Timing of Eight Binary Millisecond Pulsars Found with Arecibo in Fermi-LAT Unidentified Sources (2012.15185v1)

Abstract: We present timing solutions for eight binary millisecond pulsars (MSPs) discovered by searching unidentified Fermi-LAT source positions with the 327 MHz receiver of the Arecibo 305-m radio telescope. Five of the pulsars are "spiders" with orbital periods shorter than 8.1 h. Three of these are in "black widow" systems (with degenerate companions of 0.02-0.03 solar masses), one is in a "redback" system (with a non-degenerate companion of $\gtrsim 0.3$ solar masses), and one (J1908+2105) is an apparent middle-ground case between the two observational classes. The remaining three pulsars have white dwarf companions and longer orbital periods. With the initially derived radio timing solutions, we detected gamma-ray pulsations from all MSPs and extended the timing solutions using photons from the full Fermi mission, thus confirming the identification of these MSPs with the Fermi-LAT sources. The radio emission of the redback is eclipsed during 50% of its orbital period, which is typical for this kind of system. Two of the black widows exhibit radio eclipses lasting for 10-20% of the orbit, while J1908+2105 eclipses for 40% of the orbit. We investigate an apparent link between gamma-ray emission and a short orbital period among known binary MSPs in the Galactic disk, and conclude that selection effects cannot be ruled out as the cause. Based on this analysis we outline how the likelihood of new MSP discoveries can be improved in ongoing and future pulsar searches.

• The paper presents precision radio timing and gamma-ray detections for eight binary MSPs, establishing robust multi-wavelength ephemerides.
• It classifies five pulsars as spiders—including black widows and a redback—highlighting diverse companion types and short orbital periods.
• Detection challenges, such as heavy radio eclipses in redback systems, underscore the need for advanced search algorithms in gamma-ray surveys.

Timing Solutions of Eight Millisecond Pulsars

The paper discusses the timing solutions for eight binary millisecond pulsars (MSPs) identified using the Arecibo radio telescope and the Fermi Large Area Telescope (LAT). These discoveries were made by observing gamma-ray sources previously unidentified in the Fermi-LAT catalog. The manuscript methodically presents the radio timing solutions and corresponding gamma-ray detections, which are of particular interest given the growing recognition of MSPs as gamma-ray emitters.

The researchers use the 327 MHz receiver at Arecibo to conduct targeted searches of specific Fermi-LAT gamma-ray sources, successfully identifying eight MSPs. Notably, five of these pulsars are classified as "spiders" based on their binary nature and short orbital periods. Among these, three systems are recognized as "black widows," characterized by extremely low-mass companions, while one system is marked as a "redback," having a more massive companion. One pulsar, J1908+2105, stands at the intersection of these classifications due to its unique mass and orbital characteristics. The remaining three pulsars are paired with white dwarf companions and present with relatively longer orbital periods.

The methodology section highlights the multi-step process involving radio observations to establish initial ephemerides, leading to comprehensive timing solutions. Subsequent gamma-ray detection employs these radio results to adjust and improve positional and orbital parameters over the entire Fermi mission. Convergence of phase information across electromagnetic bands offers a robust confirmation of the pulsar-gamma-ray association.

The practical aspects of the paper noted certain detectability challenges. Particularly, the heavy radio eclipses witnessed in redback systems like J1048+2339 and J1908+2105 underscore the complexity of detecting such sources. This finding suggests room for improving search algorithms and techniques that better account for these obstacles, such as factoring in unaccounted line-of-sight acceleration, potentially unlocking additional discoveries in the pulsar population.

A vital discussion point lies in the analysis of the distribution of binary MSPs found in Fermi-LAT unidentified sources as compared to those discovered through untargeted surveys. The paper proposes that despite differences in observed populations, selection effects—more so than an intrinsic link between short orbital periods and gamma-ray emission—likely account for observed biases in discoveries. This hypothesis is bolstered by comparative statistics of orbital periods and detection metrics from various observatories.

The implications of this research are underlined by its contribution to understanding the architecture and evolution of binary MSP systems. Notably, the correlation between orbital properties and companion characteristics may aid in modeling accretion and spin-down processes, providing insights into neutron star evolution in compact binaries. The paper suggests future work could explore MSP population distribution, potentially refining search strategies for future surveys.

Overall, this research advances our understanding of MSPs as significant contributors to gamma-ray catalogs. Continued investigations will not only expand the inventory of such astrophysical objects but will enhance our grasp of their underlying physical processes and interactions within the galactic milieu.