SDSS J1536+0441: An Extreme "Double-peaked Emitter," Not a Binary Black Hole

ATel #1955; R. Chornock, J. S. Bloom, S. B. Cenko, J. M. Silverman, A. V. Filippenko (UC Berkeley), M. D. Hicks, K. J. Lawrence (JPL), P. Chang, J. M. Comerford, M. R. George, M. Modjaz, J. S. Oishi, E. Quataert, L. E. Strubbe (UC Berkeley)
on 9 Mar 2009; 22:51 UT
Credential Certification: J. S. Bloom (jbloom@cfa.harvard.edu)

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We report on optical spectra of the unusual quasar SDSS J153636.22+044127.0, obtained on 2009 March 7.52 UT with the Double Spectrograph on the Palomar 200-inch telescope. This object was recently suggested as a candidate sub-parsec supermassive black hole system by Boroson & Lauer (2009, Nature, 458, 53), based primarily on the existence of two components of the broad Balmer emission lines seen in the SDSS spectrum of the source. The red component ("r-system") shows the same radial velocity as the narrow nebular emission lines. The authors identified a blue component ("b-system") which was blueshifted by 3,500 km/s relative to the r-system, visible only in the broad Balmer lines.

We confirm the existence of both systems in our data. However, the larger wavelength coverage of our data (extending to 9900 Å) allows us to identify a third component to the broad Hα line, centered at +3,800 km/s relative to the narrow line of the r-system. The SDSS spectrum of this object analyzed in Boroson & Lauer (2009) did not extend to sufficiently long wavelengths to include this third component of the Hα profile. This third component is also visible at similar velocities in the broad Mg II 2800 Å line.

One explicit prediction of the binary black hole interpretation is that the emission connected to the less massive black hole (associated with the b-system) should show measurable orbital velocity evolution of order 100 km/s/yr (Boroson & Lauer 2009). To test this, we fit Gaussians to the peaks associated with the b- and r-systems for several lines. Both the Sloan and Palomar spectra were fit in an identical manner. The velocity splitting of the b- and r-systems in Hα is 3421 ± 31 km/sec in the SDSS spectrum and 3400 ± 20 km/sec in the Palomar spectrum. For Hβ, we measure a velocity split of 3578 ± 26 km/sec in the SDSS spectrum and 3565 ± 34 km/sec from Palomar. That is, the relative velocities of the two components appear to be consistent with being constant, with a weighted-average change of 18 ± 28 km/sec over the 0.91 yr separating the two observations. (We note that the consistently higher velocity splitting of Hβ relative to Hα is suggestive of a radiative transfer effect.)

Based on this evidence, we suggest that the most likely interpretation is that this object has complicated broad-line emission profiles similar to the multi-peaked structure seen in so-called "double-peaked emitters" such as Arp 102B (Halpern & Filippenko, 1988, Nature, 331, 46) and 3C390.3 (Eracleous & Halpern, 1994, ApJS, 90, 1). The existence of three components in the broad low-ionization lines, one centered near zero velocity and two more at similar red and blue velocities, is common to the class of double-peaked emitters. However, the ratio between the blue and red peaks in this object is extreme, as is the relatively narrow width of the blue component. Nevertheless, we believe that the presence of this third broad emission component and a lack of a velocity shift in the b-system renders the original interpretation significantly less likely.

A plot of the Balmer and Mg II line profiles in our spectrum can be seen at http://astro.berkeley.edu/~chornock/work/sdssJ1536-balmer-mgII.gif. The mean redshift in the plot was determined from the narrow lines associated with the r-system.

Acknowledgments: We thank Dan Stern for his assistance.