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Precision Pulsar Timing with the ORT and the GMRT and Its Applications in Pulsar Astrophysics

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Precision Pulsar Timing with the ORT and the GMRT and Its Applications in Pulsar Astrophysics J. Astrophys. Astr. (2018) 39:51 © Indian Academy of Sciences https://doi.org/10.1007/s12036-018-9549-y Review Precision pulsar timing with the ORT and the GMRT and its applications in pulsar astrophysics BHAL CHANDRA JOSHI1,∗ , PRAKASH ARUMUGASAMY1 , MANJARI BAGCHI3,12 , DEBADES BANDYOPADHYAY4, AVISHEK BASU1 , NEELAM DHANDA BATRA5,6 , SURYARAO BETHAPUDI7 , ARPITA CHOUDHARY3 , KISHALAY DE8 ,L.DEY2, A. GOPAKUMAR2, Y. GUPTA1 , M. A. KRISHNAKUMAR1,9 , YOGESH MAAN10 , P. K. MANOHARAN1,9 , ARUN NAIDU11, RANA NANDI14 , DHRUV PATHAK3,12 , MAYURESH SURNIS13,15 and ABHIMANYU SUSOBHANAN2 1National Centre for Radio Astrophysics (Tata Institute of Fundamental Research), Post Bag No 3, Ganeshkhind, Pune 411 007, India. 2Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Dr. Homi Bhabha Road, Mumbai 400 005, India. 3The Institute of Mathematical Sciences, C. I. T. Campus, Taramani, Chennai 600 113, India. 4Astroparticle Physics and Cosmology Division, Saha Institute of Nuclear Physics, HBNI, 1/AF Bidhannagar, Kolkata 700 064, India. 5Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India. 6Department of Physics, Birla Institute of Technology and Science Pilani, Hyderabad Campus, Shameerpet Mandal, Hyderabad 500 078, India. 7Department of Physics, Indian Institute of Technology Hyderabad, Kandi, Hyderabad 502 285, India. 8Cahill Centre for Astrophysics, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA. 9Radio Astronomy Centre (NCRA-TIFR), Ooty, India. 10ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands. 11Mcgill Space Institute, McGill University, Montreal, Canada. 12Homi Bhabha National Institute Training School Complex, Anushakti Nagar, Mumbai 400 094, India. 13Department of Physics and Astronomy, West Virginia University, P. O. Box 6315, Morgantown, WV, USA. 14Department of Nuclear and Atomic Physics, Tata Institute of Fundamental Research, Mumbai 400 005, India. 15Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, WV, USA. ∗Corresponding author. E-mail: [email protected] MS received 12 July 2018; accepted 16 August 2018; published online 29 August 2018 Abstract. Radio pulsars show remarkable clock-like stability, which make them useful astronomy tools in experiments to test equation of state of neutron stars and detecting gravitational waves using pulsar timing techniques. A brief review of relevant astrophysical experiments is provided in this paper highlighting the current state-of-the-art of these experiments. A program to monitor frequently glitching pulsars with Indian radio telescopes using high cadence observations is presented, with illustrations of glitches detected in this program, including the largest ever glitch in PSR B0531+21. An Indian initiative to discover sub-μHz gravitational waves, called Indian Pulsar Timing Array (InPTA), is also described briefly, where time-of-arrival uncertainties and post-fit residuals of the order of μs are already achievable, comparable to other international pulsar timing array experiments. While timing the glitches and their recoveries are likely to provide constraints on the structure of neutron stars, InPTA will provide upper limits on sub-μHz gravitational waves apart from auxiliary pulsar science. Future directions for these experiments are outlined. Keywords. Equation of state—gravitational waves—pulsars: general—stars: neutron. 51 Page 2 of 10 J. Astrophys. Astr. (2018) 39:51 1. Introduction of the paper is as follows. The pulsar timing technique is briefly described in section 2. Current constraints on Highly magnetized rotating neutron stars, discovered neutron star structure and EoS are discussed in sec- first as radio pulsars emitting a train of narrow periodic tion 3 followed by a description of the glitch monitoring pulses (Hewish et al. 1968), provide excellent celes- program using the ORT and the uGMRT and its pre- tial clocks, primarily due to their massive and compact liminary results. A brief review of PTA experiments is nature (mass ∼ 1.4M;radius∼10km). A stability presented in section 4 followed by details and current of their periods up to one part in 1020, allows mea- status of Indian PTA experiment called Indian Pulsar surements in astrophysical experiments with precision, Timing Array (InPTA). unprecedented in astronomy, for constraining Equation of State (EoS) of these stars and detecting sub-μHz gravitational waves (GW). 2. Pulsar timing About 2600 pulsars have been discovered so far1 (Manchester et al. 2005), which are broadly classified Pulsar timing involves comparison of the prediction of as normal pulsars, young pulsars and millisecond pul- pulse time-of-arrival (ToA) from an assumed rotational sars, based on their rotation period and magnetic field model of star with the observed ToA to refine the model strength. The latter two of these classes are relevant parameters. The key point of this technique is keeping for this paper. Young pulsars with high magnetic dipo- track of rotation cycles of pulsar, which improves pre- lar surface field (1012 < B < 1014 G) and/or short cision as a function of time. As discussed below, the periods (P ∼ 100 ms) show rotational irregularities, assumed model is usually complex involving several such as abrupt spin-ups, also called glitches (Radhakr- parameters, all of which are estimated in this process. ishnan & Manchester 1969; Lyne et al. 2000; Krawczyk As pulsars are weak sources, ToAs are measured from et al. 2003; Espinoza et al. 2011; Yu et al. 2013), as an average pulse, obtained after averaging the pulsed well as slow wander in rotation rate, known as tim- time series over several thousand pulses. A fiducial ing noise (Boynton et al. 1972; Cordes 1980; Cordes point on the pulse is chosen, which usually comes with & Helfand 1980). Measurements of these rotational a random shift from a noise-free template in a given irregularities are useful for characterizing the internal observation. Template matching is used to find this shift structure of neutron star and constraining its EoS (Link and adjust time of observations (Taylor 1992)togetthe et al. 1999, 1992; Haskell & Melatos 2015; Ho et al. ToA of the pulse up to a precision of few nano-seconds 2015). On the other hand, older pulsars with relatively using an atomic clock, usually a hydrogen maser syn- lower magnetic fields (108 < B < 1011 G) and mil- chronized to an international time scale, called Temps lisecond periods (P ∼ 1.5–30 ms) exhibit a much Atomic Internationale. smaller rotational slow-down and highly stable rota- These observed ToAs are first referred to Solar tion rates. An ensemble of these millisecond pulsars System Barycentre (SSB – an inertial frame) and then (MSPs) is useful as a celestial detector to measure small onwards to an inertial frame for the pulsar as shown metric perturbation caused by GW passing near the below: Earth (Foster & Backer 1990; Joshi 2013). High pre- cision observations of such ensembles are carried out 2 tp = ttopo + tclock − D/f + R + S + E, (1) by the Pulsar Timing Arrays (PTA), which are experi- μ ments for detection of sub- Hz GW (Manchester et al. where ttopo are observed topocentric ToAs, tclock are 2013; Demorest et al. 2013). High-sensitivity observa- clock corrections, D is the dispersion constant account- tions using large collecting area radio telescopes, such ing for dispersive delay in the inter-stellar medium and as the Ooty Radio Telescope (ORT: Swarup et al. 1971) R, S and E are the Roemer, Shapiro and Ein- and the upgraded Giant Meterwave Radio Telescope stein delays respectively (see Stairs (2003) for details (GMRT, uGMRT: Swarup et al. 1991; Gupta et al. 2017) on these delays). Additionally, the timing model may are analysed with pulsar timing technique to obtain high also include dynamics of the star itself and that of its precision measurements for such experiments. companion if the pulsar happens to be in a binary sys- In this paper, a brief review of state-of-the-art in this tem. Finally, the timing model also assumes a rotational field is presented followed by a description of our efforts model of the neutron star given by in this direction using the ORT and the GMRT. The plan 1 2 ν(t) = ν0 +˙ν(t − t0) + ν(¨ t − t0) , (2) 1http://www.atnf.csiro.au/people/pulsar/psrcat/. 2 J. Astrophys. Astr. (2018) 39:51 Page 3 of 10 51 MJD Timing Noise in PSR B0531+21 56660 56680 56700 56720 56740 56760 0.004 20 Fermi-LAT ( 0.1-300 GeV) GMRT (1390 MHz) 0 ORT (334.5 MHz) ASTROSAT (20-150 KeV) -0.004 10 -0.008 2e-08 0 0 Residuals (sec) -10 0 -2e-14 -20 57400 57500 57600 57700 -4e-14 56660 56680 56700 56720 56740 56760 MJD MJD Figure 1. Timing noise seen in the rotation rate of Crab pul- Figure 2. A small glitch detected in PSR B0740−28 at the sar (PSR B0531+21) from radio- to high-energies. The data ORT on MJD 56727. The top panel shows the pre- and post- are from the ORT, the legacy GMRT and the ASTROSAT. glitch timing residuals as a function of MJD. The middle We also used archival data obtained by Fermi telescope. The panel shows pre- and post-glitch spin frequency (ν) and the timing noise, which is a slow wander of rotation rate, is seen bottom panel shows the frequency derivative (ν˙). The pulsar as systematic deviation from zero residuals. The timing resid- was observed at 334.5MHz. uals for the four telescopes are offset from each other by the amount of relative offset in the data acquisition pipeline our high cadence monitoring of this pulsar with the ORT and the GMRT. Another rotational irregularity is pulsar glitch, seen as an abrupt increase in rotation rate of the where ν, ν , ν˙ and ν¨ are the rotational frequency and 0 star, illustrated in Fig. 2 for PSR B0740−28. Also rele- its higher order derivatives assumed at an epoch t .The 0 vant to this paper is systematics in the timing residuals timing model is used to predict the pulse number N, of an ensemble of pulsars due to correlated unmodeled N = ν(t − t0).
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