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

2. 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

3. 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

4. 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

5. 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

6. 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. GRB Piro et al 1997)
L. Piro - Moriond 2005

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

8. 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

9. 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

10. Iron features GB970508 (Piro et al 1999)
GB (Antonelli et al 2000)
GB (Piro et al 2000)
GB (Amati et al 2000)
GB (Yoshida et al 1999)
L. Piro - Moriond 2005

11. 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 GRB (by XMM, Reeves et al 02), GRB (by Chandra, Butler et al 03) and GRB (by XMM, Watson et al 03)
Soft X-ray lines
L. Piro - Moriond 2005

12. 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

13. X-ray absorption
X-ray absorption column densities in the afterglow: NH= 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

14. 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= cm-2 in GMC
Since the typical density in a GMC is n= 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

15. 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

16. 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: (Gendre et al A&A, 2004), GRB011121, XRR (Piro et al ApJ05), XRF (Galli tak)
L. Piro - Moriond 2005

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

18. 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

19. 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

20. Deviations from standard model
GRB (in’t Zand 2002) and GRB (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

21. 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

22. 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

23. 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

24. OT GRB
Dark
L. Piro - Moriond 2005

25. 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

26. 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

27. XRF
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: 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

28. 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

29. 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

30. 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, , 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

31. Conclusions (II)
Late-time ( 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

32. 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