Long-period Pulsars as Possible Outcomes of Supernova Fallback Accretion

Abstract

For about half a century, the radio pulsar population was observed to spin in the ∼0.002–12 s range, with different pulsar classes having a spin-period evolution that differs substantially depending on their magnetic fields or past accretion history. The recent detection of several slowly rotating pulsars has reopened the long-standing question of the exact physics, and observational biases, driving the upper bound of the period range of the pulsar population. In this work, we perform a parameter study of the spin-period evolution of pulsars interacting with supernova fallback matter and specifically look at the fallback accretion disk scenario. Depending on the initial conditions at formation, this evolution can differ substantially from the typical dipolar spin-down, resulting in pulsars that show spin periods longer than their coeval peers. By using general assumptions for the pulsar spin period and magnetic field at birth, initial fallback accretion rates, and including magnetic field decay, we find that very long spin periods (≳100 s) can be reached in the presence of strong, magnetar-like magnetic fields (≳1014 G) and moderate initial fallback accretion rates (∼1022−1027 g s−1). In addition, we study the cases of two recently discovered periodic radio sources, the pulsar PSR J0901–4046 (P = 75.9 s) and the radio transient GLEAM-X J162759.5–523504.3 (P = 1091 s), in light of our model. We conclude that the supernova fallback scenario could represent a viable channel to produce a population of long-period isolated pulsars that only recent observation campaigns are starting to unveil.

Constraining the Nature of the 18 min Periodic Radio Transient GLEAM-X J162759.5-523504.3 via Multiwavelength Observations and Magneto-thermal Simulations

“…In this case, to explain the 18 min spin period, we would need a constant field of B ∼ 10 15 G for ∼10 8 yr (or B ∼ 10 16 G for ∼10 6 yr), which is rather extreme when compared to the known magnetar population in our Galaxy. In contrast, assuming fall-back accretion, the spin period can easily be reconciled with a 18 min value (Ronchi et al 2022). Although the core-dominated field-decay interpretation or a fast cooling scenario are in principle viable, they are intriguing, as we have no evidence of other pulsars that require either hypothesis to explain their emission and spin period.…”

“…In the magnetar scenario, the upper limits we have derived on the X-ray luminosity (L X < 10 30 erg s −1 ) imply that GLEAM-X J1627's age should be >1 Myr for any reasonable crustal magnetic field (B > 10 13 G). The 18 min spin period (assuming a fast rotating pulsar at birth) would necessarily require a strong magnetic field and a phase of fossil-disk accretion (see Ronchi et al 2022), but in any case the age of GLEAM-X J1627 is constrained to be two orders of magnitude higher than that of typical radio-loud magnetars (which have ages <20 kyr). At this age, the bright radio bursts emitted by GLEAM-X J1627 would be unusual for such an old magnetar.…”

“…However, as extensively studied by Ronchi et al (2022), the 18 min periodicity of GLEAM-X J1627's radio emission cannot be reconciled with the pulsar scenario when only dipolar losses and a typical crust+core field configuration are considered. This would require the assumption that this pulsar has an unreasonably large magnetic field (B ∼ 10 16 -10 17 G) that does not decay in time (something unseen in the pulsar population; see also Figure 1 by Ronchi et al 2022).…”

“…However, as extensively studied by Ronchi et al (2022), the 18 min periodicity of GLEAM-X J1627's radio emission cannot be reconciled with the pulsar scenario when only dipolar losses and a typical crust+core field configuration are considered. This would require the assumption that this pulsar has an unreasonably large magnetic field (B ∼ 10 16 -10 17 G) that does not decay in time (something unseen in the pulsar population; see also Figure 1 by Ronchi et al 2022). In a typical crust+core field configuration, the magnetic field is expected to decay on a timescale of 10-100 Kyr depending on its intensity (the stronger the field, the faster it decays; see the spin-period evolutionary curves in Figure 1).…”

“…In this scenario, the supernova fossil-disk is now inactive (because the disk is now too cold or has been completely disrupted), so the source had resumed its dipolar-driven rotation and normal radio-loud magnetar activity. However, its spin period has been driven at a longer value than that of its peers at the same age and field (∼10 14 -10 15 G; for detailed simulations, see Ronchi et al 2022;Gençali et al 2022, andTong 2022).…”