Gamma-Ray Bursts and X-ray afterglows Luigi Piro Istituto Astrofisica Spaziale Fisica Cosmica, INAF, Roma L. Piro - Moriond 2005

Outline Prompt vs afterglow observational signatures. Fireball model Environment and progenitor: X-ray lines X-ray absorption, Wind vs ISM Late-time rebursting Precursors Deviations from standard fireball model Dark GRB X-Ray Flashes L. Piro - Moriond 2005

The deepest mystery Out of 40 GRB localized by BSAX, about 30 went off during Italian night time, week end, holydays One (GRB980703) during the penalties of the world championship soccer game ITALY vs FRANCE Terrestrial origin? L. Piro - Moriond 2005

GRB970228: the 1st X-ray and O afterglow Triggered by GRBM and localized by WFC Fast follow up with NFI in 8 hrs: a bright unknown X-ray source A second pointing 3 days after the GRB: fading X-ray counterpart (Costa et al 1997) Optical fading source (van Paradijs et al 1997) L. Piro - Moriond 2005

Power laws: the hallmark of afterglows F≈ t-d n-a dx≈ 1.4; ax ≈ 1.0 GB000926:Piro et al, 01 Costa et al 97 Piro et al 99 L. Piro - Moriond 2005

Prompt vs afterglow emission: internal vs external shock In contrast with the afterglow, the prompt emission is characterized by strong hard-to-soft spectral evolution from X- to Gamma rays (e.g. GRB960720 Piro et al 1997) L. Piro - Moriond 2005

Prompt vs afterglow emission Afterglow: power-law a=1; no spectral variation Prompt: hard-to-soft L. Piro - Moriond 2005

Testing the fireball model 1016 cm taft X-ray afterglow 1013 cm 106 cm g RB tGRB G1 G2 t-1 L. Piro - Moriond 2005

The progenitors of GRB The nature of the progenitor can be inferred from the environment NS- NS merging NS-NS (BH-NS & BH-WD) travel far from their formation sites before producing GRB’s (Fryer et al 2000) => “clean environment”: no lines Hypernova Hypernovae/collapsar evolve much faster, going off in their formation site => “mass-rich environment” => lines L. Piro - Moriond 2005

Iron features GB970508 (Piro et al 1999) GB000214 (Antonelli et al 2000) GB991216 (Piro et al 2000) GB990705 (Amati et al 2000) GB980828 (Yoshida et al 1999) L. Piro - Moriond 2005

The GRB-SN connection furtherly confirmed by the detection of He/H-like Mg, Si, S, Ar metal lines blueshifted at v/c=0.1 in the afterglow spectra of GRB011211 (by XMM, Reeves et al 02), GRB020813 (by Chandra, Butler et al 03) and GRB030227 (by XMM, Watson et al 03) Soft X-ray lines L. Piro - Moriond 2005

Line models vs time behaviour In the distant reprocessor scenario (=preSN explosion= Supranova): line intensity=const => line EW (vs continuum) increases with t: lines detectable after t>few hours (not easily detectable by SWIFT) In the “local” reprocessor scenario (Meszaros& Rees2000,2003): line EW should be roughly constant = visible after few minutes (SWIFT:OK) L. Piro - Moriond 2005

X-ray absorption X-ray absorption column densities in the afterglow: NH=1021-22 cm-2 (Stratta et al, ApJ03) Powerful probe of the environment of GRB and of the medium in the line of sight to the GRB L. Piro - Moriond 2005

GRB, star forming region & standard fireball model Association of GRB with star-forming regions: X-ray lines Distribution of OT location in their host galaxies (Bloom et al) SN-GRB connection X-ray absorption column densities consistent with NH=1021-22 cm-2 in GMC Since the typical density in a GMC is n=102-104 cm-3 why the density derived from the standard fireball (e.g. Panaitescu, Kumar et al..) model is 3-4 orders of magnitude lower ? Wind ejection by progenitor. Wind environment is expected from progenitor (collapsar, in particular) but most afterglows are consistent with constant density profile .. L. Piro - Moriond 2005

GRB (high G) XRF (low G) X-ray lines (local reprocessor scenario (Mesaros & Rees 00)); Thermal precursor (Ramirez-Ruiz et al 02) Collapsar model L. Piro - Moriond 2005 Woosley et al

Wind vs ISM Wind ISM Telltale of wind: X-ray decay SLOWER than optical (no< nc < nx ) by 0.25 Distribution of dx-dO From BSAX + Optical ISM preferred Wind cases: 040106 (Gendre et al A&A, 2004), GRB011121, XRR011211 (Piro et al ApJ05), XRF011030 (Galli tak) L. Piro - Moriond 2005

GRB011121 X-ray precursor Hard prompt emission X-ray rebursting Late afterglow Piro et al. 05, ApJ, in press. L. Piro - Moriond 2005

GRB011121 Wind suggested e.g. by Price et al (O+R) BSAX data (Piro et al, ApJ 05) Afterglow: a=1 Prompt: hard-to-soft L. Piro - Moriond 2005

GRB011121: late afterglow onset in a WIND Setting t0 at the beginnig of the rebursting: Power law dX=1.29 ± 0.04 vs dO=1.66 ± 0.06 (Price et al 02), consistent only with WIND Interpretation + other examples: talk by A. Galli L. Piro - Moriond 2005

Deviations from standard model GRB010222 (in’t Zand 2002) and GRB990123 (Maiorano et al 2005, Corsi et al 2005) do not fulfill closure relationship AND the X-ray data are above the extrapolation of the optical spectrum IC component? (Corsi et al 2005) L. Piro - Moriond 2005

Dark GRB With BeppoSAX: 30 X-ray afterglow candidates out of 36 GRB follow up observations in X-rays (Piro et al 2004, De Pasquale et al 2004) X-ray afterglows > 90% Optical afterglows ~ 40% - 50% Radio afterglows ~ 35% - 40% L. Piro - Moriond 2005

Ly-a forest by intergalactic H clouds Spectrum of high-z quasar (z=5.73, Djorgovski et al 2001) Ly-a forest by intergalactic H clouds GRB00013C: z=4.5 (Andersen et al 01) L. Piro - Moriond 2005

Are there really dark GRB? Yes From the BSAX sample we find (De Pasquale, LP etal 03, ApJ) that Dark GRB are on average 6 times fainter in X-rays than OTGRB (explaining why HETE2 SXC localization lead to OTGRB) about 20% of dark GRB are “truly dark”, being their ratio of optical-to-X-ray fluxes smaller by a factor of about 6 compared to OTGRB Not consistent with the fireball model unless: OT heavily absorbed by star forming region ? Or located at z>5 (such that intragalactic gas will absorb photons below Lyman limit) L. Piro - Moriond 2005

OT GRB Dark L. Piro - Moriond 2005

Dark GRB000210: BeppoSAX & Chandra, ESO-VLT GRB localized by BeppoSAX. Simultaneous obs of the X-ray afterglow with Chandra. No OT >23.5 Deep VLT imaging and spectroscopy: z=0.86 (Piro et al ApJ 2002) L. Piro - Moriond 2005

X-ray flashes X-ray rich GRB/ X-Ray Flash: a new class discovered by BSAX and confirmed by HETE2: about 30% GRB’s with no or very faint or gamma-ray emission (Sx/Sg>1). Several are dark A different type of GRB’s or events at z>5-10? (Few events at z<3) or GRB seen off axis (unification scenario as in AGN…) Heise et al 2001 L. Piro - Moriond 2005

XRF 031220 Trigger by HETE 2, we carried out Chandra observation to pintpoint the location of the afterglow at 1” No OT, Extremely red host galaxy: R-K’=5.3 Note: GRB000131@z=4.5 R-K=3.7 (z>5 ?) Fitting with galaxy SED (vs stellar population, internal absorption, Lya absorption): z=1.9: => dark due to dust extinction (Melandri et al05) L. Piro - Moriond 2005

Origin of X-ray flashes We compiled a homogeneous BSAX+HETE2 sample of XRF By comparing the properties of afterglows of GRB vs XRF => strong implications on off-axis (and high z) models… talk by Valeria D’Alessio L. Piro - Moriond 2005

The quest for high-z GRB Why so much excitement? They can pinpoint obscured star-forming galaxies (X-rays and gamma-rays pierce through) and probe the region z=10-20 where the first stars & galaxies formed (current record holder is a qso at z=6.7) If GRB=SFR, about 20% of them at z>5 (Bromm & Loeb 03) Events at z>5 will not be visible in the optical range, (Lyalpha forest absorption): they have to be “dark” Most of the redshift are now derived from optical obs => strong bias against high-z GRB X-ray redshift or IR photometric/spectroscopy z L. Piro - Moriond 2005

Conclusions Massive star progenitor from the environment: X-ray lines, X-ray absorption, SN association Prompt vs afterglow emission: different spectral (and temporal) behaviour Wind vs ISM: comparison with optical decay slopes shows ISM is slightly preferred. Way out: wind termination shock produces a constant density region (Chevalier &Li 99). Nonetheless we have found so far 3 cases where the wind is preferred (GRB011121, 040106, XRF011030 New features: X-ray precursors and rebursting (about 10% on BSAX bursts) Precursor is NOT thermal black body (power law) L. Piro - Moriond 2005

Conclusions (II) Late-time (200-1000 sec) rebursting (in 3 cases) is identified with the onset of the afterglow. Same spectral index and falls on the same power law decay when t0 is set at the onset of the reburst (talk by Galli) About 20% truly dark/optically faint GRB Origin of XRF (talk by D’Alessio) GRB and cosmology L. Piro - Moriond 2005

Gamma-Ray Bursts: from Astrophysics to Cosmology International Space Science School: At L’Aquila (Italy) from Sept.12-16, 2005 Advanced school for PhD, Post Doc and young researchers: tutorials, lessons, and seminars in the different branches of Physics and Astrophysics relevant to the comprehension of the GRB phenomenon. GRB in Cosmology. Experiments and methods used in the field. A session devoted to data analysis of all instruments of SWIFT. L. Piro - Moriond 2005