GALEX Measurements of the Big Blue Bump in Soft X-ray Selected AGN
📝 Abstract
We study the UV properties of Type I AGN from the ROSAT All-Sky Survey that have been selected to show unusually soft X-ray continua. We examine a sample of 54 Seyfert 1 galaxies with detections in both Near-UV and Far-UV bands of the Galaxy Evolution Explorer (GALEX) satellite. Our sample is systematically fainter in the UV than galaxies studied in similar work by previous authors. We look for correlations between their UV and X-ray properties as well as correlations of these properties with either black hole mass or Eddington ratio. The shape of the Big Blue Bump(BBB) in the GALEX regime does not appear to correlate with its strength relative to the power law continuum, which conflicts with results reported by previous authors. The strength of the BBB is correlated with the shape of the X-ray continuum, in agreement with previous work, but the slope of the correlation is different than previously reported. The properties of the accretion disks of Type I AGN in the GALEX regime are relatively independent of black hole mass and Eddington ratio. We compare our measurements to the predictions of alternative theories for the origin of the soft excess, but we are unable to distinguish between Comptonization of BBB photons by a hot plasma and absorption in relativistic winds as the most likely origins for the soft X-ray excess.
💡 Analysis
We study the UV properties of Type I AGN from the ROSAT All-Sky Survey that have been selected to show unusually soft X-ray continua. We examine a sample of 54 Seyfert 1 galaxies with detections in both Near-UV and Far-UV bands of the Galaxy Evolution Explorer (GALEX) satellite. Our sample is systematically fainter in the UV than galaxies studied in similar work by previous authors. We look for correlations between their UV and X-ray properties as well as correlations of these properties with either black hole mass or Eddington ratio. The shape of the Big Blue Bump(BBB) in the GALEX regime does not appear to correlate with its strength relative to the power law continuum, which conflicts with results reported by previous authors. The strength of the BBB is correlated with the shape of the X-ray continuum, in agreement with previous work, but the slope of the correlation is different than previously reported. The properties of the accretion disks of Type I AGN in the GALEX regime are relatively independent of black hole mass and Eddington ratio. We compare our measurements to the predictions of alternative theories for the origin of the soft excess, but we are unable to distinguish between Comptonization of BBB photons by a hot plasma and absorption in relativistic winds as the most likely origins for the soft X-ray excess.
📄 Content
arXiv:0901.3566v2 [astro-ph.GA] 18 Aug 2009 GALEX Measurements of the Big Blue Bump in Soft X-ray Selected AGN David W. Atlee and Smita Mathur Department of Astronomy, The Ohio State University atlee@astronomy.ohio-state.edu ABSTRACT We study the UV properties of Type I AGN from the ROSAT All-Sky Survey that have been selected to show unusually soft X-ray continua. We examine a sample of 54 Seyfert 1 galaxies with detections in both Near-UV and Far- UV bands of the Galaxy Evolution Explorer (GALEX) satellite. Our sample is systematically fainter in the UV than galaxies studied in similar work by previous authors. We look for correlations between their UV and X-ray properties as well as correlations of these properties with either black hole mass or Eddington ratio. The shape of the Big Blue Bump (BBB) in the GALEX regime does not appear to correlate with its strength relative to the power law continuum, which conflicts with results reported by previous authors. The strength of the BBB is correlated with the shape of the X-ray continuum, in agreement with previous work, but the slope of the correlation is different than previously reported. The properties of the accretion disks of Type I AGN in the GALEX regime are relatively independent of black hole mass and Eddington ratio. We compare our measurements to the predictions of alternative theories for the origin of the soft excess, but we are unable to distinguish between Comptonization of BBB photons by a hot plasma and absorption in relativistic winds as the most likely origins for the soft X-ray excess. Subject headings: galaxies: Seyfert, ultraviolet: general, X-ray: general 1. Introduction The soft X-ray excess is a contribution to the 0.2-2 keV flux in some Type I AGN beyond that predicted by extrapolating the hard X-ray power-law. It was first reported by – 2 – Arnaud et al. (1985), who suggested that it was caused by thermal emission from the hot inner portion of the AGN accretion disk. Early observational work by Turner & Pounds (1989) using EXOSAT and subsequently by Walter & Fink (1993; WF93) using ROSAT further explored the properties of the soft excess in an attempt to conclusively determine its origin. Turner & Pounds (1989) found their results to be consistent with the soft excess arising from the high-energy tail of the thermal accretion disk emission for AGN, but WF93 found that their measurements using a combination of X-ray measurements from ROSAT and Ginga and UV fluxes from the International Ultraviolet Explorer (IUE) were inconsistent with simple thick or thin accretion disk models. They also discovered that the ROSAT spectral indices (αx) of AGN with soft excesses were strongly correlated with the strength of the excess. The results of WF93 were later verified by Walter et al. (1994) using simultaneous IUE and ROSAT observations. Several alternative theories for the origin of the soft excess have since been proposed. The current models tend to favor either reprocessing of thermal disk emission via Compton scattering in thermal plasmas (e.g. Kawaguchi, Shimura & Mineshige 2001, Nied´zwiecki & Zdziarski 2006) or relativistically broadened absorption (e.g. Schurch & Done 2007). Models invoking atomic processes to generate the soft excess were originally proposed by Gierli´nski & Done (2004), who noted that the soft excess shows very consistent “temperature” across AGN with a wide variety of black hole masses. Schurch & Done (2006) recently proposed an alter- native picture to the usual wind model for atomic origins. Their “failed” wind model does not require the massive outflow from the accretion disk usually required to make an atomic origin viable. Several additional theories have also been proposed, including Compton reflection of hard X-ray photons by the dense, low-ionization gas in the accretion disk, resulting in an emergent spectrum that is very steep in the soft X-ray regime (Ross & Fabian 1993; Sobolewska & Done 2007; Done & Nayakshin 2007). Alternatively, hard X-ray photons could be absorbed by the disk instead of being reflected, and the absorbed energy would be re- emitted as soft X-ray photons with spectral indices that depend on the properties of the disk (R´o˙za´nska et al. 2002). Another popular class of model is the slim disk model, originally proposed by Muchotrzeb & Paczy´nski (1982), in which super-Eddington accretion causes changes in the properties of the standard thin disk, causing it to become geometrically thick and optically thin in its inner region and emit high-energy photons (Chen & Wang 2004). For more recent theoretical treatments of slim disks, see e.g. Heinzeller, Mineshige & Ohsuga (2006) and Heinzeller & Duschl (2007). After WF93, much of the observational work on the soft excess focused on the X-ray properties at the expense of the UV. The seminal paper from Boller, Brandt & Fink (1996), – 3 – which first reported the different distributions of Γx seen in Narrow-Line Seyfert 1 galaxies (NLS1s) and normal Seyfert
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