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Searching for nature's best clocks and extragalactic millisecond transients with large interferometric arrays: from the GMRT to the SKA

Final Report Summary - CLOCKS AND BURSTS (Searching for nature's best clocks and extragalactic millisecond transients with large interferometric arrays: from the GMRT to the SKA)

Neutron stars are one of the densest objects in the universe. They are born when stars which are between 8 and 20 times the mass of our sun explode when they run out of fuel. Ultra compact neutron stars are formed after a explosion called supernovae explosion of the massive stars. Some of these objects can be seen as pulsars. Pulsars are rapidly rotating strongly magnetised neutron stars that can be used as extremely precise celestial clocks and thus provide a valuable means for probing the most extreme states of matter. Studies of pulsars yield a better understanding of a variety of physics problems, from acceleration of particles in ultra strong magnetic fields (primarily via study of 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). Millisecond pulsars (MSPs) with period of very stable rotation period (change only by ~1s in 100 Giga years) are thought to have been spun up by the transfer of matter and angular momentum from a low-mass companion star during an X−ray emitting phase. This stability, a compactness second only to black holes, and the fact that they are often in binary systems, makes them ideal laboratories to test the physics of gravity and detectors for long-wavelength gravitational waves.

In addition to the regular radio emission from pulsars, fast radio bursts are observed at the locations of energetic, and possibly cataclysmic events, making them useful probes for extreme states of gravity, pressure, temperature and magnetic fields. Magnetar flares, super-giant pulses from neutron stars, and pulsar-planet systems, are some of the models for the extragalactic origin of fast radio bursts at cosmological distances.

Radio pulsars and fast radio bursts are extremely faint radio sources. Our understanding of these extreme objects is therefore hindered by their radio faintness and demands deeper observations with larger telescopes and we are reaching the limit of what is possible with fully steerable single dishes. Large arrays of many smaller telescopes are therefore the future for large radio telescopes leading ultimately to the world’s largest telescope, the Square Kilometre Array (SKA). In the design of the SKA electronics is replacing steel i.e. many small dishes and fast computers will be used to create larger telescope with more sensitivity. The Giant Metrewave Radio Telescope (GMRT) is present the largest multi-element array telescope operating in the mid frequency range (300−1450 MHz) and is currently undergoing a major upgrade leading to a significant sensitivity improvement. Pulsars are relatively rare astronomical objects in the sky and till now 2300 pulsars are discovered (~ one pulsar in 20 square degrees). Considering the extremely large field-of-view at low observing frequencies like 322 MHz (~1.8 square degrees), the GMRT can play a very significant role in carrying out sensitive surveys for pulsars and transients at mid and high Galactic latitudes (where sensitivity is not limited by the sky background). The GMRT interferometric array provides a unique opportunity for localisation of the detected pulsars and fast radio bursts. In addition, since pulsars can have millisecond rotation periods and the widths of the fast radio bursts could be in milliseconds one needs to have very fast time resolution while observing these objects. This requirement can be efficiently fulfilled with the flexible GMRT soft-ware backend.

The Marie Curie Fellowship for the project "Clocks and Bursts" allowed us to embark on a survey for pulsars and transients with the GMRT. The GMRT High Resolution Southern Sky (GHRSS) survey is an off-Galactic-plane (|b| > 5) survey with a sky coverage having declination range −40 degrees to −54 degrees at 322 MHz. The target sky is complementary to the other ongoing low-frequency surveys with other telescopes over the world, e.g. Low frequency array (LOFAR) in Netherlands and the Green Bank Telescope (GBT) in USA. With the high time (up to 30.72 μs) and frequency (up to 0.016275 MHz) resolution observing modes, we achieve a sensitivity of ~0.5 mJy for a 2 ms pulsar with 10% duty cycle at 322 degrees . The GHRSS sky coverage of 2866 square deg, will result from 1953 pointings, each covering 1.8 square deg. The 10-sigma detection limit for a 5 ms transient burst is 1.6 Jy. With 35% of the survey completed (i.e. 1000 square deg), we have already discovered 10 pulsars. This is one of the highest pulsar per square degree discovery rates for any off-Galactic plane survey. One of the pulsars that we have discovered is a millisecond pulsar. We re-detected 23 known in-beam pulsars. Utilising the imaging capability of the GMRT we also localised 4 of the GHRSS pulsars (including the MSP) in the gated image plane within ± 10 arc second. We demonstrated rapid convergence in pulsar timing with a more precise position than is possible with single dish discoveries. We also exhibited that we can localise the brightest transient sources with simultaneously obtained lower time resolution imaging data, demonstrating a technique that may have application in the SKA.The simultaneous time-domain and imaging study for localising pulsars and transients, efficient candidate investigation with machine learning (i.e neural network based binary classifier) are some of the unique features of the GHRSS survey, which are also finding application in the SKA design methodology. The Hydrus super-computing cluster of University of Manchester equipped with 20 Nvidia GPU cards, partially procured from my Marie Curie grant, and IBM cluster at the National Centre for Radio Astrophysics are acting as the major data centre and processing host. Currently two PhD students of University of Manchester are involved in pulsars and transient search analysis of the GHRSS survey. The full 3000 square degree of survey coverage will result in about 80 Tera-byte of data within next 1 year, which will likely result in discovery of ~ 20 more pulsars and ~ 4 fast radio bursts at fluence of 3 Jy ms. Even in case of non-detection of fast radio bursts , this yields a 2-sigma upper limit of 2000 events /sky/day at 322 MHz, which can put an upper-limit on the fast radio bursts spectral index and thus constrain the emission mechanisms of the fast radio bursts. Moreover, with advancement of compute power re-processing the data with wider parameter space (increased acceleration range) is likely to result in some more interesting discoveries.

Survey web page:

References for further reading:
Pulsars and Gravitational waves: Lee et al. 2012
FRB discovery : Lorimer et al. 2007, Thornton et al. 2014, Champion et al. 2015
FRB origin : Thornton et al. 2014, Kulkarni et al. 2014, Falcke et al. 2014, Cordes et al. 2015, Mottez & Zarka 2014
Machine Learning :Lyon et al. 2015
Precise localisation of Pulsars :Roy and Bhattacharyya 2013
GHRSS survey : Bhattacharyya et al. 2016