Formation and nature of "Huntsman" binary pulsars

TL;DR

The paper tests the Huntsman pulsar hypothesis by computing explicit binary-evolution tracks with and without irradiation feedback (IFB) for a donor of $M_{2,i}=1.25\,M_{\odot}$ and a neutron star of $M_{NS,i}=1.3\,M_{\odot}$ across initial orbital periods and metallicities. It demonstrates that hydrogen-shell burning detachment (HSBD) can produce a detached Huntsman phase lasting a few Myr within well-defined $P_{orb,i}$ ranges, with metallicity shifting these ranges ($1 \le P_{orb,i} \le 12.83$ d for solar and $1.44 \le P_{orb,i} \le 26.62$ d for $Z=10^{-3}$). IFB introduces pulsed mass transfer and detached episodes that align with the Redback stage, while HSBD and IFB operate independently, indicating a unified, robust Huntsman scenario within the broader spider pulsar family. Overall, the results advance a cohesive picture where Huntsman is a natural evolutionary stage under general conditions, bridging Redbacks, Black Widows, and related systems through shared physical processes.

Abstract

Spider systems are a class of close binaries in which a neutron star first accretes from a normal companion, and later ablates it in some cases. New observations have expanded this category, with the addition of a Huntsman group, tentatively linked to a short donor phase along the red bump in the secondary evolutionary track. We present explicit evolutionary tracks that support the Huntsman nature recently suggested, and discuss how the whole class of spiders emerges from the full consideration of irradiation and ablating winds. We address the irradiation feedback (IFB) effects and the hydrogen-shell burning detachment (HSBD) simultaneously, and show that they act independently and do not interfere with each other, supporting a physical picture of the Huntsman group. We employ our binary evolution code to compute a suite of binary systems formed by a donor star and a neutron star for different initial orbital periods, assuming solar composition and Z=0.01. Although many models do not consider IFB, we also present the evolution with IFB for one system as an example. We found that the recently suggested association of Huntsman pulsar with the evolutionary stage where (as a consequence of the dynamics of HSBD) the system remains detached for a few million years is plausible. However, this feature alone is unable to account for the occurrence of Redback spider pulsars. Meanwhile, models including IFB, with pulsed mass transfer, display detachment episodes that can be naturally associated with the Redback stage. Irradiation feedback does not preclude or modify HSBD and in fact, Huntsman systems were already present as an implicit prediction in our earlier calculations. We conclude that Huntsman is an expected stage of the spider systems under quite general conditions. This is another step towards a unified picture of spider pulsars as a group.

Figures (4)

• Figure 1: The orbital period as a function of the donor mass for a suite of systems formed by a donor star of 1.25 $M_{\odot}$, a NS of 1.3 $M_{\odot}$ with $P_{orb,i}$ logarithmically evenly spaced with steps of 20%. The orbital evolution was computed from the ZAMS. Black (magenta) lines denote results corresponding to solar metallicity ($Z=10^{-3}$). The detached stages, in which Huntsman pulsars are expected, are denoted in blue for solar metallicity and in pink for $Z=10^{-3}$. Horizontal bars correspond to the measurements of the system PSR J1947-1120 (green) and PSR J1417-4402 (black). MB0 denotes the standard magnetic braking prescription.
• Figure 2: HR diagram showing the trajectories and the intervals where the Huntsman states occur (blue). The first initial period that allows the mass transfer (the so-called case B) is clearly seen on the bottom, producing the ample and slanted "V" track is $P_{orb, i} = 1 \, d$, while all shorter periods end in downward tracks. It is important to remind the reader that this is somewhat sensitive to the treatment of the physics and small variations may occur for different choices of the braking and other features (see, e.g., Thomas1967Mis).
• Figure 3: $R_{2}/R_{L}$ filling factor for some of the solar composition models presented in Fig. \ref{['Fig:masa_vs_periodo']} as a function of age. Models with larger $P_{orb,i}$ that undergo HSBD are not included since they behave very similar to the case of $P_{orb,i}= 3.58$ d. Labels on each curve correspond to $P_{orb,i}$. The fractions marked with heavy blue lines correspond to conditions in which the system is detached, allowing for the detection of the pulsar companion.
• Figure 4: $R_{2}/R_{L}$ filling factor for a system formed by a solar composition donor star of $1M_{\odot}$, a NS of $1.4M_{\odot}$ with $P_{inic,i}=1\ d$. We considered moderate IFB assuming $\alpha_{irr}= 0.10$. Blue lines represent detached conditions whereas black lines depict stages with a mass transfer rate $\dot{M} \geq 10^{-11}\ M_{\odot}\ y^{-1}$. IFB-driven pulsed mass transfer and HSBD occur in the model.