRT andCZTI to be − ± µ s and that between GMRTand LAXPC to be − ± µ s. The measured o ff setbetween the GMRT and ORT is − ± µ s, be-tween GMRT and Fermi-
LAT is − ± µ s. Theseclock o ff sets have been further tabulated in Table 4. Thephase aligned pulse profile after accounting for the o ff -sets has been presented in the Figure 1. In the bottompanel of Figure 2 we present the timing residuals ob-tained after removing the timing o ff sets between them.The trend-free residuals imply that all the pulsar param-eters and clock o ff sets have been properly modelled.The clock o ff set between the LAXPC and the CZTI in-struments aboard AstroSat is found to be 969 ± µ s.The uncertainties in the o ff sets are obtained from thoseof the parameters fitted to the TOA using the JUMPfunction. The results presented here meet the desiredaccuracy (see Section 1.) for a multi-wavelength in-vestigation of the GRP from the Crab pulsar with theinstruments used in this paper. J. Astrophys. Astr. (0000) : − P o s t - fi t R e s i du a l ( µ s ) F ermi -LATGMRTORTAstroSat-CZTIAstrosat-LAXPC − P o s t - fi t R e s i du a l ( µ s ) Figure 2 . The timing residuals from di ff erent telescopes. Upper panel:
The green, grey, red, black and yellow data pointsrepresent the TOAs obtained from GMRT, ORT,
Fermi -LAT, LAXPC and CZTI observations, respectively. The systematicpattern is called as timing noise and the parallel tracks indicate the timing o ff set incorporated by di ff erent telescopes backend as explained in the text. Lower Panel:
The TOA residuals from all the observations after modelling the timing noise, thee ff ect of dispersion measure and clock o ff sets (detailed description is given in the text). . Astrophys. Astr. (0000) : Instrument Clock-o ff sets in µ sAstroSat-CZTI -4716 ± ± ± ± Table 2 . The table summarises the clock o ff sets of di ff erenttelescopes given in the first column with respect to theGMRT. Acknowledgements
We thank the anonymous referees for their valuablesuggestions to improve the presentation of the paper.This publication made use of data from the Indian as-tronomy mission AstroSat, archived at the Indian SpaceScience Data Centre (ISSDC). The LAXPC data wereprocessed at the Payload Operations Centre at TIFRMumbai. The CZT Imager instrument was built bya TIFR-led consortium of institutes across India, in-cluding VSSC, ISAC, IUCAA, SAC, and PRL. TheIndian Space Research Organisation funded, managedand facilitated the project. We thank the sta ff of theOoty Radio Telescope and the Giant Metrewave Ra-dio Telescope for taking observations over such a largenumber of epochs. Both these telescopes are operatedby National Centre for Radio Astrophysics of Tata In-stitute of Fundamental Research. PONDER backend,used in this work, was built with TIFR XII plan grants12P0714 and 12P0716. BCJ acknowledges support forthis work from DST-SERB grant EMR / / References
Antia, H. M., Yadav, J. S., Agrawal, P. C., et al . 2017,Astrophysical Journal Supplement Series, 231, 10Atwood, W. B., Abdo, A. A., Ackermann, M., et al .2009a, ApJ, 697, 1071—. 2009b, ApJ, 697, 1071Basu, A., Joshi, B. C., Krishnakumar, M. A., et al .2019, Monthly Notices of the Royal AstronomicalSociety, 491, 3182Basu, A., Joshi, B. C., Bhattacharya, D., et al . 2018,Astronomy & Astrophysics, 617, A22 Bhalerao, V., Bhattacharya, D., Vibhute, A., et al . 2017,Journal of Astrophysics and Astronomy, 38, 31Bhattacharya, D. 2017, Journal of Astrophysics and As-tronomy, 38, 51Cordes, J. M., & Helfand, D. J. 1980, AstrophysicalJournal, 239, 640Edwards, R. T., Hobbs, G. B., & Manchester, R. N.2006, Monthly Notices of the Royal AstronomicalSociety, 372, 1549Heiles, C., Campbell, D., & Rankin, J. 1970, Nature,226, 529Hobbs, G., Edwards, R., & Manchester, R. 2006,Monthly Notices of the Royal Astronomical Society,369, 655Hotan, A. W., van Straten, W., & Manchester, R. N.2004, Publications of the Astronomical Society ofAustralia, 21, 302Hutchings, J. 2014, Astrophysics and Space Science,354, 143Krishnakumar, M. A., Joshi, B. C., & Manoharan,P. K. 2018, in Pulsar Astrophysics: the Next FiftyYears, Proceedings of the International Astronomi-cal Union, IAU Symposium, Volume 337, 364–365Kuiper, L., Hermsen, W., Walter, R., & Foschini, L.2003, Astronomy & Astrophysics, 411, L31Lyne, A., Pritchard, R., & Graham Smith, F. 1993,Monthly Notices of the Royal Astronomical Society,265, 1003Manchester, R. N., Hobbs, G. B., Teoh, A., & Hobbs,M. 2005, Astronomical Journal, 129, 1993Molkov, S., Jourdain, E., & Roques, J. P. 2009, Astro-physical Journal, 708, 403Naidu, A., Joshi, B. C., Manoharan, P. K., & Krish-nakumar, M. A. 2015, Experimental Astronomy, 39,319Ray, P. S., Kerr, M., Parent, D., et al . 2011, Astrophys-ical Journal Supplement Series, 194, 17Rots, A. H., Jahoda, K., & Lyne, A. G. 2004, Astro-physical Journal Letters, 605, L129Singh, K. P., Tandon, S., Agrawal, P., et al . 2014,in SPIE Astronomical Telescopes + Instrumenta-tion, International Society for Optics and Photonics,91441S–91441S
J. Astrophys. Astr. (0000) :
Singh, K. P., Stewart, G. C., Chandra, S., et al . 2016, inSociety of Photo-Optical Instrumentation Engineers(SPIE) Conference Series, Vol. 9905Smith, D. A., Guillemot, L., Camilo, F., et al . 2008,A&A, 492, 923Surnis, M. P., Joshi, B. C., McLaughlin, M. A., et al .2018, Astrophysical Journal, 870, 8Swarup, G., Ananthakrishnan, S., Kapahi, V., et al .1991, Current science, 60, 95Swarup, G., Sarma, N., Joshi, M., et al . 1971, NaturePhysical Science, 230, 185Taylor, J. H. 1992, Philosophical Transactions: Physi-cal Sciences and Engineering, 117Terada, Y., Enoto, T., Miyawaki, R., et al . 2008, Pub-lications of the Astronomical Society of Japan, 60,S25Vadawale, S. V., Chattopadhyay, T., Mithun, N. P. S., et al . 2018, Nature Astronomy, 2, 50Yadav, J., Agrawal, P., Antia, H., et al . 2016, in SPIEAstronomical Telescopes ++