We have measured dramatic flux and spectral variations through the 0.193d orbit of the optical counterpart of the unidentified gamma-ray source 0FGL J2339.8-0530. This compact object companion is strongly heated, with T_eff varying from ~6900K (superior conjunction) to <3000K at minimum. A combined fit to the light curve and radial velocity amplitudes implies M_1 ~ 0.075M_sun, M_2 ~ 1.4 M_sun and inclination i ~ 57deg. Thus this is a likely `black widow' system with Edot~10^{34-34.5} erg/s pulsar driving companion mass loss. This wind, also suggested by the X-ray light curve, may prevent radio pulse detection. Our measurements constrain the pulsar's reflex motion, increasing the possibility of a pulse detection in the gamma-ray signal.
The Large Area Telescope (LAT) on the Fermi satellite has revolutionized our understanding of the γ-ray sky, with particular success in detecting spin-powered pulsars and blazars. After only three months of observation, a preliminary 0FGL 'bright source list' of 205 objects detected at > 10σ significance was presented in Abdo et al. (2009a). Remarkably, over 95% of these sources now have been associated with lower energy counterparts (Ackerman et al. 2011). Moreover, comparison with the γ-ray properties of the identifications make it possible to classify many of the remaining sources; likely blazars show significant variability, while likely pulsars are steady, but show strongly curved spectra, with cut-offs above a few GeV.
At present, there are two 0FGL sources at |b| > 1 • with pulsar-like properties and no identification. One, 0FGL J2339.8-0530 was a ∼ 12σ detection with steady, hard spectrum emission. It has been the subject of both radio pulsar counterpart searches and ‘blind’ searches for γ-ray pulsations (eg. Abdo et al. 2009b). To date no pulsed emission has been seen and the source remains unidentified in the latest catalog (Abdo et al. 2011). There it is listed as 2FGL J2339.6-0532, localized to l = 81.357 ± 0.03, b = -62.467 ± 0.02, with a variability index of 15.7 (indicating steady emission) and a flux of 3.0 ± 0.2 × 10 -11 erg/cm 2 /s, with a hard Γ = 1.96 spectrum showing 6σ evidence of a spectral cutoff. It thus remains one of the best unidentified pulsar candidates. Indeed, Kong et al. (2011ab), have drawn attention to this source, finding that there was an optically variable star coincident with a CXO source in the LAT uncertainty region, arguing that this was likely a ‘radio-quiet’ millisecond pulsar (MSP) and suggesting that it may be an LMXB in quiescence, similar to the LAT-associated LMXB/MSP transient FIRST J102347.6+003841 (Archibald et al. 2010;Tam et al. 2010). We report here on optical imaging and spectroscopy of this source which support the millisecond pulsar hypothesis, but instead show that this is a ‘black-widow’ type pul- sar, evaporating a low mass companion, similar to the original of such systems, PSR B1957+20, but 2-3× closer at d ∼ 1 kpc. We suggest that strong outflow in this evaporating system inhibits detection of radio pulsations. If the pulsed emission can be detected in the LAT photons, the resulting orbital information will make this a double-line spectroscopic binary and should allow an unusually precise determination of the neutron star mass, comparable to the important high M PSR = 2.40 ± 0.12M ⊙ determination recently made for PSR B1957+20 (van Kerkwijk, Breton & Kulkarni 2011).
Examination of plots of white light CCD photometry made on Oct. 31 and Nov. 1, 2010 with the 1m Lulin Telescope (Taiwan) and on November 11, 2010 with the 0.81m Tenagra Telescope (Arizona) allowed us to infer an orbital period of ∼0.19d. The source appears in the SDSS DR8 data release with colors u=20. 85, g=19.00, r=18.61, i=18.25, z=18.23, suggesting a G-type spectral class. Optical extinction at this high latitude is very small (A V ≈ 0.1 from the Schlegel et al. 1998 dust maps). Based on the large (> 2.5mag) variation it seemed likely that this is a nearby, short-period black-widow type pulsar, with a strongly heated companion and thus a suitable target for detailed optical spectroscopy.
The 9.2 m Hobby*Eberly Telescope (HET) has a spherical primary with a tracking corrector and can follow a source for ∼ 1hr/night during a transit. This is a small fraction of the estimated 4.6 h orbital period. However the HET is dynamically queue scheduled (Shetrone et al. 2007), and with an ephemeris one can obtain full orbital coverage. During HET Low Resolution Spectrograph (LRS) observations one obtains short direct images for target acquisition. We used these 3-15 s ‘pre’ images, augmented by ‘post’ images in several cases, to monitor the flux of J2339-0533. These images, through a Schott GG385 long-pass filter, approximate the ‘white light’ of the 2010 photometry. We measured simple differential aperture magnitudes, calibrated to the SDSS r ′ magnitudes of nearby stars. These photometric data covering Aug. 5 through Sept. 6 2011 confirmed the dramatic optical modulation of J2339-0533. Conditions were variable, but detections were always of high S/N, even at the r ′ ∼ 21 minimum. Since the target is very bright, we obtained additional photometry with the 0.61 m Cassegrain telescope at the Stanford Student Observatory (SSO) on Aug. 28, 2011 (MJD 55801.287 -.399) andOct. 20, 2011 (MJD 55855.140 -.381). Using 300 s unfiltered CCD exposures with an Apogee AP8 camera, we made differential flux measurements, again normalized to SDSS r ′ magnitudes. These data helped in absolute phasing and improving the binary period estimate.
Finally, we obtained SDSS g ′ r ′ i ′ frames at the 3.6 m WIYN telescope using the MiniMo camera on Sept. 27, 2011 (55832.290 -.424). These data, covering inferior con
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