The First Constraint on the Reionization from GRBs: the Case of GRB 050904

1. 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

2. 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

3. GP trough flux red damping wing wavelength 1215 (1+z) Å 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 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 effect • complicated intrinsic spectra • GRBs are an ideal object to search this red damping wing! (Milarda-Escude 1998)

4. GRB 050904, 3.4 days (Kawai et al. ’06) Haislip et al. ‘05 GRB 050904 @ z=6.3 • Swift Detection • Cusumano et al. ’05 • 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: • clear Lyman break • metal absroption line systems at z=6.295 ± 0.002 • Signature of the Damping Wing!

5. GRB 030323 at z=3.372 (Vreeswijk et al. 2004) 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 • IGM neutral hydrogen (damping wing of GP trough) • If detected, it would give a crucial information on xHI

6. 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

7. 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)

8. Mirabal+ ‘03 Vreeswijk et al. ‘04 zDLA versus zmetal • zmetal = 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 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) • -0.005 < zDLA– zmetal < +0.07

9. 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

10. Fitting Results: DLA? IGM? (1) Totani et al. 2005 DLA model with z=6.295, log NHI = 21.62

11. Fitting Results: DLA? IGM? (2) • Both the DLA and IGM models can explain the damping wing (degeneracy!) • marginally zDLA=zmetal allowed for DLA • If IGM, zmetal must be blueshifted by about 3,000 km/s • possible in GRBs! zmetal = 6.295±0.002 First detection of almost neutral IGM!?

12. 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 • the plausible model: zDLA=zmeteal = 6.295 • xHI = 0 consistent with the data • but xHI=1 with zIGM,u=6.295 would affect the damping wing DLA (zDLA=6.295) IGM (zIGM,u=6.36)

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

14. 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

15. GRB 050904 LAE statistics Malhotra & Rhoads ’05 Stern et al. ’05 Haiman & Cen ‘05 Comparison with other constraints • The constraint of xHI<0.6 at z=6.3 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 line Fan et al. 2006

16. solid: DLA long-dashed: IGM+DLA(20.0) Vreeswijk et al. ‘04 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

17. GRB050904 Panaitescu & Kumar 01 Fynbo et al. 01 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 990123 （Boer et al. 05)  rare object? • NIR spectroscopy necessary at z>7  lower sensitivity than optical

18. 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

19. Identified Absorption Lines Kawai et al. 2006

20. solid: DLA long-dashed: IGM+DLA(20.0) Vreeswijk et al. ‘04 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 • GRB host galaxies may have strong Lyα emission (in EW) (Jakobsson et al. 2005) • 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