Radio afterglows of Gamma Ray Bursts Poonam Chandra National Centre for Radio Astrophysics - Tata Institute of Fundamental Research Collaborator: Dale Frail and many others

Radio Afterglows O Late time follow up. O Accurate energetics instead of “isotropic equivalent” energy. O Radio scintillation: Constraints on fireball size (Goodman 1997). O Radio VLBI – Fireball expansion. O Reverse Shocks: 6 times more prominent in radio afterglows than optical afterglows. O Density estimation O Detectable at high redshifts.

Multiwaveband Modeling

Radio Afterglows O Late time follow up. O Accurate energy instead of “isotropic equivalent” energy. O Radio scintillation: Constraints on fireball size O Radio VLBI – Fireball expansion. O Reverse Shocks: 6 times more prominent in radio afterglows than optical afterglows. O Density estimation O Detectable at high redshifts.

Negative K-correction (detectable at high redshifts) Chandra et al. 2012, Frail et al. 2006

Radio Afterglows: GRB Frail et al. 2000, 1997, Waxman et al O F irst radio afterglow detection. O Relativistic expansion measurement of fireball through diffractive scintillation. O Measured flux lower than spherical prediction (jet like geometry) O Bright and long lived afterglow followed over a year, E 0 =5 x ergs. O Density ~0.5 cm -2, O Equipartition  B ~  E ~0.5

GRB radio afterglows O GRB : First afterglow with reverse shock detection in radio band (Kulkarni et al. 1999). O GRB : evidence of a constant density medium around massive star (Berger et al. 2003). O GRB (Frail et al. 2005) and (Chandra et al. 2010) : highest redshift bursts discovered in radio. O GRB : radio afterglow with scintillation, chromatic break, uniform density (Chandra et al. 2008).

Radio afterglows: van der Horst et al. 2008, Pihlström et al. 2007, Taylor et al O Very bright radio burst. O Constant density medium. O Non-relativistic transition ~ days O VLBI- relativistic expansion of fireball.

Radio Afterglows: Statistics O 304 GRBs observed in radio bands from O 123 bursts in pre-Swift and 181 in post-Swift. O Sample includes 33 SHBs, 19 XRFs and 26 SN/GRBs (9 with confirmed SN and rest possible). O 28 SHBs detected by Swift itself. O 17 SN/GRBs detected pre-Swift and 9 post- Swift.

Radio Detection Statistics O 95 out of 304 GRBs detected in radio – 31% O Pre-Swift radio detection 42/123 – 34% O Post-Swift radio detection 53/181 – 29% O X-ray detection rate 42% to 93% (bias). O Optical detection rate 48% to 75% (bias) O No strong redshift dependence O z 2=21/43. Chandra et al. 2012, ApJ 746, 156

Detection Statistics Chandra et al. 2012, ApJ 746, 156

Radio Detection Biases detection Upper limits Chandra et al. 2012, ApJ 746, 156

Radio Detection Biases Chandra et al. 2012, ApJ 746, 156

Canonical Light Curve of cosmological long afterglows Chandra et al. 2012, ApJ 746, 156

Bursts of different Classes Chandra et al. 2012, ApJ 746, 156

Detectability of radio afterglows - redshift Chandra et al. 2012, ApJ 746, 156 Kolmogorov-Smirnov test P=0.61

Detectability of radio afterglows - fluence Chandra et al. 2012, ApJ 746, 156 Nysewander et al. 2009, Swirt XRT repository P=2.6x /206 (85%) non- detections fluence <1x10 -6 erg cm -2 82/95 (86%) detections fluence >1x10 -6 erg cm -2

Detectability of radio afterglows - Energy Chandra et al. 2012, ApJ 746, 156 P=9x10 -7 k-corrected bolometric in 1 keV-10 MeV range 144 grbs 60/95 detections Energy >1x10 53 erg Only 9/206 non- detections Energy >1x10 53 erg

Detectability of radio afterglows - Energy Chandra et al. 2012, ApJ 746, 156 Beaming corrected bolometric energy Where f b is the beaming fraction P=3.5x10 -3

Detectability of radio afterglows – X-ray and optical Chandra et al. 2012, ApJ 746, 156 Gehrels et al. 2008, de Pasquale et al. 2006, Sakamoto et al.2008, 2011 P=3x10 -6 P=1x10 -9

What determines radio flux? Fluence R-index=0.02 Optical flux R-index=0.62 Isotropic Energy R-index=0.12 X-ray flux R-index=-0.05

Synthetic Light Curve  e =0.1  B =1%, E KE =10 53 erg, p=2.2 Chandra et al. 2012, ApJ 746, GHz light curve matches with sample. 1.4 GHz challenges: JVLA, ASKAP, WSRT/Apertif will not detect. Higher frequencies favored. JVLA (high freq) and ALMA ideal. Expected large increase in detection.

Synthetic Light Curve: density  e =0.1  B =1%, E KE =10 53 erg, p=2.2 Chandra et al. 2012, ApJ 746, 156 Radio sample biased for n=1-10 cm -3. Weak emission at lower n. Higher self-absorption for higher n. Explains why some bright GRBs dim in radio.

Synthetic Light Curve: density  e =0.1  B =1%, E KE =10 53 erg, p=2.2 Chandra et al. 2012, ApJ 746, 156 Afterglow in mm strong function of n. Effects of self- absorption weak in mm bands. ALMA (3-sigma=42  Jy in 1 hr at 250 GHz) may detect all mm afterglows for n>0.1 cm -3.

Reverse shocks

Reverse shocks in radio Kulkarni et al. 1999

Radio Reverse Shocks O Possible RS in 24 GRBs. O But 87 GRBs with no early radio data for t<3 days. O About 1:4 radio AG may be RS

Reverse shocks in Radio GRBs

Reverse shocks in radio afterglows O Only has a confirmed optical and radio reverse shock. O Low incidence of optical reverse shocks, i.e. < 4% (Gomboc et al. 2009). O Radio RS is 1 every 4 bursts, i.e. 6 times more than optical. O Magnetization, poynting dominated, SSC, dust extinction, wind density O Mundell et al. 2007, electron freq drop ~t -73/48. O RS freq is lower by (Lorentz factor) 2 than FS. O If m < opt then no RS in optcal band O For , optical RS is seen but no radio RS emission (Synchrotron self absorbtion???)

Future of radio afterglows

Future: Atacama Large Millimeter Array Accurate determination of kinetic energy

Future: ALMA: Wind versus ISM

Summary O Radio afterglows explore unique territory. O Detection rate unchanged in pre- and post- Swift phase. O Radio detections sensitivity limited. O Other prompt and afterglow emission parameters can be useful in determining detectability. O JVLA and ALMA are goldmines