Swift Observation of the Candidate Supermassive Black Hole Binary SDSS J153636.22+044127.0

ATel #1931; Z. Arzoumanian (CRESST/USRA/NASA-GSFC), M. Loewenstein (CRESST/UMCP/GSFC), R. F. Mushotzky (GSFC), K. C. Gendreau (GSFC)
on 12 Feb 2009; 04:31 UT
Credential Certification: Zaven Arzoumanian (zaven.arzoumanian@nasa.gov)

Tweet

The X-Ray Telescope (XRT) and Ultraviolet/Optical Telescope (UVOT) on board the Swift satellite observed the candidate supermassive black hole binary SDSS J153636.22+044127.0 (hereafter, J1536; Boroson & Lauer 2009, arXiv:0901.3779v1) on 2009-02-04 and 2009-02-05 for total exposures of 3738 s and 3544 s, respectively. A ROSAT Faint Source Catalog source (1RXS J153635.6+044118; countrate 0.03 ± 0.01 s^–1) with a positional error-circle radius of 18 arcsec was previously known to lie 12.7 arcsec from the SDSS coordinates.

The XRT data reveal an unresolved source from which 142 background-subtracted counts were collected (countrate 18.8 ± 1.6 ks^–1) within its 90% encircled-energy extraction circle. The two single-day countrates differed by less than 2σ, and the average XRT countrate is consistent with the cataloged ROSAT value for spectral models similar to the one described below, suggesting weak long-term variability. The centroided position of the XRT source is 3.5 arcsec from the SDSS position with an error-circle radius, including systematic uncertainties, of 4.0 arcsec.

The XRT source spectrum was fit to a model typical of X-ray-bright AGN: a redshifted power law with intrinsic (redshifted) and Galactic absorption. The Galactic absorbing column was held fixed at 4×10²⁰ cm^–2, as was the redshift at 0.388. Although not statistically well-constrained, the resulting power-law index (~1.5) and intrinsic absorbing column (<~ 2×10²¹ cm^–2) are consistent with the properties of broad-line AGN (e.g., Carrera et al. 2004, A&A 420, 163). The implied rest-frame (H₀ = 70, Λ₀ = 0.73) 0.5–10 keV luminosity of 5×10⁴⁴ erg s^–1 is consistent with Seyfert galaxies. Together, the close positional coincidence, spectrum, and luminosity of the XRT source suggest that it is the counterpart to the SDSS quasar.

UVOT data were obtained in image mode using the standard filter sequence; J1536 was detected in all six bands at greater than 18σ. Analysis of the co-added level II images yielded a U-band flux of 0.189 ± 0.007 mJy and a U–B color of –0.63. In a spectral fit, the UVOT fluxes are consistent with a power-law model with an unusually steep slope (Richards et al. 2006, ApJS 166, 470), 3.2 ± 0.15, in agreement with cataloged GALEX measurements extending into the far UV. The derived optical/UV-to-X-ray flux ratio α_ox (Steffen et al. 2006, AJ 131, 2826) is –1.29.

It is unclear from this modest dataset whether the proposed binary black-hole nature of this quasar results in unusual X-ray properties. Our measurement of the broadband X-ray and UV fluxes will help to justify future observations to constrain, e.g., the existence of pairs of UV or X-ray spectral lines displaced in redshift due to orbital motion, as are seen in the optical.

We acknowledge the use of public data from the Swift data archive.