Gamma Ray Bursts Poonam Chandra National Centre for Radio Astrophysics Tata Institute of Fundamental Research

Poonam Chandra What are Gamma Ray bursts (GRBs)? Most energetic events in the universe Long duration GRBs (t>2s) (Massive star explosions?) Short duration GRBs(t<2s) (NS-NS merger, SGRs?)

Ionized f(HI) ~ 0 Neutral f(HI) ~ 1 Reionized f(HI) ~ 1e-5

DEATH OF MASSIVE STARS Poonam Chandra

Evolution of stars Poonam Chandra

Massive Stars, 25 M 

Nuclear reactions inside a heavy star

Evolution of stars Poonam Chandra

8M Θ ≤ M ≤ 30M Θ Supernova M ≥ 30M Θ Gamma Ray Burst Poonam Chandra10

Barkana and Loeb (2007) Initially formed from dark matter mini-halos at z=20-30 before galaxies Pop III: M~100 M sun L~10 5 L sun T~10 5 K, Lifetime~2-3 Myrs Dominant mode of star formation below Z solar Can be found only via stellar deaths

Gamma Ray Bursts Meszaros and Rees 1997

Challenges (Gamma Ray Bursts) Localization Galactic or Cosmological Central Engine, fireball model? Origin (massive star?)

GRB Missions BATSE BeppoSAX 16

Major breakthrough BeppoSAX: first detection of X-ray counterpart of GRB Optical detection after 20 hours. Poonam Chandra18

Afterglows: GRB , (z=0.83) Frail et al. 2000, 1997, Waxman et al O Diffractive scintillation size constraint (<10 17 cm). O Energetics from long lived afterglow E 0 =5 x ergs. O Density ~0.5 cm -2,

GRB / SN 1998bw first GRB/SN association

Crisis: GRB Assuming isotropy, the  ray isotropic energy ~ 3×10 54 erg Central engine energy requirements??

The GRB Energy Crisis circa Stan Woosley says “I’m a very troubled theorist.” Piran, Science, 08 Feb 2002 ApJ 519, L7, 1999

Jet Break due to collimation: GRB Poonam Chandra23

GRBs: Jets and Geometry GRB emission is not spherical but in relativistic jets Due to relativistic beaming, only small fraction of jet. As jet slows down, lateral expansion. Jet break, geometrical effect. Simultaneous in all electromagnetic bands.

The GRB Energy Crisis Resolved Frail et al (2001 )

That was then… The GRB energy crisis was resolved GRB outflows are highly beamed ( θ ~ 1-10 degrees) Geometry measured from jet break signature in light curves Beaming-corrected radiated energies are narrowly distributed around a “standard” value of ~10 51 erg A host of other measurements (X-ray afterglows, broadband modeling, calorimetry) support this energy scale This energy scale is consistent with models of GRB central engines 26

SWIFT AVERAGE REDSHIFT = 2.7 Poonam Chandra27

Poonam Chandra28 FERMI AGILE

This is now… POST-SWIFT 1.The mystery of the missing/chromatic jets in the Swift era. 2.The emerging population of hyper-energetic events. 3.The established class of sub-energetic gamma-ray bursts.

GRBs: Energetics E rel =E gamma +E inj +E rad +E kin Two models collapsar, magnetar, upper limit ~1E+52 ergs Cenko et al. 2011

Gamma Ray Bursts Meszaros and Rees 1997 Poonam Chandra32

Multiwaveband modeling Long lived afterglow with powerlaw decays Spectrum broadly consistent with the synchrotron. Measure F m, m, a, c and obtain E k (Kinetic energy), n (density),  e,  b (micro parameters), theta (jet break), p (electron spectral index).

Radio Observations Late time follow up- accurate calorimetry Scintillation- constraint on size VLBI- fireball expansion Density structure- wind-type versus constant

RADIO TELESCOPES VLA GMRT

Energetics from Radio Radio long lived afterglow emission The outflow reaches sub-relativistic regime Quasi-spherical geometry Energetics are independent of uncertain beaming angles

Afterglows: GRB , (z=0.83) Frail et al. 2000, 1997, Waxman et al O Energetics from long lived afterglow E 0 =5 x ergs.

Density estimation GRB N=50 cm -3 or A*=2.5 (n=3E35A*r -2 ), Chandra et al. 2008

GRB : Chromatic jet break Chandra, P. et al. 2008

Scintillation puts constraints (Goodman 1997) GRB Limit of 3 microarcsec on the angular size. R~1E17cm

GRB : Scintillation theta=~2.8+/-0.5  arcsec R~2E17cm Chandra et al 2008

Other Inputs in radio bands Radio VLBI. Direct constraints on fireball size Confirmation on relativistic expansion GRB : Size 0.07mas (0.2pc) 25 days and 0.17mas (0.5p) 83 days. Confirmation of relativistic fireball expansion (Taylor et al. 2004)

Other Inputs in radio bands: Detectability at high redshift Chandra et al. 2012, ApJ 746, 156

GRBs detected at high redshift GRB (Chandra et al. 2010) GRB (Frail et al. 2009)

Swift had expected to find many RS At most, 1:25 optical AG have RS Favored explanation –Ejecta are magnetized (i.e. σ>1). –Do not need to be fully Poynting-flux dominated –Suppresses RS emission Does not explain why prompt radio emission is seen more frequently. About 1:4 radio AG may be RS Possible Explanation: The RS spectral peak is shifted out of the optical band to lower frequencies 45 Kulkarni et al. (1999) Reverse shock in radio GRBs Chandra et al. 2010b

Reverse shocks in radio afterglows Only has a confirmed optical and radio reverse shock. Low incidence of optical reverse shocks, i.e. < 4% (Gomboc et al. 2009). Radio RS is 1 every 4 bursts, i.e. 6 times more than optical.

Reverse shocks in Radio GRBs : : Chandra et al., , to be submitted soon (hopefully )

Reverse shock emission from GRB (Chandra et al. 2010) Reverse shock seen in GRB (z=6.26) too RS seen in PdBI data too on day 1.87

GRB A: Evidence of RS Laskar et al Observations with VLA, GMRT, CARMA and combined with optical/IR/UV and X-ray bands. Most detailed modeling of RS. Wind medium with low density prefered.

Radio Detection Statistics 95 out of 304 GRBs detected in radio – 31% Pre-Swift radio detection 42/123 – 34% Post-Swift radio detection 53/181 – 29% X-ray detection rate 42% (pre-Swift) to 93% (post-Swift). Optical detection rate 48% (pre-Swift) to 75% (post-Swift). Chandra et al. 2012, ApJ 746, 156

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

Sample bias or different population? (Hancock et al. 2013) Chandra et al. (2012) sensitivity limited. Hancock et al. (2013):visibility stacking- two different populations. No more than 70% of GRB afterglows are truly radio- bright. Radio quiet GRBs are intrinsically weak GRBs at all wavelengths. Gamma-ray efficiency of the prompt emission is responsible for the difference between the two populations. One magnetar-driven, and one black-hole-driven, as gamma ray efficiency inversely proportional to magnetic field.

A seismic shift in radio afterglow studies The VLA got a makeover! More bandwidth, better receivers, frequency coverage 20-fold increase in sensitivity Capabilities started in 2010 Poonam Chandra53

Future of GRB Physics Poonam Chandra54

In meter wavebands

Atacama Large Millimeter Array 56 Poonam Chandra

Future: Atacama Large Millimeter Array (ALMA) Accurate determination of kinetic energy Poonam Chandra57

Future: ALMA Debate between wind versus ISM solved Poonam Chandra58

Future: ALMA Reverse Shock at high redshifts mm emission from RS is bright, redshift- independent (no extinction or scintillation) (Inoue et al. 2007). ALMA will be ideal. Poonam Chandra59