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Register a new userThe Spectral Energy Distributions of AGN with Double-Peaked Balmer Lines
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I. Strateva
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We summarize the optical, UV, and X-ray properties of double-peaked emitters -- AGN with double-peaked Balmer emission lines believed to originate in the AGN accretion disk. We focus on the X-ray spectroscopic results obtained from a new sample of the 16 broadest Balmer line AGN observed with Chandra and Swift.
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Double-peaked Balmer-line profiles originate in the accretion disks of a few percent of optically selected AGN. The reasons behind the strong low-ionization line emission from the accretion disks of these objects is still uncertain. In this paper, we characterize the X-ray properties of 39 double-peaked Balmer line AGN, 29 from the Sloan Digital Sky Survey and 10 low optical-luminosity double-peaked emitters from earlier radio-selected samples. We find that the UV-to-X-ray slope of radio-quiet (RQ) double-peaked emitters as a class does not differ substantially from that of normal RQ AGN with similar UV monochromatic luminosity. The radio-loud (RL) double-peaked emitters, with the exception of LINER galaxies, are more luminous in the X-rays than RQ AGN, as has been observed for other RL AGN with single-peaked profiles. The X-ray spectral shapes of double-peaked emitters, measured by their hardness ratios or power-law photon indices, are also largely consistent with those of normal AGN of similar radio-loudness. In practically all cases studied here, external illumination of the accretion disk is necessary to produce the Balmer-line emission, as the gravitational energy released locally in the disk by viscous stresses is insufficient to produce lines of the observed strength. In the Appendix we study the variability of Mrk 926, a double-peaked emitter with several observations in the optical and X-ray bands.
AGN with double-peaked narrow lines (DPAGN) may be caused by kiloparsec scale binary AGN, bipolar outflows, or rotating gaseous disks. We examine the class of DPAGN in which the two narrow line components have closely similar intensity as being especially likely to involve disks or jets. Two spectroscopic indicators support this likelihood. For DPAGN from Smith et al. (2010), the equal-peaked objects (EPAGN) have [Ne V]/[O III] ratios lower than for a control sample of non-double peaked AGN. This is unexpected for a pair of normal AGN in a galactic merger, but may be consistent with [O III] emission from a rotating ring with relatively little gas at small radii. Also, [O III]/H-beta ratios of the redshifted and blueshifted systems in the EPAGN are more similar to each other than in a control sample, suggestive of a single ionizing source and inconsistent with the binary interpretation.
Here we present results of the long-term (1987-2010) optical spectral monitoring of the broad line radio galaxy Arp 102B, a prototype of active galactic nuclei with the double-peaked broad emission lines, usually assumed to be emitted from an accretion disk. To explore the structure of the broad line region (BLR), we analyze the light curves of the broad Halpha and Hbeta lines and the continuum flux. We aim to estimate the dimensions of the broad-line emitting regions and the mass of the central black hole. We use the CCF to find lags between the lines and continuum variations. We investigate in more details the correlation between line and continuum fluxes, moreover we explore periodical variations of the red-to-blue line flux ratio using Lomb-Scargle periodograms. The line and continuum light curves show several flare-like events. The fluxes in lines and in the continuum are not showing a big change (around 20%) during the monitoring period. We found a small correlation between the line and continuum flux variation, that may indicate that variation in lines has weak connection with the variation of the central photoionization source. In spite of a low line-continuum correlation, using several methods, we estimated a time lag for Hbeta around 20 days. The correlation between the Hbeta and Halpha flux variation is significantly higher than between lines and continuum. During the monitoring period, the Hbeta and Halpha lines show double-peaked profiles and we found an indication for a periodical oscillation in the red-to-blue flux ratio of the Halpha line. The estimated mass of the central black hole is sim 1.1 times 10^8 Modot that is in an agreement with the mass estimated from the M-sigma* relation.
We present results from spectroscopic observations of AT 2018hyz, a transient discovered by the ASAS-SN survey at an absolute magnitude of $M_Vsim -20.2$ mag, in the nucleus of a quiescent galaxy with strong Balmer absorption lines. AT 2018hyz shows a blue spectral continuum and broad emission lines, consistent with previous TDE candidates. High cadence follow-up spectra show broad Balmer lines and He I in early spectra, with He II making an appearance after $sim70-100$ days. The Balmer lines evolve from a smooth broad profile, through a boxy, asymmetric double-peaked phase consistent with accretion disc emission, and back to smooth at late times. The Balmer lines are unlike typical AGN in that they show a flat Balmer decrement (H$alpha$/H$betasim1.5$), suggesting the lines are collisionally excited rather than being produced via photo-ionisation. The flat Balmer decrement together with the complex profiles suggest that the emission lines originate in a disc chromosphere, analogous to those seen in cataclysmic variables. The low optical depth of material due to a possible partial disruption may be what allows us to observe these double-peaked, collisionally excited lines. The late appearance of He II may be due to an expanding photosphere or outflow, or late-time shocks in debris collisions.
We present infrared (IR) to X-ray spectral energy distributions (SEDs) for 44 red AGN selected from the 2MASS survey on the basis of their red J-K$_S$ color (>2 mag) and later observed by Chandra. In comparison with optically-, radio-, and X-ray selected AGN, their median SEDs are red in the optical and near-IR with little/no blue bump. It thus seems that near-IR color selection isolates the reddest subset of AGN that can be classified optically. The shape of the SEDs is generally consistent with modest absorption by gas (in the X-ray) and dust (in the optical-IR). The levels of obscuration, estimated from X-rays, far-IR and our detailed optical/near-IR color modeling are all consistent implying N_H < few*10^{22} cm^{-2}. We present SED models that show how the AGN optical/near-IR colors change due to differing amounts of reddening, AGN to host galaxy ratio, redshift and scattered light emission and apply them to the sources in the sample. We find that the 2MASS AGN optical color, B-R, and to a lesser extent the near-IR color, J-K$_S$, are strongly affected by reddening, host galaxy emission, redshift, and in few, highly polarized objects, also by scattered AGN light. The obscuration/inclination of the AGN allows us to see weaker emission components which are generally swamped by the AGN.
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