Radio continuum research using the GMRT: A compact overview

1. Radio continuum research using the GMRT: A compact overview South Africa – India Ground Based Astronomy Workshop Capetown (6th , 7th August 2012) Gopal Krishna National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Pune (India)

2. Selection of Topics: To highlight the Radio Continuum work using GMRT • (Reported in over 200 publications) • Extragalactic: Clusters of Galaxies Radio Galaxies Normal Galaxies Wide-Area Multi-band Fields (Deep Radio Imaging) Extragalactic Supernovae Gamma-Ray Burst Afterglows • Galactic: Exoplanets Centre of our Galaxy Supernova remnants Novae Active Stellar Binaries – Micro-Quasars

3. Science with GMRT Highlights of Extragalactic Radio-Continuum Research

4. Clusters of Galaxies Detection of radio emission filling ghost cavities in the hot intra-cluster gas (ICM) • X-ray cavities seen in the ICM of many clusters are believed to be arise from the episodic injections of mechanical power by AGN jets. This energy input is believed to dominate the cluster evolution itself. Such energy input can be vastly underestimated if we are unable to detect the true extent of the radio emission filling such cavities. • Here, sensitive imaging at low frequencies can play a vital role, because the radiation at lower frequencies takes much longer to decay. An example is seen in Abell 262 (Clarke et al. 2009)

5. A new class of radio halo detected in a cluster of galaxies • Such haloes have extremely steep radio spectra and are undetected above 1 GHz. • The first good example has been found using GMRT images of the cluster Abell 521 (z = 0.247). • Even below 1 GHz, its α = -2.1 (much steeper than the typical α of -1.2 to -1.3) (Brunetti et al. 2008, Nature, 455, 944)

6. Discovery of the most powerful radio halo • MACS J0717.5+3745 (z = 0.555), P (1 GHz) ~ 5 x 1025 W/Hz, size 1.2 Mpc • A 1.2 Mpc radio halo is seen coincident with the hotter part of the ICM. • Spectral Index is -1.24 +/- 0.05 • Also seen is a radio relic between two merging sub-structures. (van Weeren et al. 2009)

7. GMRT Radio Halo Survey • About 40 X-ray detected clusters (z < 0.4) imaged at 610MHz (Venturi et al. 2007; 2008) • Discovery of bi-modality of cluster radio halos • Giant radio haloes occur in dynamically disturbed (merging) clusters (Brunetti et al 2007; Cassano et al. 2010) • Evidence for relativistic particle acceleration by cluster • merger shocks (Bagchi et al, 2011) • A 4.4 Mpc giant double radio arc detected in an exceptionally hot merging cluster showing Sunyaev-Zeldovich effect. (Brunetti et al 2007) (Bagchi et al, 2011)

8. Clearest evidence for spectral ageing of the particles accelerated in cluster merger shocks • If indeed, relativistic electrons are accelerated at merger shocks,then a moving shock should leave behind a trail of aged synchrotron plasma. This has been confirmed by the discovery of spectral gradient across a few radio relics. The clearest example is the 2 Mpc long, extremely narrow radio relic at the outskirt of the cluster J2242.8+5301 (z=0.19), which has been imaged with GMRT/WSRT/VLA. (van Weeren et al. 2010, Science 330, 347)

9. Spectral gradient of the famous radio halo in Abell 2256 • If the kinetic energy of merging clusters is responsible for accelerating the relativistic electrons creating radio haloes, then the geometry of cluster merger must leave an imprint in the form of spectral index gradient across the halo. • Here, imaging at metre-wavelength can play a pivotal role, provided high resolution can be achieved without sacrificing sensitivity to diffuse radio emission. (Kale & Dwarakanath 2010)

10. Radio Galaxies • Several interesting results on spectral index mapping (and ageing analysis) of unusual types of radio galaxies: • Double-double radio galaxies and X-shaped radio galaxies. Discovery of a single-lobed radio galaxy with the largest jet. 440 kpc long radio jet which is initially two-sided But, then why only one radio lobe has formed ?? (Bagchi, Gopal-Krishna, Krause & Joshi, 2007)

11. Normal Galaxies • First maps of “radio-to-farIR” ratio across 4 spiral galaxies. (Basu, Roy & Mitra, 2012) “Radio-to-farIR” correlation has different slopes for spiral arms and inter-arm regions. The flatter slope in the inter-arm region is probably caused by propogation of low energy (~1 GeV) long-lived (~108 yr) cosmic-ray electrons from the spiral arms.

12. Green Peas: A new class of galaxies(Cardamone et al, 2009) • First direct radio detection of “green peas” with GMRT. (Chakraborti et al, 2012) Two green peas galaxies were detected at 617 MHz (~1 mJy with 5σ) • Setting synchrotron loss time ≈ diffusion time scale, leads to B ~ 36 μG. Comparable to equipartition magnetic field. (B ~ 50 μG) • But much higher than the value for the Milky Way. (B ~ 5 μG) • General Properties of Green Peas Galaxies : • Small size (~ 3 kpc). • Irregular appearance in HST snapshots. • Low mass (109 Solar mass) • Relatively low metallicity • Under densed galaxy environment • Extremely high“specific star-formation rate” (10-8 per year) { inferred from [OII] line }. • Very short stellar mass doubling time (108 year). • Mean flux is 0.12 mJy at 1.4 GHz (from stacking of FIRST images).

13. Deep imaging of several wide-area, multi-band fields, mainly at 610 MHz & 150 MHz • These fields have already been imaged at other bands (X/UV/Optical/N-IR/sub-mm/radio) • Main Objective of GMRT Imaging: • To find the composition of sub-mJy population (where radio source counts flatten) • To find sub mJy Ultra-Steep-Spectrum Radio Sources (best high-redshift candidates) (George & Steven, 2008)

14. Fields imaged at 610 MHz • Spitzer Extragalactic First Look Survey {610 MHz, σ ~ 30 μJy } (Garn et al, 2007) • ELAIS-N1 {610 MHz, σ ~ 50μJy, 2500 sources} (Garn et al, 2007; 2008) {325 MHz, σ ~ 40μJy, 1286 sources} (Sirothia et al, 2010) • ELAIS-N2 {610 MHz, σ ~ 80μJy} (Garn et al, 2009) • XMM-LSS {610/235 MHz, σ ~ 0.3/2.5 mJy, ~700 sources} (Tasse et al, 2007) • VVDS-VLA {610 MHz, σ ~ 50 μJy} (Bondi et al, 2007) • Lockman Hole {610 MHz, σ ~ 60 μJy, 2845 sources} (Garn et al, 2008) {610 MHz, σ ~ 15 μJy, 928 sources} (Ibar etal, 2010; Afonso et al, 2011) • Summary of Main Results: • Differential counts do flatten near 1 mJy. • No significant variation of <α> between 10 to 0.1 mJy.(-0.6 to -0.7) • 2-3 % of sub-mJy sources have ultra-steep spectrum (α<-1.3). • About 6% of sub-mJy sources have inverted radio spectrum. • Most sub-mJy sources are resolved but smaller than 4“. Hence they are not core dominated or GPS sources. • Main contributors to the sub-mJy excess are : 1. star forming galaxies. 2. radio-quiet AGN (X-ray detected). 3. small double radio sources. • Only 10% of sub-mJy sources remain unidentified in the 3.6μm SERVS survey. (limiting magnitude ~23). High-z FR I? Ibar et al, 2009

15. Fields imaged at 150 MHz • Eridanus {150 MHz, σ ~ 3.1 mJy, 113 sources (81 in NVSS) } (George & Steven, 2008) • LBDS-Lynx {150 MHz, σ ~ 0.7 mJy, 765 sources} (Ishwar-Chandra et al, 2010) 150 sources have α < -1.0 and ~ 1/3 undetected in SDSS (high-z) • BOOTES {150 MHz, σ ~ 1.0 mJy, 598 sources, (Intema et al , 2011) compared with WSRT 1.4GHz images} 16 sources have α < -1.3 Median α flattens from -0.90 +/- 0.12 (at 0.5 mJy) to -0.7 +/- 0.25 (at 0.1 mJy)

16. Extragalactic Supernova: SN 1993J • Radio spectrum was measured using GMRT and VLA (9 years after the explosion) First evidence for a spectral break in a young supernova. Since age is known, synchrotron aging analysis provided an estimate of magnetic field (~300 mG), which is independant of the equipartition assumption ! (Chandra, Ray & Bhatnagar 2004) • Type Ib SN in the nearby galaxy NGC 2270, detected as a X-ray transient • From day 11 to 571 it was observed on 24 epochs with GMRT at 325/610/1240 MHz. On day ~400 a peak of around 3 mJy at 325 MHz was reached. This delayed emission was interpreted in terms of a mildly relativistic OFF-AXIS jet. (van der Horst et al 2011) Gamma-ray burst (GRB 030329 afterglow) Monitored with GMRT at 325 MHz in the non-relativistic expansion phase. First radio detection of GRB afterglow below 1 GHz. (van der Horst et al 2008)

17. Science with GMRT Highlights of Galactic Radio-Continuum Research

18. Exoplanets (So far no radio detection reported) • Radio light curves at 150 MHz of 4 nearby transiting exoplanets • (Lecavelier, A.; Sirothia, S.; Gopal Krishna; Zarka, P.) • HD 189733 b and HD 209458 b : no detection of radio eclipse (planetary emission < 2-4 mJy at 150 MHz) • HAT-P-11 (26 Earth mass, D = 37.5 pc, Orbital period = 5 days, Eclipse duration = 2 hr) A hint of planetary eclipse

19. Galactic Centre Region • Sgr A (the source at the galactic centre): Spectral Variation of Sgr A Roy & Rao (2004) An et al (2006) • First detection of Sgr A below 1 GHz 0.5 +/- 0.1 Jy at 620 MHz • June 17, 2003 : Simultaneous imaging of Sgr A at 620 MHz (GMRT) and 325 MHz & 1400 MHz (VLA) Spectral cutoff at λ ~ 30 cm (in 2003) Changed from λ ~ 100 cm in 1975 (Davies, Walsh & Booth, 1976)

20. Radio transients in the galactic centre region • GMRT 325 MHz observations of the transient J1745-3009 in 2003 & 2004 detected a 2-minute long burst with extremely steep spectrum. (α = -13.5 +/- 3.0 across the bandpass) (Hyman et al, 2007) • J1742-3001: A new transient discovered and monitored at 235/610 MHz on 24 epochs during 2006-08. At 235 MHz, a 100 mJy flare was recorded, on month-like time scale. (α < -2.0) (Hyman et al, 2009) • CSS 100217: This known optical transient was detected with GMRT (~ 1 mJy at 610 MHz) and also with eVLA at 5 GHz (α ≈ 0.4) (Drake et al, 2011)

21. Recurring Nova RS Ophiuchi (a binary of M giant and a white dwarf) First time detection below 1 GHz (~ 44 mJy at 325 MHz) Spectral Index between 325 and 1500 MHz measured to be -1.0, establishing non-thermal nature of the outburst (Kantharia et al, 2007)

22. Micro-Quasars • TeV binary HESS J0632+057 (D=1.5 kpc, Be star) Observed at 610/235 MHz (3 sessions) and 1280 MHz (3 sessions) Size < 2” (3000 AU), α = -0.6 +/- 0.2 (non-thermal) SED modeling gave B ~ 70 mG & Emin ~ 2 GeV (Skilton et al , 2009) • TeV binary LS5039 (O6.5 star with P ~ 3.9day) GMRT revealed spectral turnover below 1 GHz, which remains independent of orbital phase. (disfavouring synchrotron self-absorption) (Bhattacharyya et al, 2012) Skilton et al, 2009