Discovery of a New Redback Millisecond Pulsar Candidate: 4FGL J0940.3-7610 (Planetary Science)

Swihart and colleagues have discovered a new candidate redback millisecond pulsar binary near the center of the error ellipse of the bright unassociated Fermi-LAT γ-ray source 4FGL J0940.3–7610. The candidate counterpart is a variable optical source that also shows faint X-ray emission. Optical photometric and spectroscopic monitoring with the SOAR telescope indicates the companion is a low-mass star in a 6.5-hr orbit around an invisible primary, showing both ellipsoidal variations and irradiation and consistent with the properties of known redback millisecond pulsar binaries.

Pulsars form the largest population of Galactic FermiLAT γ-ray sources with clear associations. Follow-up studies of as-yet unidentified γ-ray sources continue to reveal new compact binaries, especially those containing millisecond pulsars (MSPs) spun up to fast periods through accretion from a companion. γ-ray emission from these objects may be ubiquitous and can serve as a signpost for MSPs difficult to find or study at other wavelengths, such as “spider” (black widow or redback) MSP binaries with non-degenerate companions that have extensive eclipses in the radio.

A class of extreme binary pulsars with semi-degenerate companion stars is dubbed “spider pulsars.” These objects are further categorized as “black widows” if the companion has extremely low mass (less than 0.1 solar masses), while if the secondary star is heavier they are called “redbacks.”

Finding the multi-wavelength (optical, X-ray, or radio) counterpart of unidentified Fermi-LAT γ-ray sources can be challenging. The γ-ray error ellipses are relatively large (often dozens of square arcminutes on the sky), potentially containing a substantial number of X-ray or radio sources and hundreds or more optical sources.

Characteristic multi-wavelength behavior can help narrow down possible associations for spider MSPs. Most of these show a characteristic hard X-ray spectrum with rapid stochastic and orbital variability, likely due to an intrabinary shock that occurs between the wind from the companion star and the pulsar wind. In addition, γ-ray emitting compact binaries with secondaries that substantially fill their Roche lobes should have detectable periodic optical variability due to the tidal deformation (and sometimes irradiation) of the secondary star.

Now, Swihart and colleagues presented the discovery of an X-ray and variable optical source that they argued is likely a new redback MSP binary associated with the unidentified Fermi source 4FGL J0940.3–7610. The results show that the companion is a low-mass star (late-G to early-K type dwarf star) orbiting an invisible primary every 0.27 days. The object showcases ellipsoidal variations and irradiation, consistent with the properties of other known redback MSP binaries.

“We have discovered a short-period (6.5 hr) compact binary with X-ray emission near the center of the error ellipse of the unassociated Fermi γ-ray source 4FGL J0940.3–7610. The optical and X-ray properties of the source are well-explained as a redback MSP but are not consistent in a straigtforward manner with any other class of source. Hence we think the binary is likely to be a redback millisecond pulsar and the counterpart to 4FGL J0940.3–7610.”

— said Swihart, lead author of the study.

Compared to known (or strong candidate) redbacks, the orbital period, high-energy properties, and distance are typical. The inclination appears relatively edge-on. The mass of the neutron star appears to be low, closer to ∼ 1.4 M as for the redbacks PSR J1723–2837 and PSR J2039–5617 rather than ∼ 1.8 M as found for a typical (median) redback.

“While, the mass of the secondary star turns out to be greater than 0.4 solar masses. The system has an edge-on inclination and is estimated to be some 7,500 light years away from the Earth.”

— wrote authors of the study.

There is also a hint that the secondary star might be on the more massive side for redback companions (perhaps & 0.4 solar masses M). This has potential relevance for the fact that a radio pulsar has not yet been detected toward this region despite extensive searches. There are several other systems that have compelling optical and X-ray evidence for being redbacks but in which pulsars have also not been found, such as 1FGL J0523.5–2529 and 3FGL J0212.1+5320, which have secondaries that are more massive than typical for redbacks. According to authors, this could suggest that the pulsars in these systems are more difficult to detect through radio observations, perhaps due to more extensive eclipses. This makes 4FGL J0940.3–7610 a strong candidate for a focused search for γ-ray pulsations.

Although all their available evidence points towards a redback classification, this needs to be confirmed with additional data. Ultimately this requires a detection of a pulsar in either radio or γ-ray observations. However, much deeper X-ray data than the shallow Swift/XRT observations (presented in their paper) could allow the detection of orbital variability or a hard X-ray spectrum, which would provide compelling supporting evidence for their classification, and they were recently approved for XMM – Newton observations for AO-20 (2021 May–2022 Apr).

The discovery of yet another redback candidate associated with a persistent γ-ray source that has been known since the first year after Fermi’s launch suggests a substantial population of compact binaries still awaits detection, and that multi-wavelength follow-up of unassociated γ-ray sources remains a fruitful route to find new candidate MSPs.

Featured image: Optical Digitized Sky Survey image of the field showing the positions and overlapping 95 percent error ellipses from the 1FGL, 2FGL, and 3FGL catalogs corresponding to the gamma-ray source 4FGL J0940.3–7610 (magenta), along with the position of the Swift X-ray source (blue circle). Credit: Swihart et al., 2021.

Reference: Samuel J. Swihart, Jay Strader, Elias Aydi, Laura Chomiuk, Kristen C. Dage, Laura Shishkovsky, “Discovery of a New Redback Millisecond Pulsar Candidate: 4FGL J0940.3-7610”, Astrophysical Journal, 2021.

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