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Bhaswati Bhattacharyya, Jayanta Roy, Yashwant Gupta, Bhal Chandra Joshi

Pulsars have tremendous untapped potential to probe the behaviour of matter, energy, space and time under extraordinarily diverse conditions. Even though pulsars are frequently getting discovered with ongoing surveys at major telescopes over the world, the presently known pulsar population is only 1\% of that predicted by stellar synthesis models. Modeling by Faucher-Giguere et al. (2006) indicates that the Galaxy contains about 100,000 pulsars, while only a few thousand are known today! This implies that the vast majority of pulsars is waiting to be discovered.

Studies of pulsars yield a better understanding of a variety of physics problems, from acceleration of particles in ultra strong magnetic fields (primarily via the study of the emission properties of normal pulsars, having spin period > 30 ms) to probes of ultra-dense matter (mostly via studying the timing properties of millisecond pulsars, having spin period <30 ms, that are very stable rotators). The fractional rotational stability of millisecond pulsars (MSPs), one part in a million billion, is comparable to that of atomic clocks (Lorimer et al. 2004). Such rotational stability, compactness second only to black holes, and their presence in binary systems, make MSPs ideal laboratories to test the physics of gravity and as detectors for long-wavelength gravitational waves. Moreover, MSP evolutionary processes can be tracked through individual interesting discoveries of MSPs in special evolutionary phases (e.g. Roberts et al. 2011, Archibald et al. 2009; Roy et al. 2015). In addition to the regular radio emission from pulsars, millisecond transient bursts, called Fast Radio Bursts (FRBs; Lorimer et al. 2007, Thornton et al. 2013) are observed at the locations of dynamic events, making them useful probes for extreme matter states.

The known pulsar population is increasing with the surveys running at major single dish and array telescopes around the world: e.g. the High Time Resolution Universe (HTRU) survey at Parkes (Keith et al. 2010), the HTRU-North survey at Effelsberg (Barr et al. 2011), the SUrvey for Pulsars and Extragalactic Radio Bursts (SUPERB) survey at Parkes, the Pulsar survey Arecibo L-band Feed Array (PALFA, Cordes et al. 2006), the Green Bank Telescope drift scan survey (Boyles et al. 2013), the Green Bank Northern Celestial Cap (GBNCC) survey (Stovall et al. 2014), the Arecibo all-sky 327 MHz drift pulsar survey (AO327, Deneva et al. (2013)), the LOFAR Pulsar Pilot Survey (LPPS) survey (Coenen et al. 2014), the LOFAR Tied-Array All-Sky Survey (LOTAAS), and the Fermi-targetted MSP surveys (Ray et al. 2012). Because of the generally steep spectral nature of pulsars, lower frequencies are an obvious choice for searching for fainter pulsars away from the Galactic plane, where the search sensitivity is not severely affected by sky temperature and increased scattering. Such surveys away from the Galactic plane will detect relatively older pulsars.

The high sensitivity of the GMRT at low frequencies (<~ 800 MHz) makes it an outstanding telescope for pulsar surveys; such surveys are hence an important research area at NCRA. With the aid of reduced quantised noise and high time resolution supported by the flexible GMRT Software Backend (GSB; Roy et al. 2010), the GMRT search sensitivity for MSPs improved significantly, resulting in the discovery of 8 MSPs from Fermi-directed searches (Bhattacharyya et al. 2013, Roy et al. 2015).

The GMRT High Resolution Southern Sky survey (GHRSS)

The outstanding GMRT potential for low-frequency pulsar surveys has been recently emphasized by the GMRT High Resolution Southern Sky (GHRSS) survey, a low-frequency survey for pulsars and transients away from the Milky Way's plane. The GHRSS survey covers Galactic latitudes |b|>5 degrees, scanning the southern sky, with declination -40 degrees to -54 degrees. This declination coverage is complementary to the coverage of other ongoing low-frequency sky surveys around the world. The first phase of the GHRSS survey was carried out using the narrow bandwidths of the GMRT Software Backend, at 322 MHz, and has already resulted in the discovery of 13 new pulsars (Bhattacharyya et al. 2016). The second phase, using the GMRT Wideband Backend and the 250-500 MHz receivers of the upgraded GMRT is now under way.