The Ultraviolet Peak of the Energy Distribution in 3C 273: Evidence for an Accretion Disk and Hot Corona around a Massive Black Hole

Abstract

We present absolutely calibrated far-ultraviolet spectrophotometry of the quasar 3C 273 covering the 900-1800 Å range. Our ∼ 3 Å resolution spectra were obtained with the Hopkins Ultraviolet Telescope during the Astro-1 mission in December 1990 and during the Astro-2 mission in March 1995. Both spectra exhibit a change in slope near the Lyman limit in the quasar rest frame. At longer UV wavelengths, the continuum has a power-law index of 0.5-0.7, while shortward of the Lyman limit it is 1.2-1.7. The energy distribution in ν f ν therefore peaks close to the quasar Lyman limit. The short wavelength UV power law extrapolates well to match the soft X-ray excess seen in simultaneous observations with the Broad-Band X-ray Telescope and nearly simultaneous ROSAT observations.The general shape of the broad-band spectrum of 3C 273 is consistent with that of an optically thick accretion disk whose emergent spectrum has been Comptonized by a hot medium. Our UV spectrum is well described by a Schwarzschild black hole of 7 × 10 8 M ⊙ accreting matter at a rate of 13 M ⊙ yr −1 through a disk inclined at 60 degrees. Superposed on the intrinsic disk spectrum is an empirically determined Lyman edge of optical depth 0.5. The Comptonizing medium has a Compton parameter y ≈ 1, obtained with an optical depth to electron scattering of unity and a temperature of 4 × 10 8 K.This overall shape is the same as that found by Zheng et al. and Laor et al. in their UV and X-ray composite spectra for quasars, giving physical validity to the composite spectrum approach. When combined with those results, we find that the generic ionizing continuum shape for quasars is a power law of energy index 1.7-2.2, extending from the Lyman limit to ∼1 keV. The observational gap in the extreme ultraviolet for these combined data describing the quasar continuum shape is now only half a decade in frequency.

Constraining the location of rapid gamma-ray flares in the flat spectrum radio quasar 3C 273

“…The molecular torus luminosity can be considered as a 0.1−0.5 fraction of the accretion disk luminosity (L AD ). For 3C 273, we have L AD ∼ 1.6 × 10 46 erg s −1 (Kriss et al 1999;Soldi et al 2008;Giommi et al 2012). Using L MT,45 ∼ 1.6, Γ bulk,10 = 10.6 ± 2.8 (Jorstad et al 2005) and Δt max < 6 h, we obtain r γ < 1.6 pc.…”

The associated system of HE 2347-4342

“…Zheng et al 1997;Telfer et al 2002). Even though there are hints that a peak of the energy spectrum of quasars occurs near the Lyman limit (an example is the z = 0.158 quasar 3C 273, Kriss et al 1999), individual sources can show substantially different energy distributions in the extreme-UV (e.g. Reimers et al 1998).…”

A possible interpretation of the Seyfert-like component in 3C 273 X-ray spectrum

“…1 the continuous line shows the adopted EF E spectral distribution (for this first type of "seed" photon) and the extension of the bars takes into account both the flux ranges observed at a specific energy at different times and the errors affecting each observation. The data are from von Montigny et al (1997); Kriss et al (1999), and from the 3C 273 database cited by Türler et al (1999). At longer wavelengths (namely for λ > ∼ 1.5 × 10 4 Å), as explained before, the thermal emission comes from the dust located at larger distances (see Table 2 of Türler et al 2006); hence the related photons are geometrically diluted and their number is negligible for the reprocessing.…”

“…The complexity of both time dependence and shape of its spectrum suggests that 3C 273 emission is due to the contribution of several physical mechanisms at work in different parts of the source. In fact, the optical-UV range of the spectrum, exhibiting a Big Blue Bump (as well as strong emission lines), may indicate the contribution of a thermal-type emission typically attributed to an accretion disk (see for instance von Montigny et al 1997;Kriss et al 1999;Impey et al 1989) while the X-ray portion of the spectrum, which we are interested in, is generally interpreted as mainly due to beamed non-thermal emission produced by a jet that was first discovered in the radio domain and has been observed down to a fraction of pc scale thanks to the VLBA (Savolainen et al 2006). The corresponding jet structure on a much larger scale (i.e., on the Kpc -few tens of Kpc scale) has been recently observed also in the optical (Jester et al 2001) and in the X-ray Sambruna et al 2001;Marshall et al 2001;Jester et al 2006) domain.…”