Gamma Ray Bursts

S. R. Kulkarni

California Institute of Technology

Acknowledgements

• Alicia Soderberg

• Caltech/NRAO/Carnegie gang– Berger, Cenko, Fox, Frail, Harrison, Price,

Schmidt

• T. Sakamoto & R. Yamazaki

Quasars: A Historical Analogy, I

• Astonished & Impressed: The immense power and energy of quasars resulting from Schmidt’s discovery of redshift.

• Amused and Educated: Relativistic effects such as super-luminal motion were anticipated by Rees.

• Ruthless Exploitation: Ask not why quasars quase but simply use them as light beacons to study the IGM.

Quasars: A Historical Analogy, II

• Scintillation: Interplanetary Scintillation showed that quasars were compact

• The Central Engine: After three decades we have a working model involving black holes

• The Pesky Jets: Questions remain– FRI and FRII– What is the difference between radio quiet and radio loud AGN?

• Unification: The desire to unify various classes of quasars drove much of quasar research.

Unification: Working Model

“Torus”BLR

NLR

Jet

AGN: Empirical Classification

• Radio Luminosity:– Radio Loud– Radio Quiet

• Optical Emission Lines:– Broad Emission Lines (Type 1) – Narrow Emission Lines (Type 2)

Roughy speaking these two may map to the type of host galaxy and the type of black hole

What are the equivalent physical parameters for GRBs?

Outline

• GRB Phenomenon• Long Duration GRBs

– Jets– Energetics– SN Connection

• Jets in nearby SNe?• Where do we stand at unification?

Two classes of GRBs

Short - Hard

Long - Soft

Jets• Decrease Total Energy (by beaming fraction)

• Increase the event rate (by inverse beaming fraction)

Light Curves provide Evidence for Collimation

t < tjet

high log f

log t

|

tjett > tjet

low

log f

log t|

tjetRhoads

GRB Energetics: Tiger becomes Lamb

Before the beaming correction (isotropic)

After the beaming correction

(Frail et al.)

Radio Light Curves at 8.5 GHz

Radio Afterglows: Angular Size and Calorimetry

and the latest ….

• GRB 030329, 24 days after the burst– VLBA+Bonn at 22 GHz

• Marginally resolved at 0.08 milliarcsec

• In line with expectations from the fireball model– superluminal expansion (5c)0.45 x 0.18 mas

Taylor et al.

GRB 980703: Non-relativistic Transition

Complications

• Evidence for continued or additional injection of energy

• Evidence for additional components in the jet (wide angle, low Gamma)

Early Light Curves

Fox

The second nearest GRB 030329 is peculiar

Berger et al in prep.

A possible solution:

(1) a narrow, ultra-relativistic jet with low energy which produces X-ray & optical

(2) a wide, mildly relativistic jet carrying the bulk of the energy and powering the radio

Jet break

Berger et al. 2003

Puzzle: A single fireball does not account for radio & X-ray emission

Long Duration GRB-SN Connection

SN 1998bw/GRB 980425, a severely underluminous GRB

Galama et al. E~1048 erg (isotropic)

Mildly Relativistic Ejecta in SN 1998bw

Kulkarni et al

E~1048 erg

Mildly relativistic ejecta vastly exceeds gamma-ray energy relese

Direct Spectroscopic Evidence

MMT (Stanek et al) VLT (Hjorth et al)

X-ray Flashes

Heise

XRF 020903: First redshift is low (z=0.25)

Energy in the Explosion (Prompt): 1049 erg (low compared to GRBs)

Soderberg et al

No evidence for off-axis model (optical flux declines)However, evidence for mildly relativistic ejecta from radio afterglow

Collapsar Model

Woosley, Heger, MacFadyen

GRB-SN: Grand Unification

All core collapse events are the same. – GRBs are explosions viewed on axis– XRFs are explosions viewed off axis– GRB 980425 is an off-axis GRB– In all cases, underlying SNe Lamb, Nakamura, Yamazaki…

In favor:SimplicityPeak energy-luminosity correlation

SN-GRB: Meek Diversity

• GRBs are not standard explosions (energy, opening angle)• XRFs are not GRBs viewed sideways and likely lower

energy explosions• SN 1998bw is an engine driven SN but with a weak engine• In most core collapses the influence of engines is likely to

be small or subtle.In favor: The existence of sub-energetic events (e.g. 031203, SN

1998bw). No evidence for early rise in the afterglow

Kulkarni, Soderberg, Sakamoto

Putting it altogether: Engine

Soderberg

SUMMARY: Peak SN magnitudes

(Soderberg et al. 2005b)

Do nearby core collapse SNe have strong jets that materially affect the

explosion?

VLA & ATCA Program

• Radio emission traces both relativistic and mildly relativistic ejecta (cf SN 1998bw)

• Relativistic aberration is less of an issue for mildly relativistic ejecta

• Motivated by 1998bw we began a program of monitoring all known nearby Ib/c

• Monitored SNe from day to a yearSoderberg thesis

RadioLight-curves

ofCosmic

Explosions

Ibc Survey:11 detections

73 upper limitsNo GRBs or 98bw’s

< 1.2% GRB/SNc.f. ~1 % beaming fraction for GRBs

~5 % hypernova rate

Explosion Energies of Local Ibc & GRBs

2003

L &

200

3bg

Conclusion: SN 1998bw-like events are rare

Was GRB 980425 an off-axis event?

• Six years of radio monitoring: No evidence for off-axis jet.

• Off-axis jet (if present) requires a very low mass rate: A* ~ 0.03, not consistent with inferred density

(Soderberg, Frail, Wieringa 2004)

Progenitors of Ibc SNe: A Hot Result

Progenitor of SN 2004gt (Ic SN)

Gal-Yam

Mv > -5.5

Summary: Cosmological GRBs

• Long duration GRBs are highly collimated explosions and possess central engines which drive the explosion

• Searches with good sensitivity have almost always found associated SNe of type Ib/c (or at least not of Type II)

• Not all associated SNe are bright (-19 mag)• XRFs are likely simply low energy explosions

(relative to cosmological GRBs but comparble to low energy GRBs)

Summary: Nearest Events

• There is growing evidence of underenergetic GRBs (e.g. 980425, 030329, 031203) with engines releasing a mix of ejecta: – ultra-relativistic ( >100),– relativistic( >10) &– mildly relativistic ( >2) ejecta

• Some of these events are dominated by mildly relativistic ejecta (GRB 030329). Some are X-ray Flashes (I.e. dominated by X-ray and not gamma-ray emission).

GRBs as 2-parameter Explosions

• GRBs clearly manifest an essentially spherical explosions (supernova) and narrow jets (few to tens of degrees)

• There is wide variation in properties of both components.

• There is little evidence for “universal” jet or “universal” supernova model.

Nearest Ib/c SNe

• Other than SN 1998bw we have not identified a single similar example

• No strong emission (indicative) is seen in any of the nearly one hundred local Ib/c SNe on timescales of days to years.

• Significant variation in peak optical emission as well as spectro-velocity peculiarities (e.g 2003jd, Mazzali et al)

Open Issues

• What accounts for the variation in opening angles of GRB jets?

• Do jets play a significant role in exploding typical core collapse events?– Attractive hypothesis but little evidence (so far)– Alternate explanations must be sought for

variation in optical diversity.

• Are short hard bursts strongly jetted?

GRB050509b: Short Hard Burst

• Rapidly fading X-ray afterglow (Gehrels et al)

• No optical/radio afterglow• Seen against z=0.22 cluster

GRB 050509b: Constraining an associated “moderate nova”