Gravitational radiation from precessing accretion disks in gamma-ray bursts

We study the precession of accretion disks in the context of gamma-ray burst inner engines. Our aim is to quantitatively estimate the characteristics of gravitational waves produced by the precession of the transient accretion disk in gamma-ray bursts. We evaluate the possible periods of disk precession caused by the Lense-Thirring effect using an accretion disk model that allows for neutrino cooling. Assuming jet ejection perpendicular to the disk plane and a typical intrinsic time-dependence for the burst, we find gamma-ray light curves that have a temporal microstructure similar to that observed in some reported events. The parameters obtained for the precession are then used to evaluate the production of gravitational waves. We find that the precession of accretion disks of outer radius smaller than $10^8$ cm and accretion rates above 1 solar mass per second could be detected by Advanced LIGO if they occur at distances of less than 100 Mpc. We conclude that the precession of a neutrino-cooled accretion disk in long gamma-ray bursts can be probed by gravitational wave astronomy. Precession of the disks in short gamma-ray events is undetectable with the current technology.Comment: 5 pages, 5 figures, accepted for publication in A&

1WGA J2223.7-0206: a Narrow-Line Quasi-Stellar Object in the XMM-Newton field of view of 3C445

We report the discovery of a Narrow Line QSO located at about 1.3' from the Broad Line Radio Galaxy 3C445. The source,1WGA J2223.7-0206, although already revealed by ROSAT has never been optically identified previously. An XMM-Newton observation of 3C445 has allowed, for the first time, an accurate X-ray spectral study of 1WGA J2223.7-0206, revealing an ultra-soft spectrum and fast flux variations typical of Narrow Line AGN. The 0.2-10 keV spectrum is well represented by a power law (Gamma=2.5) plus a black body component (kT = 117 eV) absorbed by Galactic NH. About 80% of the X-ray flux is emitted below 2 keV. The 0.2-2 keV flux is observed to decrease by about a factor 1.6 in about 5000 s. The optical observations, triggered by the X-ray study, confirm the Narrow Line AGN nature of this source. The continuum is blue with typical AGN emission lines, pointing to a redshift z=0.46. The full width half maximum of H_beta is 2000 km/secand the flux rat io [OIII}]/H_beta=0.21. The optical luminosity (M_R=-23.2) and the point-like appearance in the optical images identify 1WGA J2223.7-0206 as a Narrow Line QSO. From the optical-UV-X-ray Spectral Energy Distribution we obtain a lower limit of the bolometric luminosity of 1WGA J2223.7-0206 (L_bol > 3 10^{45} erg/sec) implying, for accretion rates close to the Eddington limit, a black hole mass M_BH > 2.4 x 10^{7} M_{\odot}Comment: 6 pages, 5 figures, accepted by A&A, removed pag. 7 containing a duplication of Fig.