Abstract
The γ-ray source was first detected as 2FGL J0846.0+2820 (Nolan+ 2012, J/ApJS/199/31), listed in the second full catalog of Fermi-LAT sources, based on the first two years of LAT data obtained from 2008 August to 2010 August using the earlier P7V6 instrument response functions (IRFs). See section 2.1. The γ-ray source 2FGL J0846.0+2820 was selected for follow-up study based on a search of positional coincidences of periodic optical variables found in the Catalina Sky Surveys Data Release-1 (CSS -DR1; Drake+ 2014, J/ApJS/213/9) catalog with Fermi-LAT sources. To analyze the CSS photometry of this variable, we retrieved 471 CSS photometric measurements of the variable optical counterpart taken between 2005 April 10 and 2013 September 25. See section 2.2.1. We obtained time series photometry of the optical source in B, V, and R bands with the 16-inch PROMPT-5 telescope at Cerro Tololo International Observatory between 2015 February 4 and 2015 June 7. Each observing night consisted of multiple 60s exposures of the target field, which included the candidate optical counterpart to 2FGLJ0846.0+2820 as well as five nearby comparison stars. Our final sample includes 1643 photometric measurements in B, 1038 in V, and 500 in R, with mean magnitudes B=16.73, V=15.95, and R=15.40. See section 2.2.2. We began spectroscopic monitoring of the source with the Goodman Spectrograph on the Southern Astrophysical Research (SOAR) 4.1m telescope on 2014 December 8, continuing through 2016 December 31. We obtained 19 independent SOAR epochs of velocities. We also obtained some low-resolution spectra with OSMOS on the Hiltner 2.4m telescope at the MDM Observatory at Kitt Peak. These spectra were obtained in two to three exposures of 20 minute each on eight epochs from 2016 October 20 to 2017 January 8. See section 3.1. We obtained a high-resolution spectrum with HIRES on the Keck I telescope on 2016 September 25 (R~36000, wavelength range ~3900-8100Å). See section 3.2. Forthcoming X-ray observations will help confirm the connection between the Fermi γ-ray source 2FGL J0846.0+2820 and the optical binary CSS J084621.9+280839. (2 data files).
