Radio afterglows of Gamma Ray

BurstsPoonam Chandra

National Centre for Radio Astrophysics - Tata Institute of Fundamental Research

Collaborator: Dale Frail and many others

Radio AfterglowsO 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 AfterglowsO 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 estimationO Detectable at high redshifts.

Negative K-correction(detectable at high redshifts)

Chandra et al. 2012, Frail et al. 2006

Radio Afterglows: GRB 970508Frail et al. 2000, 1997, Waxman et al. 1998

O First 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, E0=5 x 1050 ergs.O Density ~0.5 cm-2,

O Equipartition eB~eE~0.5

GRB radio afterglowsO GRB 990123: First afterglow with reverse

shock detection in radio band (Kulkarni et al. 1999).

O GRB 020405: evidence of a constant density medium around massive star (Berger et al. 2003).

O GRB 050904 (Frail et al. 2005) and 090423 (Chandra et al. 2010): highest redshift bursts discovered in radio.

O GRB 070125: radio afterglow with scintillation, chromatic break, uniform density (Chandra et al. 2008).

Radio afterglows: 030329van der Horst et al. 2008, Pihlström et al. 2007, Taylor et al.

2004

O Very bright radio burst. O Constant density medium.O Non-relativistic transition ~ 80-200

daysO VLBI- relativistic expansion of

fireball.

Radio Afterglows: StatisticsO 304 GRBs observed in radio bands

from 1997-2011.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 dependenceO z<2=47/88 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=2.6x10-7

• 176/206 (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 >1x1053

erg• Only 9/206 non-

detections Energy >1x1053 erg

Detectability of radio afterglows - Energy

Chandra et al. 2012, ApJ 746, 156

Beaming corrected bolometric energy

Where fb 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?

FluenceR-index=0.02

Optical fluxR-index=0.62

Isotropic EnergyR-index=0.12

X-ray fluxR-index=-0.05

Synthetic Light Curveee=0.1 eB=1%, EKE=1053 erg, p=2.2

Chandra et al. 2012, ApJ 746, 156

• 8 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: densityee=0.1 eB=1%, EKE=1053 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: densityee=0.1 eB=1%, EKE=1053 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 mJy 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 ShocksO 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 990123 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 n~t-73/48.O RS freq is lower by (Lorentz factor)2 than FS.O If nm<nopt then no RS in optcal bandO For 021004, 021211 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

SummaryO 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