Enigmatic Gamma-ray Bursts

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Enigmatic Gamma-ray Bursts Powered By Docstoc
					GRBs: Recent progress and new mysteries


   • 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
  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.

• Determine the nature of
  short duration GRBs.

• Find GRBs at high z
• The transition from -ray .
  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)
                                                   np  t -1
                                  t =R 2/2       fn  t -2-b

                                  t =R -2/2

                                        t ~ -2-b
                         t ~ -1

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/202 ;       ct2 ≈ Rfs/2fs2
     Since fs < 0         R > (t1/t2) Rfs
                                  Rfs = [3ct2Eiso/2mpc2n0]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     Afterglow

 Inner      Relativistic    Internal
 Engine                                        External
             Outflow        Shocks              Shock

       106cm           1013-1015cm         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:
 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 fnn+ve

• Synchrotron peak frequency = 100 kev      Bi2 = 1013

• Electron cooling:
    c/i ~ 10-17 i3 /tGRB(1+Y)

    Compton Y ~ eci
             c/i ~ 10-9 [ i/(tGRB e)]1/2

      Therefore, c/i <<1  fnn-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.

    • Is the poynting that produces -rays baryonic
             GRB jet

    • 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                   ISM
                                   Forward shock
             Reverse shock

    Ejecta                   ISM
                    New puzzles posed by Swift data
                    (Problem when you have good data!)

  Flares lasting                                         Chromatic plateau
for hours - short                                          In x-ray LCs
 and long GRBs

                                                             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)


                          v =5c
                                                    v   1 bsin 

                                                        ≈ 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                         R
            1 1

Area visible to observer = (R/)2
       Area visible to observer = (R)2  (R/)2 ()2  (R/)2 t -3/4

                            t   ~ -2                                            t ~ -2
            t ~ -1                                          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.)
                                          Absorption lines at different velocities
       Temporal fluctuations            (spectrum at ~ 1 day -- McDonald HET)

Bersier et al. 2002, astro-ph/0211130            Schaefer et al. 2002
Nakar & Piran, 2003, ApJ 598, 400         (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:

                           SN 1998bw:
                           local, energetic,
                           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: fn(t)  n - t-b
   (~1 and b~1-3 at early times)