ガンマ線バーストによる宇宙再電離の研究： GRB 050904 _ z=6.3 からの制限

							The First Constraint on the Reionization
 from GRBs: the Case of GRB 050904
                Tomonori Totani (Kyoto)
      Nobuyuki Kawai, George Kosugi, Kentaro Aoki,
         Toru Yamada, Masanori Iye, Kouji Ohta,
                   Takashi Hattori

            To Appear in PASJ (astro-ph/0512154)

         STScI workshnop “The End of the Dark Ages”
                     2006, March 13-15
     Why GRBs? a Unique Reionization Probe
   Brightness
        detectable to very high redshift of z>~10
        much brighter than LAEs/LBGs and even than QSOs if observed
         quickly enough


   less biased, probing more normal IGM
        most likely, simply tracing SFR
        QSOs and bright galaxies normally in strongly biased, clustered regions
        host galaxy luminosity irrelevant for GRB detection
        No proximity effect that is a major problem for quasars

   Clean spectrum
        almost single power-low SED, in contrast to QSOs or LAEs
    GRBs are the best astronomical source for searching the red
    damping wing of HI absorption in IGM
     Probing the Reionization by the Red Damping
                         wing
   GP trough in QSOs only gives a lower
    limit of xHI >~10-3

   The red damping wing, if detected, would




                                                          flux
                                                                 GP trough
    give a precise measurement of xHI
        since optically thin, it is insensitive to any                            red damping wing
         clumpiness of IGM


   However, this method is problematic in                                              wavelength
    QSOs, since:                                                    1215 (1+z) Å
        proximity effect
        complicated intrinsic spectra


   GRBs are an ideal object to search this
    red damping wing! (Milarda-Escude
    1998)
                  GRB 050904 @ z=6.3
   Swift Detection
       Cusumano et al. ’05
                         GRB 050904, 3.4 days
                         (Kawai et al. ’06)
   photometric colors of optical afterglows indicating z~6
       Haislip et al. ’05, Price et al. ’05, Tagliaferri et al. ’05


   optical spectroscopic observation by Subaru
       3.4 days after the burst, z’ = 23.7
       confirmed z=6.3 by: Haislip et al. ‘05
            clear Lyman break
            metal absroption line systems at z=6.295 ± 0.002
       Signature of the Damping Wing!
    Analysis of the Red Damping Wing to
         Constrain the Reionization

   two possibilities for the origin of the damping wing:

       A damped Lyα system (DLA) associated with the host
        galaxy
            DLAs often found in GRB afterglows
            log NHI (cm-2) ~ 21-22         GRB 030323 at z=3.372
                                                (Vreeswijk et al. 2004)

       IGM neutral hydrogen (damping wing of GP trough)
            If detected, it would give a crucial information on xHI
                              Model Fitting
   Intrinsic GRB afterglow spectrum
       β= - 1.25±0.25 power-law (Fν∝ν－β)
            ~0.5 day observations by Haislip et al., Tagriaferri et al.
       spectral change by cooling frequency break passage is unlikely
       no evidence for extinction  assume AV=0 at host (check later)
       Galactic Extinction: E(B-V) = 0.066 mag

   Absorptions by DLA and IGM
       4 model parameters: zDLA, NHI, zIGM,u, xHI
       DLA: τ= NHI σ[(1+z)νobs]
            damping width >> reasonable velocity dispersion (<~200 km/s)
       IGM: Formula of Miralda-Escude 1998
            uniform IGM distributed from zIGM,l to zIGM,u
            zIGM,l = 6.0 assumed in the baseline model
      redshift range of IGM contributing the
               damping wing at z~6.3
   more than 80% (half) of IGM
    optical depth is by IGM at z > 6
    (6.2)




                                       (zIGM,u = 6.3)
                          zDLA versus zmetal
   zmetal = 6.295 ± 0.002 (no other redshift around z~6.3) ‘03
                                                       Mirabal+
        fine-structure Si* detected  ~pc scale from the burster


   zDLA vs. zmetal?
        zDLA = zmetal : straightforward, the baseline model
             [S/H] ~ -1.3, [Si/H] ~ -2.9 for log NHI ~ 21.6
             (large Si depletion is not rare in GRB afterglows)
        zDLA = zhost, but different from zmetal
             host galaxy must be very low metallicity of [Si/H] < -3 ... likely?
             | zDLA – zhost | < ~ 0.005 (~200 km/s)
             zmetal – zhost < ~ 0.07 (~3,000 km/s, by accelerated metal absorbing shell
              around the GRB)
                                    Vreeswijk et al. ‘04
        -0.005 < zDLA – zmetal < +0.07
                       zIGM,u versus zmetal
   zIGM,u vs. zmetal?
       zIGM,u vs. zhost
            ionized bubble size: zIGM,u – zhost ~ 0.02 (LAEs), 0.003 (theory for
             typical GRBs at z~6; Barkana & Loeb 2004)
            IGM gas infall to GRB host galaxies: negligible (Δz < 0.001)


       -0.02 < zIGM,u – zmetal < +0.07
            +0.07 comes from the possible accelerated shell for zmetal
         Fitting Results: DLA? IGM? (1)




Totani et al. 2005
                     DLA model with z=6.295, log NHI = 21.62
               Fitting Results: DLA? IGM? (2)
   Both the DLA and IGM models can
    explain the damping wing
    (degeneracy!)                               zmetal = 6.295±0.002
       marginally zDLA=zmetal allowed for
        DLA
       If IGM, zmetal must be blueshifted by
        about 3,000 km/s
            possible in GRBs!




                                                First detection of almost
                                                neutral IGM!?
               Discrimination by LyβFeature
   The degeneracy can be broken
    by Lyβ profile!
       The IGM model inconsistent
        with the data
       upper limit zIGM,u < 6.314
       DLA is dominant under this
        constraint
                                              DLA
   the plausible model:                      (zDLA=6.295)
    zDLA=zmeteal = 6.295                       IGM
       xHI = 0 consistent with the data       (zIGM,u=6.36)
       but xHI=1 with zIGM,u=6.295
        would affect the damping wing
                             Constraint on xHI?
   What xHI is preferred, when NHI is
    treated as a free parameter without any
    prior?                                                zero level: best fit with xHI=0
                                                          solid:      best fit with xHI=1
       zIGM,u = zDLA = 6.295

       best fit xHI = 0.00 ± 0.17           The data obey to Gaussian well
       xHI < 0.60 (95% C.L.)

       The first upper limit on xHI at z>6!
                   Uncertainty Check
   check done for spectral index, dust in host galaxy, redshift
    parameters, weak absorption lines, and time variability




                                           Totani et al. 2006, PASJ in press
            Comparison with other             Malhotra & Rhoads ’05
                                              Stern et al. ’05
                constraints                   Haiman & Cen ‘05
   The constraint of xHI<0.6 at z=6.3        LAE statistics
                                             GRB 050904
    is consistent with those derived
    from QSO (HII region size etc.)
    and LAE statistics

   The strength of the GRB
    constraint :
        derived directly from absorption
         optical depth (like GP trough)
        model uncertainty is very small
        insensitive to clumpiness, giving
         mass-averaged xHI along the sight
                                               Fan et al. 2006
         line
        Prospects for future observations

   low NHI GRBs?
       only weak constraint on xHI when log NHI > 21.5
       However, there are GRBs with log NHI <~ 20
       promising chance for a better constraint on xHI




                                                        solid: DLA
                                                        long-dashed:
                                                         IGM+DLA(20.0)
                                 Vreeswijk et al. ‘04
    Prospects for future observations (2)
   How often do we expect events like GRB 050904?
       spectrum of GRB 050904 was 3.4 days. It would be x10 brighter if
        0.5 day
       If GRB 050904 occurred at z=1, R=17.9, F~0.2 mJy at 1 day  one of
        the brightest optical afterglows
       NIR flash of GRB 050904 was as bright as the opt. flash of GRB
                                             GRB050904
        990123 （Boer et al. 05)  rare object?
       NIR spectroscopy necessary at z>7  lower sensitivity than optical




                                                        Panaitescu & Kumar 01




        Fynbo et al. 01
                           Conclusions
   GRB 050904 gave us the first constraint on reionization from
    GRBs
      Opened a new era of GRB cosmology



   degeneracy: the red damping wing can be explained by DLA
    or IGM
       Lyβ feature can be used to break this degeneracy
       In the case of GRB 050904, the DLA is dominant


   Lower xHI at z~6-6.3 is preferred from our data
       xHI = 0.00 ± 0.17， xHI < 0.60 (95% C.L.)
       The IGM was largely reionized at z=6.3
       The first quantitative upper limit on xHI at z>6 by a direct method
Identified Absorption Lines
                  Kawai et al. 2006
        Prospects for future observations

   low NHI GRBs?
       only weak constraint on xHI when log NHI > 21.5
       However, there are GRBs with log NHI <~ 20
       promising chance for a better constraint on xHI


   Lyα line emission from host galaxies?
       line emissivity affected by xHI , giving a probe for reionization
                                                         solid: DLA
       GRB host galaxies may have strong Lyα emission (in EW) (Jakobsson
                                                         long-dashed:
                                                          IGM+DLA(20.0)
        et al. 2005)
                                    Vreeswijk et al. ‘04
       Lyα emission search for GRB host galaxies may also be interesting
       For GRB 050904, we set upper limit on Lyα corresponding to SFR <
        0.8 Msun/yr