{
"subject": "Tentative redshift of GRB100704A from Swift-XRT data",
"bibcode": "2010GCN.10940....1C",
"createdOn": 1278431120000,
"circularId": 10940,
"submitter": "Sergio Campana at INAF-OAB ",
"email": "sergio.campana@brera.inaf.it",
"body": "S. Campana (INAF-OAB) and D. Grupe (PSU) report on behalf of the \nSwift-XRT team:\n\nSwift XRT observed the bright GRB 100704A (Grupe et al. 2010, GCN 10929)\nstarting 86 s after the BAT trigger. After strong X-ray flares the light \ncurve started decaying\nfollowing a power law (Grupe 2010, GCN 10935).\nWe selected the time interval 4,510-28,896 s after the burst onset to \ncollect the Photon\nCounting (PC) spectrum. During this interval the spectrum is almost \nconstant as testified by\na constant power law photon index (chi2=102 with 123 degrees of freedom).\n\nWe fit the PC spectrum (comprising 2002 counts) with a (Galactic plus \nintrinsic) absorbed\ncutoff power-law model (tbabs*ztbabs*cutoff) within XSPEC (the cutoff \npower-law model\nprovides much better results in terms of column density evaluation with \nrespect to a simple\npower-law model when small spectral variations are present).\nWe assume a Galactic column density of 1.1x1021 cm-2 (Kalberla et al. \n2005, A&A 440 775)\nand allowed for a 30% variability given its high value (and sampling all \nthe values obtained\nwithin 1 degree from the source).\nWe fit the spectrum using C-statistics. In the intrinsic column density \nvs. redshift plane there\nis a low significance (3 sigma) solution at low redshifts but the \nminimum lies at higher redshifts\nwith a 2 sigma confidence level solution of z=3.6^+1.1 _-1.2 and N_H \n(z)=(9.7^+9.9 _-5.6 )x10^22 cm^-2 .\nThus, even if we cannot exclude that GRB100704A is at low redshift, a \nhigh redshift (z~3.5),\nhighly absorbed (N_H (z)~1x10^23 cm^-2 ) solution is preferred.\nThis compares well with the non-detection with UVOT (Kuin and Grupe \n2010, GCN 10934)\nand MOA robotic telescope (Omori et al. 2010, GCN 10931).\nWe also note that if no interviening systems are present along the line \nof sight a 90% lower\nlimit on the metallicity of 0.03 solar can be set (based on the \nassumption that the absorbing\nmedium is not Thomson thick).\n\nThe contour plot is available at\nhttp://www.brera.inaf.it/utenti/campana/100704.gif"
}