Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Standard solar model wikipedia , lookup
Cosmic distance ladder wikipedia , lookup
Metastable inner-shell molecular state wikipedia , lookup
History of X-ray astronomy wikipedia , lookup
X-ray astronomy wikipedia , lookup
Astrophysical X-ray source wikipedia , lookup
GRBs: Recent progress and new mysteries Outline: • Summary of main results. Prompt and afterglow emissions. (our current understanding or lack there of) • • Unsolved problems. Ohio, September 26, 2007 Gamma-ray bursts are short pulses of radiation that come from random directions a few times a day. GRB Duration Short burst Long burst Swift GRB mission Swift was launched on Nov 20, 2004. Swift has accurate localization (~2’) rapid response (~1 min) and excellent temporal coverage for a few days. the nature of • Determine short duration GRBs. • Find GRBs at high z transition from -ray • The prompt to afterglow emission. What Have We Learned About GRBs? Long duration GRBs: • Energy: ~ 1051 erg (wide dispersion) • The outflow is collimated: j ~ 3o 30o • Occur in late type galaxies and associated with star formation. • At least a few of them have SNa Ic associated with them; these supernove were more energetic than average Ic. • Medium is uniform (often) with density a few/cc • Median redshift for Swift bursts is ~ 2.5; the lowest Z is 0.033 & the highest is 6.29 Short duration GRBs Swift has detected 15 and HETE 1 short bursts: • 5 GRBs are located near early type galaxies whereas 2 are in late type galaxy; the offset varies from 0.3 to 13 Rg & SFR < 0.1 Mo yr-1 for two of the host galaxies (Nakar , 2007). • Very stringent limit on any underlying SN for two GRBs (L<4x1040 erg/s) between 7-20 days (Fox et al.). • Low density of the circum-stellar medium; For 2 of the GRBs n< 10-2 cm-3; Panaitescu (2006) • Lower Eiso (E) by ~ 103 (10) & median z 0.25. Associated with older stellar population, possibly binary n-star (but we lack a firm proof). Nakar, 2007 Evidence for Relativistic outflow • Superluminal motion in 030329: Rt ~ 3x1017cm at 25d vt =Rt/t=5c ≈7 at 25 days (Taylor et al. 2004). • Diffractive scintillation quenched at 30d for 970508 R~1017cm V~R/t~C; Goodman 1997; Frail et al. • Afterglow modeling gives >4.5 at 1 day for 10 bursts (Panaitescu & Kumar, 2002). Synchrotron from FS fits late afterglow data Panaitescu & Kumar EGRB ~ 1051 erg; jet Uniform ISM; nISM ~ 10 cm-3 Early Afterglow Results for Swift Bursts • Rapidly decaying flux in the x-ray; likely the remnant of decaying -ray source (before the onset of FS emission). • Slowly decaying lightcurve in the x-ray. Sudden increase in flux (flare) during the “afterglow” (long lived central engine activity) Nousek et al. 2005 Because of smearing due to curvature dt/t ~ 1 in FS. Many of the flares have t/t << 1 which suggests late time engine activity. Some of the basic unanswered questions about GRBs • How does the central engine operate: accretion, B-Z…? The central engine is hidden -- opaque to EM signals. So our best hope is to try to model the -ray emission and the x-ray flares. • How is the relativistic jet produced? • Is the GRB outflow baryonic or magnetic? Understanding the -ray emission mechanism and detecting RS emission from GRB-ejecta would help. Prompt -ray generation mechanism O’Brien et al., 2006 Factor ~ 103 drop in flux! • The early time data from Swift shows that -rays are produced by a distinct - short lived - source. • We exploit this steep falloff to determine -ray source distance from the center of explosion. The fastest decay of LCs (Off-Axis Emission) (Kumar & Panaitescu 2000) t =R 2/2 t =R -2/2 1 fn R t ~ -2-b t ~ -1 t np t -1 fn t -2-b Nousek et al. 2005 Gamma-ray Generation (distance) • The distance from the center of the explosion where -rays are produced, R , can be determined from the early x-ray lightcurve: ct1 ≈ R/202 ; ct2 ≈ Rfs/2fs2 Since fs < 0 R > (t1/t2) Rfs Rfs = [3ct2Eiso/2mpc2n0]1/4 0: -ray source Lorentz factor fs: forward shock LF at t2 t1: time when -ray emission ends, t2: time when steep x-ray decline ends. • For 10 Swift bursts (t2/t1) is between 5 & 25 ; the mean is ~ 14 same for FRED & non-FREDs. -ray source lies within a factor ~10 of FS radius. • Or -rays are produced at a distance of ~ 1016 cm from the center of explosion. This distance is much larger than what was expected for internal shocks and of order the distance suggested for poynting model. The Internal-External Fireball Model -rays Inner Engine Relativistic Internal Shocks Outflow 106cm 1013-1015cm Afterglow External Shock 1016-1018cm Piran et al. 1993; Rees & Meszaros 1994; Paczynski & Xu 1994 Understanding -ray emission (Kumar et al. 2007) Synchrotron & IC from a relativistic source emission can be completely described by 5 parameters: Constraints Flux spectral index below the peak of spectrum frequency at peak of spectrum burst/pulse duration 5 unknowns and 3 constraints gives 2-D solution surface. -rays produced via the synchrotron process? (Ep =100kev; flux=1mJy; t=1s; low energy spectral index -ve) Synchrotron solution is also ruled out when fnn+ve • Synchrotron peak frequency = 100 kev Bi2 = 1013 • Electron cooling: c/i ~ 10-17 i3 /tGRB(1+Y) Compton Y ~ eci c/i ~ 10-9 [ i/(tGRB e)]1/2 Therefore, c/i <<1 fnn-1/2 -rays produced via the SSC process? (Ep =100kev; flux=1mJy; t=1s) SSC gives consistent solutions. It predicts bright, prompt, optical which we see in a few cases: 041219 ~ 14 mag. GRB jet that produces -rays baryonic • Is the or poynting outflow? • For baryonic outflow we should see RS emission. Reverse shock emission? One of the things Swift was going to do is find many more bright optical flashes like GRB 990123 where are they? Roming et al. 2006 Ejecta Forward shock ISM Reverse shock • Ejecta ISM New puzzles posed by Swift data (Problem when you have good data!) Flares lasting for hours - short and long GRBs Chromatic plateau In x-ray LCs RS emission? Do we have the FS AG right? Jet breaks? A sudden drop in x-ray flux in a few cases! Troja et al. astro-ph/0702220 Summary of Results 1. Long duration GRBs are associated with collapse of a massive star (at least in several cases!). 2. The short GRBs have much less energy and are associated with old stellar population. 3. The rapid fall off of the early x-ray afterglow suggests that -ray emission is produced by a short lived source; we find that it is most likely SSC at a distance of ~ 1016 cm baryonic jets have a few problems. 4. X-ray lightcurves show flares on time scale of minutes to a day suggesting that the central engine of GRBs can be active for a period of order ~ 1 day. Unsolved Problems 1. The nature of the central engine is not understood. 2. Is the energy from the explosion carried outward by magnetic field, e±, or baryonic material? 3. No firm evidence for r-2 density structure (except perhaps in 1 or 2 cases). And very low density found in several cases is puzzling. 4. Collisionless shocks, particle acceleration, magnetic field generation etc. poorly understood. Superluminal motion in GRB 030329 (Taylor et al., 2004, 609, L1) ≈7 v=3c v=5c v 1bbsincos ≈ 50 Solid line: Spherical outflow in a uniform ISM; E52/n0 =1 Dashed line: jet model with tj =10 days & E52/n0 =20. Nakar, 2007 Host of short-GRBs) Tagliafferi et al. 2005 t-0.72 t-2.4 Break in the LC at 2.6 days implies: j ~ 3o E ~ 4x1051 erg Determining Jet Angle from Break in LC (Rhoads 1999, Sari et al. 1999, Kumar & Panaitescu 2000) At late time: -1 ≥ At early time: -1 ≤ R 1 1 R Area visible to observer = (R/)2 Area visible to observer = (R)2 (R/)2 ()2 (R/)2 t -3/4 t t ~ -1 t ~ -2 ~ -2 t ~ -1 Future Missions GLAST, due for launch in 2008, will cover 10 Kev – 300 Gev, and detect > 200 GRBs yr-1. AGILE (an Italian mission) 30 Mev – 30 Gev & 10 – 40 kev is expected to launch in 2005. • ICECUBE, ANTARES will explore Neutrino emission from GRBs: 10 Gev – 105Tev. • Gravitational waves from GRBs? GRB 021004 (HETE II: Shirasaki et al.) Temporal fluctuations Absorption lines at different velocities (spectrum at ~ 1 day -- McDonald HET) Bersier et al. 2002, astro-ph/0211130 Nakar & Piran, 2003, ApJ 598, 400 Schaefer et al. 2002 (similar velocity features are also seen in 050505; Berger et al.) Long GRBs - collapse of massive stars (Woosley and Paczynski) GRB 030329/SN2003dh SN 2003dh/ GRB 030329: z=0.166 (afterglow-subtracted) SN 1998bw: local, energetic, core-collapsed Type Ic Stanek et al., Chornock et al. Eracleous et al., Hjorth et al., Kawabata et al. Detectability of Bursts at high Z The peak flux for GRB 050904 was ~ 3x10-8 erg cm-2 s-1 (BAT sensitivity, 15-150 kev, is 0.25 photons cm-2 s-1 or 1.2x10-8 erg cm-2 s-1 for fn n-1/2) So Swift can detect bursts like 050904 to Z~10. Price et al. (2005) claim that 8 out of 9 Swift bursts (at z>1) could be detected at z=6.3 and 3 of these could be detected at z~20. Detectability of Afterglows at high Z 10 min after GRB 050904 the 0.2-10 kev flux was ~ 10-9 erg cm-2 s-1 and the luminosity (isotropic equivalent) was ~ 1050 erg s-1 (the flux at earlier time scaled as t-2). Swift/XRT detection limit is 10-13 erg cm-2 s-1 for 100s integration time. At 1 hr the J-band flux was 17th-mag and the luminosity (isotropic equivalent) was ~ 1047 erg s-1 Negative k-correction helps: f (t) n- t-b n (~1 and b~1-3 at early times)