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Gamma Ray Bursts

VIEWS: 1 PAGES: 20

									Gamma Ray Bursts
GRB 050505: A high redshift burst
      discovered by Swift.

                       C. P. Hurkett


       J.P. Osborne, K.L. Page, E. Rol, M.R. Goad, P.T. O’Brien,
   D. Burrows, J. Hill, J. Kennea, V. La Parola, M. Perri & P. Romano.
                     Overview
• A brief introduction to Gamma Ray Bursts.
• The importance of high redshift bursts.
• GRB 050505
   – Multiwavelength observational data from Swift.
   – Summary of Ground Based observations.
   – Origin of the break in the X-ray lightcurve.
   – Inferred GRB properties.
• Summary.
• Future work…
            An Introduction to GRBs.
                (The largest explosions of the Cold War)

• GRBs were first discovered by the Vela Satellites in the
  1960s.




•   Extremely violent events.
• GRBs are now known to be extra-galactic events.
   • Detection of X-ray afterglows by Beppo-Sax in 1997 - positional
   accuracy few arcmin rather than degrees.
   • Allowed for optical follow-up and detection of optical afterglows.
   • Host galaxies!
• Extra-galactic, isotropic release of energy in the range of
~1053 ergs.
              GRBs Continued…
• What could cause this release of energy?
• The short rise time of the bursts multiplied by the
  speed of light results in a characteristic length
  comparable to the size of neutron star (NS), ctrise
  ~30km.
• However, NSs or even a Black hole (BH) could not
  produce 1053 ergs.
• GRBs are beamed and outflow is relativistic → Eγ ~
  1051 ergs.
                  The Suspects.
• There are two ‘populations’ of GRBs; ‘Long’ (T90 >
  2s) and ‘Short’ (T90 < 2s) duration.




    SHORT BURSTS                 LONG BURSTS
NS-NS or NS-BH collisions.        Hypernovae.
                   High redshift GRBs.
• GRBs are extremely energetic events and are expected to be
  visible out to z ~ 15-20 (Lamb & Reichart, 2000, ApJ, 536, 1), which is
  further than that obtainable by quasars (zmax ~ 6).
• Allow us to;
    – Locate high redshift host galaxies.
    – Map out star formation, since long duration GRBs are likely caused by
      the core collapse of massive stars.
    – Probe the environment immediately around the GRB.
    – Composition of the host galaxy.
    – Probe the IGM.
    – Potential evolution of GRB properties and therefore progenitors.
    – Potential use of GRBs to derive an extended z Hubble-diagram.
CURRENT WORK:
 GRB 050505
                                 BAT Data
• γ-ray lightcurve shows a
  multi-peaked structure.
• T90 = 63 ± 2 s
• Total fluence in the 15-350
  keV band = (4.1±0.4) x10-6
  ergs cm-2.


                            T90 15-150 keV spectrum;

                                            Cutoff PL (χ2/DOF = 45/55)
   Simple PL   (χ2/DOF   = 48/56)
                                            Photon index = 1.02 (+0.51/-0.57)
   Photon index = 1.56 ± 0.12.
                                            Epeak > 52 keV.
                           UVOT Data
• Initial data (limited to 100s exposures) found the following limits;
   – V > 17.7
   – U > 18.4
   – UVW1 > 18.9
   – UVM2 > 19.7

• A deeper exposure (2527 s) in V gave a limiting mag of > 20.35 (3σ).


               Ground Based Follow-up
• The first reported detection of an optical counterpart for GRB 050505 was
made by Cenko et al. (GCN 3366) – Keck I.

• Same collaboration also reported the redshift of 4.27 (Berger et al, GCN
3368).
                         XRT Data
• The automated slew was
  delayed by ~47 minutes due to
  an Earth limb observing
  constraint.

• No evidence of spectral
  variation during Swift
  observations.


• There is significant excess absorption in the spectrum (χ2/DOF =
  99/96).
   – Photon index = 1.90 ± 0.08.
   – Excess NH from Host galaxy = 1.28 (+0.61/-0.58) x1022 cm-2.
Break in the X-ray lightcurve
           1  0.66 0..11
                     0 09




                                         2  1.76   0.09
                                                     0.07


      1.10  0.19
    214 ks
      3           (Observer’s frame)

    3.9 0..7 ks
        0 5
                   (Burst frame)
      Break: End of continuous energy injection?
•      Joint BAT – XRT
       lightcurve appears to fit
       the canonical structure.
• Shallow decay;
(1) Decreasing Lorentz
    factor, or,
(2) Long lasting central
    engine activity.
      (Nousek et al. 2005, ApJ, submitted.)



    (1)   E ( )   s 1               requires s>1. Nousek et al. find s = -16.7±4.6.
    (2)   L  tlab
                q
                                         requires q>-1. Nousek et al. find q = 0.3±0.1.

            NOT THE END OF ENERGY INJECTION.
              Break: Structured jet?
• dE/dΩ = (θ/θc)-q.
• Assuming a p of 2.2 and using observed Δα, we
  can calculate a value of q (Panaitescu 2005, MNRAS, 362, 921).
• BUT eqns are only valid if q < q˜ .
• We find that q > q˜ .
• In the limit of q > q˜, Δα = ¾ (homogeneous) or
  ½ (wind).
              NOT A STRUCTURED JET.
          Break: Cooling Break?

• A cooling break should give Δα = ½
  (Sari et al. 1998, ApJ, 497, L17).




• Not consistent with observed Δα = 1.10 ± 0.19.


           NOT A COOLING BREAK.
                           Break: Jet Break?
 • A jet break occurs when the fireball Lorentz factor γ ~ 1/θj.
 • .Δα = 1.10 ± 0.19, is typical of a jet break (e.g. Chevalier & Li 2000, ApJ, 536, 195).
 • No spectral variation over break.
 • Can’t confirm a break in the optical.
 • Temporal index post-break = -p (Rhoads 1999, ApJ, 525, 737).


               p                                    p 1
              , ( x   c )                         , ( x   c )
               2                                     2
• β from measured spectral index gives p = 1.8 (νx > νc) or 2.8
  (νx < νc).
• .p = 1.8 (νx > νc) is in good agreement with α2 = 1.76 (+0.09/-
  0.07).
                Burst Properties (1)
• z = 4.27, so Eiso (15-350 keV) =
(1.45 ± 0.14) x1053 ergs.
• θj = 1.7o - 3.0o.
• Eγ = 6.3 x1049 - 2.0 x1050 ergs.
• Eγ = 1.33 x1051h65-2 ergs   (Bloom et
  al 2003, ApJ, 594, 674).

• Jet break after Swift’s
  observations: Eγ > 1.26 x1050 -
  2.24 x1051 ergs.
Burst Properties (2)

       • Epeak,obs > 52 keV.
       • Ghirlanda relation Epeak
          = .69 (+15/-14) keV for n = 1 cm-3,
          = .155 (+23/-25) keV for n =100 cm-3.


       • Amati relation Epeak = 112 ± 6
         keV.
       • Both relations consistent with
         observed lower limit.
                         SUMMARY
• Brief introduction GRBs.

• Importance of high redshift bursts.

• Presented the multi-wavelength data of GRB 050505.
• No spectral variation over observation.
    – Photon index = 1.90 ± 0.08,
    – Excess NH from Host galaxy= 1.28 (+0.61/-0.58) x1022 cm-2.
• Discussed the potential origins of the break in the X-ray lightcurve and
  we conclude that it is most likely a Jet break.
• The redshift of this burst allowed us to calculate Eγ = 6.3 x1049 – 2.0
  x1050 ergs, which is comparatively low w.r.t. previous bursts.
• Lower limit on Epeak is consistent with both the Ghirlanda and Amati
  relations.
                FUTURE WORK
• Short term:
   – Continued analysis of the GRB 050505 lightcurve →
     publication of “GRB 050505: A high redshift burst
     discovered by Swift” (first author).
   – X-ray analysis of GRB 050717 → co-authorship.

• Mid term:
   – Lightcurve flares.
   – Continuing as a Leicester ‘Burst Advocate’ and ‘X-ray
     Burst Scientist’.

• Long term:
   – With the pace of GRB research … ???

								
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