Optical Identification of Swift J1909.3+0115 as a Cataclysmic Variable

ATel #11787; J. P. Halpern (Columbia U.), J. R. Thorstensen (Dartmouth C.)
on 27 Jun 2018; 15:34 UT
Credential Certification: Jules Halpern (jules@astro.columbia.edu)

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Swift J1909.3+0115, also known as J1909.3+0124 in the Swift-BAT 105-month survey (Oh, et al. 2018, ApJS 235, 4), was observed six times by the Swift XRT between 2017 November 24 and 2018 March 7. In each observation, a variable X-ray source was detected coincident with a bright optical counterpart at (J2000.0) R.A. = 19h 09m 20.906s, decl. = +01d 12' 24.56", π = 1.414 ± 0.028 mas (Gaia DR2 position and parallax). Its maximum 0.3-10 keV count rate is ~0.15 s^-1, with an average about half that. Several of these images are contaminated by scattered X-rays from Aql X-1. The Swift-BAT source was also identified with XMMSL2 J190921.2+011225 and suggested to be Galactic by Stephen et al. 2018, ATel #11341. ROSAT X-ray detections are 1RXS J190923.2+011154 and 1WGA J1909.3+0112; evidently these are all the same object.

On 2018 March 4 we obtained a spectrum covering 3965-6875 Å using the Ohio State Multi-Object Spectrograph (OSMOS) on the 2.4-m Hiltner telescope of the MDM Observatory. Typical of a cataclysmic variable, bright emission lines of H, He I, He II, and the C III/N III Bowen blend are present, together with (more rarely seen) photospheric absorption features of an early K-type companion star.

Using the 1.3-m McGraw-Hill telescope at MDM, we obtained V-band time-series photometry on eight nights between 2018 March 16 and June 15. Six of these runs were 3-5 hours long at 33 s cadence. Flickering with full amplitude of ~0.2 mag was seen, but no coherent period was detected. Among these nights the mean magnitude ranged from V = 14.1 to 15.0.

We obtained 20 more spectra with modspec on the 1.3-m between 2018 May 9 and 13, another 20 with the 2.4-m and OSMOS on May 20 and 21, and five with modspec on the the 2.4-m on June 5 and 7. The emission-line strength varies dramatically. The emission equivalent width of Hα falls mostly in the range 15 to 25 Å, but it was as high as 43 Å in the discovery spectrum on March 4, and as low as 2 Å for some of our June data. In the latter spectra no other emission lines are detected.

The radial velocities of the absorption spectrum show a sinusoidal (almost certainly orbital) modulation at a period of 2.06(2) days with a semi-amplitude K = 107(4) km s^-1. This period is unusually long for a cataclysmic binary and requires that the companion be somewhat evolved if it fills its Roche lobe. The Gaia DR2 distance is nominally 707 ± 14 pc, and the reddening maps of Green et al. (2018, MNRAS 478, 651) give E(B-V) = 0.40 at this distance. From our spectrophotometry we estimate V = 15.5 for the companion star, which implies an absolute magnitude near +5 for the companion alone, more luminous than a main-sequence star of its early K spectral type.