Gamma-ray Eclipses and Orbital Modulation Transitions in the Candidate Redback 4FGL J1702.7-5655

Abstract

Observations with the Fermi Large Area Telescope (LAT) of the gamma-ray source 4FGL J1702.7−5655, previously classified as a candidate millisecond pulsar, show highly-significant modulation at a period of 0.2438033 days (∼5.85 hours). Further examination of the folded light curve indicates the presence of narrow eclipses, suggesting this is a redback binary system. An examination of the long-term properties of the modulation over 13 years of LAT observations indicates that the orbital modulation of the gamma-rays changed from a simple eclipse before early 2013, to a broader, more easily detected, quasi-sinusoidal modulation. In addition, the time of the eclipse shifts to ∼0.05 later in phase. This change in the orbital modulation properties is, however, not accompanied by a significant overall change in gamma-ray flux or spectrum. The quasi-sinusoidal component peaks ∼0.5 out of phase with the eclipse, which would indicate inferior conjunction of the compact object in the system. Swift X-ray Telescope observations reveal a possible X-ray counterpart within the LAT error ellipse. However, radio observations obtained with the Australia Telescope Compact Array do not detect a source in the region. 4FGL J1702.7−5655 appears to have changed its state in 2013, perhaps related to changes in the intrabinary shock in the system. We discuss how the properties of 4FGL J1702.7−5655 compare to other binary millisecond pulsars that have exhibited orbital modulation in gamma rays.

Optical Light Curve of 4FGL J0935.3+0901: A Flaring Black Widow Candidate

“…Similar candidates are often not detected in radio, sometimes due to absorption by the ablated stellar wind at an unfavorable orbital phase. However, there are a number of new redback candidates, e.g., 4FGL J2331.1−5527 (Swihart et al 2020), 3FGL J0940.6−7609 (Swihart et al 2021), and 4FGL J1702.7-5655 (Corbet et al 2022), that were previously searched a number of times without success (Camilo et al 2015(Camilo et al , 2016. It could be that they are simply beamed away from Earth.…”

Optical Light Curve of 4FGL J0935.3+0901: A Flaring Black Widow Candidate

“…However, there are a number of new redback candidates, e.g., 4FGL J2331.1−5527 (Swihart et al 2020), 3FGL J0940.6−7609 (Swihart et al 2021), and 4FGL J1702.7-5655 (Corbet et al 2022), that were previously searched a number of times without success (Camilo et al 2015(Camilo et al , 2016. It could be that they are simply beamed away from Earth.…”

Luminous Optical and X-ray Flaring of the Putative Redback Millisecond Pulsar 1FGL J0523.5$-$2529

“…The detailed properties of well-studied redbacks were summarized by Strader et al (2019), including six of the putative ones. More recent identifications of candidate redbacks among Fermi sources are 4FGL J2333.1−5527 (Swihart et al 2020), 4FGL J0935.3+0901 (Wang et al 2020), 4FGL J0940.3−7610 (Swihart et al 2021), and 4FGL J1702.7−5655 (Corbet et al 2022). Pulsations have not been detected from these last four despite dedicated searches in radio (Camilo et al 2015Zheng et al 2022).…”