Irradiated accretion disk emission from an ultrasoft AGN?

1. permitted line emission hot corona 5x1016cm Irradiated accretion disk emission from an ultrasoft AGN? Elizabeth Puchnarewicz and Roberto Soria Mullard Space Science Laboratory University College, London When Beppo-SAX measured the 0.1 to 12keV spectrum of REJ1034+396, observations in the optical (WHT), UV (HST) and EUV (EUVE) were also taken within a few weeks. This study (Puchnarewicz et al. 2001) placed very strong constraints on its unusually hot big blue bump component, which has been attributed to a high rate (~ Eddington) of accretion onto a disk surrounding a low mass black hole (Pounds et al 1996). The optical to UV continuum is flat with no evidence for host galaxy or dust contamination. This residual emission is not consistent with the cool tail of an accretion disk and its origin is not clear. We propose that the optical/UV flux is produced when the outer parts of the warped accretion disk are irradiated by the central UV/X-ray source. The X-ray spectrum is consistent with Comptonization in a surrounding, hot corona. The narrow permitted line profiles, which typify ultrasoft AGN in general (Puchnarewicz et al 1992), are consistent with emission from an irradiated accretion disk wind. The spectrum of the NLS1 REJ1034+396 from the near-IR to X-rays. Also plotted as a red curve is the model of an irradiated optically-thick, geometrically-thin accretion disk. The high energy tail includes the effects of Comptonization. Soria R. & Puchnarewicz E. M., MNRAS, in press astro-ph/0108492 The unusually hot ‘big blue bump’ A warped, or flared, accretion disk in the NLS1 REJ1034+396 In a geometrically-thin, optically-thick accretion disk, at a given radius R, the temperature Tn is proportional to R-3/4. However, if the disk is irradiated, then the temperature, Tirr, is proportional to R-1/2 and irradiative heating will dominate over viscous heating at large radii. If the disk is flared or warped then emission due to irradiation will be relatively strong in the optical/UV, where it intercepts more of the ionizing continuum. The spectrum will then flatten at these wavelengths. This provides a straightforward solution to the origin of the flat optical/UV continuum. The spectrum of REJ1034+396 is best represented by a flared disk with inner radius, Rinn~6x1011cm and outer radius, Rout~5x1016cm ( = 3x105 GM/c2 ; see top left). The region of the disk which is dominated by irradiative heating is at R > 80Rinn. The black hole mass, M ~ 106 Msun. At this mass, the value of the innermost stable orbit from this fit, Rinn, is consistent with a Kerr black hole. The fits allow a range of viewing angles so that the disk may be viewed close to the disk axis (up to ~20degrees away). This removes the problem of having to view the AD through a co-planar molecular torus and is consistent with suggestions that ultrasoft AGN are seen relatively face-on (Puchnarewicz 1992). An illustration (not drawn to scale!) of a flared accretion disk in REJ1034+396. The optical/UV continua of most AGN rise towards the blue with a slope a~0.4 while the soft X-ray spectrum falls towards high energies with a~2 (e.g. Laor et al. 1997). The spectrum seems to peak in the unobservable EUV and this continuous, optical to soft X-ray feature, known as the ‘big blue bump’, is believed to represent the emission from a geometrically thin, optically thick accretion disk. However, the optical to X-ray continuum of REJ1034+396 is highly unusual for a Seyfert 1 galaxy. The optical/UV continuum is flat (a~1) with no sign of the big blue bump down to Lya.At ~0.1keV, the soft X-ray spectrum is very strong above the extrapolated level of the optical/UV continuum, peaking at ~0.3keV then falling steeply towards higherenergies (Puchnarewicz et al. 1998). Puchnarewicz et al. (2001) showed that the AD component is one of the hottest observed in AGN, so that its high-energy turnover is measurable in the soft X-rays rather than the EUV. They inferred a high accretion rate (L~0.3-0.7LEdd), a small black hole mass, M~106 Msun and a viewing angle of 60-70degrees. Thus they concluded that REJ1034+396 had a low-mass black hole accreting close to the Eddington limit. However, the AD model preferred an edge-on view but this line of sight would pass through a co-planar torus. Emission lines from the disk? The Balmer lines and UV lines in this NLS1 have full widths at half maximum (FWHM) of 1500-1800km/s and exhibit broad and narrow components. The disk-wind model of Murray & Chiang (1997) predicts a broad emission line component emitted from an irradiated disk, close to the surface at small radii, while a narrow component is emitted from the photoionized wind. Thus an irradiated disk may predict the permitted line profiles as well as the optical to X-ray continuum. OM References Murray & Chiang 1997, ApJ, 474, 91 Pounds et al. 1995, MNRAS, 277, L5 Puchnarewicz et al 1992, MNRAS, 256, 589 Puchnarewicz et al 1995, MNRAS, 276, 20 Puchnarewicz et al 2001, ApJ, 500, 644 Soria & Puchnarewicz 2001, MNRAS, in press www.mssl.ucl.ac.uk