The First Constraint on the Reionization from GRBs:the Case of GRB 050904Tomonori 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 Ages2006, March 13-15

Why GRBs? a Unique Reionization ProbeBrightnessdetectable to very high redshift of z>~10much brighter than LAEs/LBGs and even than QSOs if observed quickly enough

less biased, probing more normal IGMmost likely, simply tracing SFRQSOs and bright galaxies normally in strongly biased, clustered regionshost galaxy luminosity irrelevant for GRB detectionNo proximity effect that is a major problem for quasars

Clean spectrumalmost 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 wingGP trough in QSOs only gives a lower limit of xHI >~10-3

The red damping wing, if detected, would give a precise measurement of xHI since optically thin, it is insensitive to any clumpiness of IGM

However, this method is problematic in QSOs, since:proximity effectcomplicated intrinsic spectra

GRBs are an ideal object to search this red damping wing! (Milarda-Escude 1998)

GRB 050904 @ z=6.3Swift DetectionCusumano et al. 05

photometric colors of optical afterglows indicating z~6Haislip et al. 05, Price et al. 05, Tagliaferri et al. 05

optical spectroscopic observation by Subaru3.4 days after the burst, z = 23.7confirmed z=6.3 by:clear Lyman breakmetal absroption line systems at z=6.295 0.002Signature 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 Ly system (DLA) associated with the host galaxyDLAs often found in GRB afterglowslog NHI (cm-2) ~ 21-22

IGM neutral hydrogen (damping wing of GP trough)If detected, it would give a crucial information on xHI

Model FittingIntrinsic GRB afterglow spectrum= - 1.250.25 power-law (F) ~0.5 day observations by Haislip et al., Tagriaferri et al.spectral change by cooling frequency break passage is unlikelyno evidence for extinction assume AV=0 at host (check later)Galactic Extinction: E(B-V) = 0.066 mag

Absorptions by DLA and IGM4 model parameters: zDLA, NHI, zIGM,u, xHI DLA: = NHI [(1+z)obs] damping width >> reasonable velocity dispersion (

redshift range of IGM contributing the damping wing at z~6.3more than 80% (half) of IGM optical depth is by IGM at z > 6 (6.2)(zIGM,u = 6.3)

zDLA versus zmetalzmetal = 6.295 0.002 (no other redshift around z~6.3)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 zmetalhost 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)-0.005 < zDLA zmetal < +0.07

zIGM,u versus zmetalzIGM,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 (z < 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. 2005DLA 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!)marginally zDLA=zmetal allowed for DLAIf IGM, zmetal must be blueshifted by about 3,000 km/spossible in GRBs!First detection of almost neutral IGM!?

zmetal = 6.2950.002

Discrimination by LyFeatureThe degeneracy can be broken by Ly profile!The IGM model inconsistent with the dataupper limit zIGM,u < 6.314DLA is dominant under this constraint

the plausible model: zDLA=zmeteal = 6.295 xHI = 0 consistent with the databut xHI=1 with zIGM,u=6.295 would affect the damping wingDLA(zDLA=6.295)IGM(zIGM,u=6.36)

Constraint on xHI?What xHI is preferred, when NHI is treated as a free parameter without any prior?zIGM,u = zDLA = 6.295best fit xHI = 0.00 0.17xHI < 0.60 (95% C.L.)The first upper limit on xHI at z>6!zero level: best fit with xHI=0solid: best fit with xHI=1

Uncertainty Checkcheck done for spectral index, dust in host galaxy, redshift parameters, weak absorption lines, and time variabilityTotani et al. 2006, PASJ in press

Comparison with other constraintsThe constraint of xHI

Prospects for future observationslow NHI GRBs?only weak constraint on xHI when log NHI > 21.5 However, there are GRBs with log NHI

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 dayIf GRB 050904 occurred at z=1, R=17.9, F~0.2 mJy at 1 day one of the brightest optical afterglowsNIR flash of GRB 050904 was as bright as the opt. flash of GRB 990123 Boer et al. 05) rare object?NIR spectroscopy necessary at z>7 lower sensitivity than optical

ConclusionsGRB 050904 gave us the first constraint on reionization from GRBsOpened a new era of GRB cosmology

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

Lower xHI at z~6-6.3 is preferred from our dataxHI = 0.00 0.17xHI < 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 LinesKawai et al. 2006

Prospects for future observationslow NHI GRBs?only weak constraint on xHI when log NHI > 21.5 However, there are GRBs with log NHI