About X-ray Bright Optically Normal Galaxies

We have been investigating a class of objects known as X-ray Bright Optically Normal Galaxies (XBONGs). Bright X-rays such as those emitted from these sources are usually attributed to Active Galactic Nuclei (AGN). AGN are accreting supermassive black holes in the center of galaxies which emit high-energy radiation, notably in the X-ray. They also show characteristic broad and narrow emission lines in their optical spectra, a feature that is missing from XBONGs. The broad lines are caused by Doppler shifts of fast-moving clouds in the center gap of the donut-shaped torus that absorb and reemit light of particular wavelengths. The same occurs for the narrow lines, but the clouds lie further outside of the torus and are not traveling quite as fast as the nearer ones. As the inner clouds of Type II (edge-on) AGN are obscured by the torus, their spectra show only narrow emission lines, not the especially broad ones typical of Type I AGN. XBONGs, however, lack both broad and narrow AGN emission lines in their optical spectra, yet their magnitudes in the X-ray exceed 10^42 erg/s, as would be expected of AGN.

Possible Explanations for XBONGs

Dilution – When the optical light from the stars in the galaxy is bright relative to the active nucleus, it is possible for the broad lines in the spectrum to be swamped out, while strong X-ray emissions still reach us.

Absorption/Obscuration – In the case of a Type II (edge-on) AGN, X-ray photons can be high energy enough to pass through the torus surrounding the accretion disk, while optical photons are not.

Advection Dominated Accretion Flows (ADAFs) – ADAFs, which are a low-luminosity type of Radiatively Inefficient Accretion Flows (RIAFs), have a unique AGN geometry which could alter their spectra.

X-ray Flaring – AGN are relatively variable, so when the X-ray luminosity was obtained, the galaxy may have had an accretion disk that was not there when the optical spectrum was taken.

Extend Hot Gas Emission – If the X-ray source appears extended, the X-rays could be originating from hot gas within a cluster of galaxies.

About Our Work

This summer, we worked with Dr. Daryl Haggard to study XBONGs. The five theories described above are useful in attempting to understand these objects. We have been focusing on eight sources that were selected for study by Chandra due to their high X-ray luminosities (from ROSAT) and normal optical spectra (from Sloan Digital Sky Survey, or SDSS). To compare coordinates from different servers, we created and ran a program using Python that converted the SDSS RA and Dec of each object from sexagesimal to decimal degrees, then found the offsets between the SDSS and ROSAT coordinates. Then, to learn more about these objects, we searched for data and papers written about them in SIMBAD, the NASA/IPAC Extragalactic Database (NED), the Chandra Source Catalog (CSC), and the Astrophysics Data System (ADS). A combination of fluxes, spectral analysis, and morphology can be used to ascertain which hypothesis best explains the unexpected X-ray and optical combination for each of these eight objects.