Monitoring Geminga (PSR J0633+1746) for Optical Flares over 46.64 days with the Kepler Spacecraft

ATel #8544; B. E. Schaefer, C. C. Hsu, M. L. Cherry (Louisiana State University)
on 15 Jan 2016; 01:43 UT
Credential Certification: Bradley E. Schaefer (schaefer@lsu.edu)

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We obtained 46.64 days of continuous optical photometric monitoring of the gamma-ray pulsar Geminga (PSR J0633+1746) with the Kepler spacecraft, as part of our program GO0108. This was in Cycle 0 for the K2 mission, with coverage in three time intervals over a 76.66 day interval, from 2014 March 12.03 to March 15.85, March 19.16 to March 25.66, and April 21.40 to May 27.69. The exposure times were 1625 seconds for each image (long cadence), for 2256 images, each recorded as a subarray of 21X22 pixels, with the square pixels being 3.97 arc-sec on a side. Comparison Star G is clearly visible with 4900 photoelectrons, and this star is V=20.86 (Bignami, Caraveo, and Paul 1988, A&A, 202, L1). Geminga is just north of Star G in a clear region of sky. For the normal variation in the sky level associated with slight movements in the telescope pointing, the real limit for the confident detection of any optical transient is close to V=22.0 mag in a single image. We performed aperture photometry in all the images over a 3x3 pixel aperture. No significant excess of flux was seen in any of the Kepler images. (Sixteen of the images had cosmic rays within the aperture, but these are recognized as being single pixel excesses and not at the correct position for Geminga. From these sixteen, three had positions that are marginally consistent with Geminga, and the corresponding flux is V=22.3 mag or fainter.) Further, no flares are seen in light curves binned in time, with a limit of V=22.3 mag for binning times of 3 hours or longer.

Geminga is normally very faint, at R=25.53 (Shibanov, Y. A. et al. 2006, A&A, 448, 313). As such, Kepler can see Geminga only if it has some large amplitude optical flare, and such might happen if even a very small fraction of the energy in some high energy events comes out as optical light. The Geminga pulsar has glitches (Jackson, M. S. et al. 2002, ApJ, 578, 935) and energetic fast radio transients (Maan, Y. 2015, ApJ, 815, 126), while the Crab pulsar (PSR J0534+2200), a similar gamma-ray pulsar, has rapid gamma-ray flares (ATel #2855; ATel #4239; Mayer, M. et al. 2013, ApJLett, 775, L37). If just 3 parts-per-million of the glitch energy goes into the optical band in under thirty minutes, or if just 0.8 parts-per-million of a Crab-like flare goes into the optical band over three days, then the optical transient should be visible in the Kepler images.

Our daily light curves from Earth-occultations with the Fermi GBM (http://heastro.phys.lsu.edu/gbm/) shows no flares for Geminga. Stacking our Geminga light curves for the Kepler time interval yields 3-sigma flux limits of 0.007 counts/cm^2/s (12-25 keV) and 0.0034 counts/cm^2/s (25-50 keV). The Swift BAT daily-average light curve for 15-50 keV (http://swift.gsfc.nasa.gov/results/transients/weak/Geminga.lc.txt) has no significant flare, with a limit of near 0.008 count/cm^2/sec. The Fermi LAT monthly-average light curve from 100 MeV to 200 GeV (http://fermi.gsfc.nasa.gov/ssc/data/access/lat/4yr_catalog/ap_lcs/lc_3FGLJ0633.9p1746.dmp1.out) shows no flares to a flux limit of 0.3*10^-6 ph/cm^2/s.