2. Why study electromagnetic counterparts?  Unraveling the astrophysical context of the source.  lifting degeneracies associated with the inferred binary parameters  reducing the signal-to-noise ratio for a confident GW detection  Redshift measurement: energy scale + an independent measurement of the Hubble constant or other cosmological parameters

3. Two kinds of searchesTwo kinds of searches  EM-triggered: Look for gravitational waves from EM candidates of gravitational waves  Its not known if they are time coincident.  Searching from archive required 10 4 -10 6 times more GW templates.  GW-triggered: Once GW discovered, look for EM counterpart  Can restrain searches within z~0.05-0.1.  But large area of uncertainty

4. 4 Cardinal Virtues (Metzger and Berger 2011) What is the most promising EM counterpart of a compact object binary merger? 4 Cardinal Virtues (Metzger and Berger 2011)  detectability- detectable in reasonable time  high fraction- accompany a high fraction of GW events  identifiability- distinguishable from other astrophysical events  positional accuracy- allow for arcsecond position determinations.

5. Electromagnetic counterparts of GW  Coalescence of NS-NS or NS-BH.  Potential EM counterparts of these mergers.  Gives rise to short GRBs (SGRBs).  FRBs may arise from NS-NS merger (Totani 2013).  Since FRBs tell exact time of mergers a correlated search will improve GW sensitivity

6. Merger scenarioMerger scenario

7. Challenges  Nearest SGRB is at z=0.12, LIGO sensitivity 0.05-0.1.  Rare cases.  SGRBs are beamed events, so we are missing out many, jet break seen in 5 out of ~80 SGRBs.  Prediction of an isotrpic kilonova (excess IR emission)!

8. GRB 130603B- a kilonovaGRB 130603B- a kilonova Tanvir et al. 2013 Berger et al. 2013

9. Why radio?Why radio?  Relaxed time constraints as long term afterglow at radio frequencies.  Quietness of the radio transient sky- less false postive (Frail et al. 2012).  Radio nonrelativistic- quasi spherical geometry  Precursor as well as afterglow!

10. Radio counterparts of GWRadio counterparts of GW  Radio bursts preceding final stages of binary neutron star mergers (Pshirkov & Postnov 2010).  Observable coherent radio burst emitted from a magnetically driven relativistic plasma outflow prior to NS merger.  Detection of such bursts appear to be advantageous in the low frequency radio band due to a time delay of ten to several hundred seconds required for radio signal to propagate in the ionized intergalactic medium.  This delay makes it possible to use short gamma-ray burst alerts to promptly monitor specific regions on the sky by low-frequency radio facilities.

11.  Observable non-thermal radio emission from beamed relativistic ejecta of SHBs.  Incoherent radio signatures from blastwave produced by quasi spherical subrelativistic ejecta in the ISM, for a year. Radio counterparts of GWRadio counterparts of GW

12. Giant Metrewave Radio Telescope (GMRT)

13.  The Giant Metrewave Radio Telescope (GMRT) is the largest low frequency radio telescope built and operated by the National Centre for Radio Astrophysics (NCRA) of TIFR.  Located 80 km from Pune near Khodad.  An array of 30 dishes, each of 45 m diameter, spread over a area of 25 sq-km.  Works on Interferrometric principle

15. Giant Metrewave Radio Telescope (GMRT)  Largest collecting area, and hence enhanced sensitivity at low frequencies.  The receivers at each antenna operate in five frequency bands near 1000-1400, 610, 325, 235, and 150 MHz, with a maximum instantaneous bandwidth of 32 MHz.  A novel mechanical design ‘SMART’ concept ensures low wind load, weight, and cost for the GMRT antennas.  Interferrometric mode as well as array mode.

16. Detectability of radio signalsDetectability of radio signals  Interaction of mildly relativistic outflow with the surrounding medium- radio flares peak at 1.4 GHz at sub-mJy level for weeks for z~0.1 (Nakar and Piran 2011).  Detectable flares upto 150 MHz bands by slower subrelativistic outflows for years, z~0.1 (1.4 GHz for smaller distances).

17. Upgraded GMRTUpgraded GMRT  Seamless frequency coverage from 50 to 1500 MHz.  Better sensitivity receivers.  Ten times larger instantaneous bandwidth (400 MHz).  Major improvement in infrastructure

18. Upgraded GMRTUpgraded GMRT

19. Detectability of radio signalsDetectability of radio signals  Interaction of mildly relativistic outflow with the surrounding medium- radio flares peak at 1.4 GHz at sub-mJy level for weeks for z~0.1 (Nakar and Piran 2011).  Detectable flares upto 150 MHz bands by slower subrelativistic outflows for years, z~0.1 (1.4 GHz for smaller distances).

20. GMRT parametersGMRT parameters JVLA 1GHz 0.45 sq-deg LOFAR 25 sq-deg

21. Challenges  Very large error box, ~100 square degrees.  Radio SHBs are rare (only 3 known): GRB 051221A, GRB 050724A, GRB 130603B nearest z=0.12.  Majority (off-axis) detectable in radio if E j,50 n ⅞ (v/c) 2.75 >0.2, existing SHBs do not satisfy (Mertzger and Berger 2011).  Long exposures needed.  Radio faintness, at low frequency reaches source confusion limit, host galaxy flux, faintness etc.

22. Strategies  Reduced error box, and hence more chances of detection  An all-sky gamma-ray satellite is essential for temporal coincidence detections, and for GW searches of gamma- ray-triggered events (Mertzger and Berger 2011)  Radio searches should focus on the relativistic case.  Target the 400 L>0.1L* galaxies in z<0.1 region?

23. Conclusions  Observing compact objects binary merger in both GW and radio is challenging  Localizations only few tens of sq-degrees.  Radio detections with current facilities quite difficult.  Gamma-ray and/or hard X-ray observations critical for establishing a firm connection between SGRBs and NS–NS/NS– BH mergers. Detections are likely to be limited to a rate <1/yr.

24. Thanks

25.  For on axis jet θ obs < θ jet, the SHB is detectable with usual energetics  For modest off axis θ obs 0.002.  For off axis jet, isotropic kilonova, optical magnitude 19-22, detectable with LSST. Various scenarios (Metzger and Berger 2011)

26. Ooty Radio Telescope (ORT)  A 530 m long and 30 m wide parabolic cylinder in Ooty.  It operates at a frequency of 326.5 MHz with a maximum bandwidth of about 16 MHz  Good use of India's proximity to the geographical equator -- the long axis of the cylinder is aligned in the north-south direction along a hill which has a natural slope of about 11 0, the geographical latitude of Ooty, thus making the long axis of rotation of the telescope parallel to the earth's rotation axis.  This makes it possible to track celestial objects for about 10 hours continuously from rise to set by simply rotating the antenna mechanically along its long axis.

27. Ooty Radio Telescope (ORT)