X-Ray and Optical Study of the Gamma-ray Source 3FGL J0838.8–2829: Identification of a Candidate Millisecond Pulsar Binary and an Asynchronous Polar

Abstract

We observed the field of the Fermi source 3FGL J0838.8−2829 in optical and X-rays, initially motivated by the cataclysmic variable (CV) 1RXS J083842.1−282723 that lies within its error circle. Several X-ray sources first classified as CVs have turned out to be γ-ray emitting millisecond pulsars (MSPs). We find that 1RXS J083842.1−282723 is in fact an unusual CV, a stream-fed asynchronous polar in which accretion switches between magnetic poles (that are ≈ 120• apart) when the accretion rate is at minimum. High-amplitude X-ray modulation at periods of 94.8±0.4 minutes and 14.7±1.2 hr are seen. The former appears to be the spin period, while the latter is inferred to be one-third of the beat period between the spin and the orbit, implying an orbital period of 98.3 ± 0.5 minutes. We also measure an optical emission-line spectroscopic period of 98.413 ± 0.004 minutes, which is consistent with the orbital period inferred from the X-rays. In any case, this system is unlikely to be the γ-ray source. Instead, we find a fainter variable X-ray and optical source, XMMU J083850.38−282756.8, that is modulated on a time scale of hours in addition to exhibiting occasional sharp flares. It resembles the black widow or redback pulsars that have been discovered as counterparts of Fermi sources, with the optical modulation due to heating of the photosphere of a low-mass companion star by, in this case, an as-yet undetected MSP. We propose XMMU J083850.38−282756.8 as the MSP counterpart of 3FGL J0838.8−2829.

A Multi-wavelength search for Black Widows and Redbacks counterparts of candidate $γ$-ray millisecond pulsars

“…Ten of the 23 γ-ray sources in Table 1 have already a confirmed/proposed identification: 4FGL J0359.4+5414 is identified with an isolated young pulsar (PSR J0359+5414; Clark et al 2017) (Strader et al 2014;Halpern et al 2017a;Li et al 2018;Salvetti et al 2017), and 4FGL J1653.6−0158 is a candidate BW (Romani et al 2014), where from here on we use the term "candidate" to refer to those sources for which the radio/γ-ray pulsation evidence has not been obtained yet. In particular, four of the seven γ-ray sources out of the original 48 in our starting sample which have been classified as pulsars in the 4FGL catalogue (see Sectn.…”

Variable Heating and Flaring of Three Redback Millisecond Pulsar Companions

“…With the new data, we were also able to fit a refined orbital ephemeris using the method described in Halpern et al (2017b), in which the epoch of minimum in the light curve is used as the fiducial phase φ = 0.25. The longer baseline also enables us to extrapolate backward with a precise cycle count to an observation obtained on 2015 February 18 (Figure 7 of Halpern et al 2017a). This is the earliest known light curve of the star, before its orbital period was apparent, but it contains a dip which provides another timing for phase 0.25.…”

A Deep Chandra X-ray Observatory Study of the Millisecond Pulsar Population in the Globular Cluster Terzan 5

“…Thus, further detailed modeling of the X-ray (and optical, where available) light curves of redbacks as an ensemble is require to gain further insight regarding the various factors that determine the apparent characteristics of the intra-binary shock. The X-ray flux behavior of Ter5 A is reminiscent of that observed from PSR J1048+2339, a confirmed redback in the field of the Galaxy, and XMMU J083850.38−282756.8, a putative redback binary and the likely optical and X-ray counterpart of the Fermi LAT source 3FGL J0838.8−2829 (Halpern et al 2017;Cho et al 2018). These systems have been observed to undergo short-lived X-ray and optical flaring states, which can be attributed to episodic strong magnetic activity on the face of the companion heated by the pulsar wind and/or shock.…”

“…The peak luminosity in the short flares is comparable to the level seen from other transitional systems, including CX1. An alternative interpretation for the variability is enhanced X-ray luminosity produced by intense stellar flaring from the secondary star as is seen from redbacks in the field of the Galaxy (Cho et al 2018;Halpern et al 2017). In the absence of contemporaneous optical and radio data it is difficult to establish the correct scenario, although the latter appears more plausible.…”