Gamma Ray Burst Jets: Predictions and Observations. James E. Rhoads Space Telescope Science Institute

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1 Gamma Ray Burst Jets: Predictions and Observations James E. Rhoads Space Telescope Science Institute

2 Motivation Burst energy requirements and event rates scale linearly with collimation solid angle. With the first GRB redshift, collimation became the dominant uncertainty in burst energy requirements and event rates- up to a factor of Collimation is a very common property of astrophysical outflows (e.g.,radio galaxies, quasars, microquasars, protostellar objects).

3 Why collimation is hard to see Bulk relativistic motion implies relativistic beaming of emission from afterglow ejecta. A parcel of matter emitting isotropically in its rest frame will beam its radiation into a cone of characteristic half angle 1/G in the observer s frame. We therefore see radiation only from a small patch of ejecta located near the line of sight. We cannot easily tell if ejecta exist outside that patch.

4 Tests for Collimation Various predictions for collimated GRBs: Jet breaks : Afterglows of collimated GRBs fade more rapidly at late times (Rhoads 1997, 1999a,b; Sari, Piran, & Halpern 1999). Orphan afterglows (Rhoads 1997). Polarization signatures (Sari 1999; Ghisellini & Lazzati 1999). Remnant statistics (Perna, Raymond, & Loeb 2000). Isotropic tracers: Reprocessed (Ghisellini et al 2002) or subrelativistic (Waxman et al 2000) emission.

5 Edge of the Jet Effect Consider a collimated jet with opening angle? m. While G > 1/? m, the dilution of relativistic beaming as ejecta decelerate is compensated by seeing more of the working surface. Later, there is no more working surface to see factor 1/G 2 in light curve evolution.

6 Burst Remnant Evolution Remnant dynamical evolution can be strongly affected by outflow geometry. Consider ejecta originally collimated with opening angle? m. The working surface has transverse size ~ r? m + c s t co. r? m c s t co For bulk Lorentz factors G << 1/? the second term dominates.

7 Evolution of G The bulk Lorentz factor G as a function of expansion radius r for bursts with identical G 0 but different initial opening angles. Note the much faster slowdown of the collimated ejecta. (Rhoads 1999.)

8 Sideways Expansion Animation Opening angle 0.1 radian

9 Light Curves for Collimated GRB afterglows Light curves for jetlike GRB afterglows at three frequecies (from Rhoads 1999).

10 Jet Break Applications First? b lower limit: GRB ; Rhoads 1999b First positive evidence for collimation: GRB and more; Sari, Piran & Halpern First good break detections: GRB and GRB (Stanek et al 1999, Harrison et al 1999) Most comprehensive sample: Frail et al 2001.

11 Afterglows with Breaks: GRB (Stanek et al 1999)

12 Collimation Corrected Energies Gamma ray energies before and after collimation correction. From Frail et al (2001) See also Kumar and Panaitescu 2002.

13 Jet Hydro Simulations Figure from Jonathan Granot Approximations in analytic work: Growing top hat : all properties are independent of? for?<? b (r). Sideways expansion occurs at fixed speed. Relativistic hydrodynamic simulations can relax these assumptions. Granot et al have done the first.

14 Simulated Jets, continued Opening angle evolution and light curves from Granot s simulations. (Time axis is lab frame time, ~ r/c.)

15 Light Curves from Simulations Light curves from simulations Break is sharp, achromatic. Shape depends on? m <>? jet.

16 Effects of Layered Jets If we replace the growing top hat with a more general profile, the interpretation of jet breaks can change so that we measure? v and not? j. (Rossi, Lazzati, & Rees 2002; Zhang & Meszaros 2002.) Tests: Luminosity function? (Yi 1994) Orphan afterglow rates? Etc

17 Orphan Afterglow Tests During the evolution of a GRB remnant, The Lorentz factor G decreases; The peak frequency decreases as G 4, and The max. possible beaming O ~ p G -2. So, the observed transient rate should increase with wavelength if GRBs are collimated. (Rhoads 1997; Perna & Loeb 1998)

18 Orphan Afterglows: Observational Status X-ray: Greiner (1998) searched the ROSAT all sky survey for afterglows. Found flare stars; no differential collimation (X -?). Optical: Schaefer (2001) surveyed 300 sq. degrees to magnitude R=21 over 2 weeks. No orphan afterglows => O? / O o > 0.01 Radio: Perna & Loeb (1998) combined archival surveys to infer? m > 5 o => O? >

19 Afterglows in Color-Color Space A difficulty of orphan afterglow searches is distinguishing the transients we want from other variable sources. Color information helps! Observed color-color plane for the field of GRB C. (Rhoads 2001)

20 Polarization Afterglows are synchrotron light, so should be polarized The degree of observed polarization depends on breaking symmetry, which is easier in collimated geometries. (Sari 1999; Ghisellini & Lazzati 1999). Polarization may be strongest in orphan events. (Granot )

21 Polarization Cartoon Viewing geometry for a ringlike GRB afterglow image, from Sari 1999.

22 Polarization Cartoon Polarization as a function of time, from Sari 1999.

23 GRB Remnant Statistics GRB remnants should be smaller and more numerous if bursts are collimated. Counting them will help if we can identify them! HI supershells? (Efremov and Elmegreen? 1998?, Loeb & Perna 1998) Shape distortions? (Ayal & Piran 2001) Photoionization signatures? (Perna, Raymond, & Loeb 2000) Positron annihilation lines? (Furlanetto & Loeb 2002)

24 Reprocessed Radiation and Calorimetry Reprocessed radiation is usually fairly isotropic. This allows tests of total energy and hence collimation angle using X-ray lines (Ghisellini et al 2002), thermal dust emission, perhaps optical emission lines Potential complication: Light travel time delays. Late time radio observations provide total energy estimates (Waxman et al 2000), hence another collimation constraint.

25 Non-Electromagnetic Signatures Gravity waves should be more nearly isotropic than gamma rays Neutrinos might be too Either of these might be developed as a collimation test.

26 GRB Collimation: Summary GRB collimation has gone from pure speculation to observational test in the last five years. Our prior helps guide our interpretation of the data. Characteristic jet angles ~ 3 to 5 degrees. Collimation corrections lead to a (more nearly) standard GRB energy. Interpretations of E iso t break relation not unique. Independent tests with different systematics will help resolve remaining uncertainties.