1. GRB afterglows as background sources for WHIM absorption studies A. Corsi, L. Colasanti, A. De Rosa, L. Piro IASF/INAF - Rome WHIM and Mission Opportunities - Rome, 2006 January 17-18

2. How to detect the WHIM WHIM: Numerical simulations predict that at z < 1, most of the baryons fall onto the cosmic dark matter web pattern and, heated by shock mechanisms, form filamentary and sheet-like structures, closely tracing the dark matter structures (Cen & Ostriker 1999; Dave’ et al. 2001) One promising method is searching for the narrow absorption features that the WHIM imprints on the X-ray spectrum of a bright background object e.g. of background X-ray sources:  bright blazar PKS2155-304 at z = 0.11  fluence of ~ 2.4  10 -5 erg cm -2 in 62.7 ks observation during a phase of high level flux ( ~ 4.2  10 -10 erg cm -2 s -1 in the 0.3-6 keV band, Nicastro et. al 2002);  blazar Mkn 421 at z = 0.034  fluence of ~ 1.2  10 -4 erg cm -2 in 100 ksobservation during a phase of high level flux ( ~ 1.2  10 -9 erg cm -2 s -1 in the 0.5-2.0 keV band, Nicastro et. al 2005);  blazar Mkn 421 at z = 0.034  fluence of ~ 1.2  10 -4 erg cm -2 in 100 ks observation during a phase of high level flux ( ~ 1.2  10 -9 erg cm -2 s -1 in the 0.5-2.0 keV band, Nicastro et. al 2005); WHIM: detectable in the soft X-rays since 10 5 K < T < 10 7 K Can we use GRBs as background sources?

3. GRBs: energy and distances Mean redshift: = 2.8 = 2.8 Prompt fluence: = 2.4  10 -6 erg cm -2 = 2.4  10 -6 erg cm -2  80% of GRBs have X-ray afterglows with fluences between 0.1 and 10 times the prompt fluence 10 -7 erg cm -2 < F 0.3-10 keV < 10 -5 erg cm -2 The brightest (L iso =10 53-54 erg/s) and most distant sources in the Universe (z = 0.16 - 6.3): sufficiently bright and distant to be good candidates as background sources to detect WHIM absorption features (O’Brien et al. 2006) (Jackobsson et al. 2005)

4. X-ray afterglow fluence distribution: BeppoSAX results logFx @11hrs PGRB with F  Fx Fx@60s (erg/cm2/s) Fluence (60 s < t < 60 ks) (erg/cm2) -12.2542%2.4 x 10 -9 4 x 10 -7 -11.48%1.7 x 10 -8 3 x 10 -6 -11.13%3.3 x 10 -8 6 x 10 -6 Using the BeppoSAX sample (De Pasquale et al. 2006), we can compute the fraction of bursts having an observed X-ray flux @ 11 hrs  F x  = -1.3 Using the typical shape of a BeppoSAX light curve (power- law with decay index = -1.3), we can extrapolate the flux @ 60 s and compute the fluence between 60 s and 60 ks = -1.3), we can extrapolate the flux @ 60 s and compute the fluence between 60 s and 60 ks N GRB per yr for FOV=3sr 30 10 3 Using the mean value for the ratio prompt X-ray fluence / X-ray afterglow flux @ 11 hrs and the WFC logN-logS (e.g. L. Colasanti, PhD thesis), we can compute the number of bursts per yr having a FOV of 3sr

5. Is Swift confirming BeppoSAX results on the afterglow fluence distribution? Log(Fx) @11hrs P (NGRB with F>Fx) 2 -12.25 44% -11.4 14% -11.1 2% 2 fraction of GRB with an afterglow flux larger than Fx in a sample of 36 SWIFT GRBs (O’Brien et al. 2006) 42 % 8 % 8 % 3 % SWIFT BeppoSAX With the same flux at 11 hrs, the X-ray fluence of a typical XRT light curve is ~ 2 times the BeppoSAX one (calculated using a power-law of index -1.3) because much power is in the early emission t (s)% 60074 600084 6000096 t (s)% 60050 600075 6000087 SWIFT BeppoSAX Simple parametrization of XRT light curves excluding flares: broken power-law Zhang et al. 2005 (O’Brien et al. 2006)

6. Counts NFI-TES (0.1-10 keV) Cts/eV @0.5 keV EWmin (eV) @0.5 keV 2 x 10 5 5330.3 2.6 x 10 6 37500.1 5.4 x 10 6 75000.08 logFx @11hr s PGRB with F  Fx N GRB per yr for FOV=3sr Fx@60s (erg/cm2/s) Fluence (60 s< t <60 ks) (erg/cm2) -12.2542 %302.4 x 10 -9 4 x 10 -7 -11.48 %101.7 x 10 -8 3 x 10 -6 -11.13 %33.3 x 10 -8 6 x 10 -6 Estimated using BeppoSAX results, confirmed by Swift Using the typical spectral shape of an X-ray afterglow we can compute, for different values of the afterglow luminosity, the expected counts for 60 ks integration time Knowing the number of counts from the continuum (X-ray afterglow) we can compute the minimum EW to have a detection at the 5  level, having an energy resolution  E= 2 eV: S/N = EW  (Counts/eV @ 0.5keV /  E) 0.5  5 WHIM absorption features: line @0.5 keV on a background GRB afterglow

7. GRBs as background sources for WHIM absorption studies: conclusions Narrow absorption lines expected to be produced by the WHIM at z < 2: using bright GRB afterglows as background sources gives the advantage, with respect to AGNs, to reach out much larger distances, increasing the number of filaments through the line of sight. For a burst at z> 0.5, at least one system with an EW  0.4 eV is expected along a random line of sight, while more (8) are expected for just twice weaker systems (e.g. Hellsten et al. 1998). EWmin (eV) @0.5 keV 0.3 0.1 0.08 N GRB per yr for FOV=3sr 30 10 3

8. Dark matter & WHIM: X-ray forest  Structure simulation from Cen & Ostriker (1999) Simulations of WHIM absorption features from OVII as expected from filaments (at different z, with EW=0.2-0.5 eV) in the l.o.s. toward a GRB with fluence=4  10 -6 as observed with ESTREMO (in 100 ksec). About 10% of GRB (10 events per year per 3 sr) with ~ 4  10 6 counts in the TES focal plane detector

9. WHIM emission lines detection: some estimates  Filament completely filling the FOV: N L  (A eff ) x (FOV) x T;  The contribution of photons from XRB and Galactic emission is determined by the energy resolution  E: N c   E x (A eff ) x (FOV) x T;  S/N = N L /N c 0.5  [ ((A eff ) x (FOV) x T) /  E ] 0.5 : a larger Aeff x FOV increases the number of counts for a given integration time, but a higher energy resolution gives a better S/N. A eff x FOV (deg 2 cm 2 )  E (eV) S/S XMM S/N/S/N XMM XMM(pn)2306011 Chandra231000.10.24 XEUS8.3 (210)1 (50)0.04 (0.9)1.5 (1.0) Con-X1220.051.2 DIOS5520.22.7 ESTREMO2020.092 NEW500228