Multiwavelength Scrutiny of the X-ray Weak Tail of Low-Mass Active Galactic Nuclei

The existence of very high-redshift quasars (z > 6) suggests that some supermassive black holes (M_BH ≈ 10⁶–10⁹ M_Sun) grew very quickly during the first hundreds of millions of years after the Big Bang, through accretion and/or mergers. Increasingly large samples of lower-mass galaxies hosting “massive” black holes (in the M_BH ≈ 10⁴–10⁶ M_Sun range) have been discovered over the past ~15 years, potentially offering clues to understanding how the first supermassive black holes may have grown. Herein, we focus on a large, optically selected sample of radio-quiet, low-mass active galactic nuclei (AGNs) identified by Greene & Ho (2007) from the Sloan Digital Sky Survey. This sample tends toward the faster-accreting (average Eddington ratios <L_bol/L_Edd> ≈ 0.3) and higher-mass (<M_BH> ≈ 1.3 × 10⁶) end of the low-mass AGN scale, which makes it an excellent starting point for comparison with relatively well-studied quasar populations. Low-mass AGNs are generally expected to display thermal disk emission peaking in the soft X-ray, with a hard X-ray component from inverse Compton upscattering of photons by a hot corona. Contrary to both this expectation and behavior observed in the unobscured quasar population, however, the Greene & Ho sample displays a perplexing long X-ray weak tail relative to ultraviolet luminosity (where the latter probes accretion disk emission). The reason for this X-ray weak tail is unclear. It may be an intrinsic property, possibly caused by a weak corona. Alternatively, it may be an orientation effect caused by obscuration/scattering of coronal X-rays from certain viewing angles, perhaps by the innermost region of a “slim” disk puffed up by radiation pressure at high accretion rates. We present scrutiny of the X-ray weak tail of these low-mass AGNs via tests of their radio properties across the full range of observed X-ray strengths, combining archival X-ray observations from Chandra with new and archival radio observations from the Very Large Array. While the exact physical origin of radio emission in these objects is unknown, we can remain confident that even if the X-ray weak tail is caused by an orientation effect, the radio emission will not experience the same type of obscuration/scattering as the X-rays. As a result, examination of X-ray to radio luminosity ratios provides a means to better understand the source of X-ray weakness. In this poster, we will present preliminary results and discuss implications for the growth of supermassive black holes through rapid accretion.