Search for Radio Pulsations from Neutron Star Candidates in Detached Binaries

Abstract

Recent optical astrometric and spectroscopic surveys have identified numerous neutron star (NS) candidates in non-accreting detached binary systems, but their compact-object nature remains unconfirmed. In this work, we present targeted radio observations of 31 such candidates using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), the Robert C. Byrd Green Bank Telescope, and the Shanghai TianMa Radio Telescope. Over a total of 46.65 hours of observing time, we detected neither periodic nor single-pulse radio emissions. These nondetections place stringent upper limits on the flux densities of any potential radio signals, reaching ~4 $μ$Jy for periodic emission and ~10 mJy for single pulses with FAST. Since our observations are highly sensitive and the flux density upper limits are well below the median fluxes of known Galactic pulsars, this suggests that geometric beaming is the most likely explanation for the non-detections if these objects are indeed pulsars. Alternatively, the NSs may be sufficiently old ($\gtrsim$ 10 Gyr) and have become intrinsically radio-quiet. In this case, our findings highlight the inherent difficulty of confirming NSs in such old detached binary systems through radio pulsation searches.

Figures (6)

• Figure 1: Formation and subsequent evolution of detached binaries with an NS and a low-mass star Bhattacharya+1991Tauris+2006pbse. The detached binary stage is highlighted by the blue dashed-line box. MS: main-sequence star, NS: neutron star, LMXB: low-mass X-ray binary, MSP: millisecond pulsar, WD: white dwarf, SyXRB: symbiotic X-ray binary, RG: red giant.
• Figure 2: Fourier frequency derivative $z$ required to account for orbital acceleration effects in pulsar searches in detached binaries. $z$ values are calculated for a binary system consisting of a $1.4~{M_\odot}$ NS and a $1.0~{M_\odot}$ companion in an orbit observed at a true anomaly of $A_{\rm T}=-90~\deg$ in an edge-on configuration ($i=90~\deg$) with an argument of periapsis $\omega=0~\deg$. The left and right panels correspond to integration times of $T_{\rm obs}=20~{\rm min}$ and $60~{\rm min}$. Spin frequency of the pulsar is assumed to be $1~{\rm Hz}$ and the harmonic summing set to $h=16$. Red circles mark NS candidates with known orbital periods and eccentricities (see \ref{['tab:info']}; with $e=0$ assumed when unspecified). Red dashed lines indicate the $T_{\rm obs}<0.15P_{\rm orb}$ condition, below which an orbital jerk search is necessary.
• Figure 3: The period--period derivative ($P$--$\dot P$) diagram for known pulsars ATNF. Gray dots represent isolated pulsars while black circles denote pulsars in binary systems. The black dashed line correspond to pulsar death line model proposed by Chen+1993. The black dash-dotted and dotted lines correspond to pulsar death line models base on curvature radiation from vacuum gap (VG) and space-charged-limited flow (SCLF) Zhang+2000. The gray dashed, and dotted lines represent the lines of constant magnetic field and constant characteristic age hpa, respectively. The blue, orange and green curves show spin-down evolutionary tracks with an initial period of 20 ms and $B_{\rm 0}=10^{11},$$10^{12}$ and $10^{13}~{\rm G}$, respectively.
• Figure 4: Diagnostic plot for PSR J1503$-$2111. Upper left: Summed profile. Lower left: time--phase plot. Upper middle: frequency--phase plot. Lower middle: reduced $\chi^2$ vs. DM. Upper right: reduced $\chi^2$ vs. period derivative. Middle right: reduced $\chi^2$ vs. period. Lower right: reduced $\chi^2$ map in the period derivative and period plane. Observing and derived parameters are listed in the inset.
• Figure 5: FFA diagnostic for PSR J1503+2111. Upper left inset: observing parameters and detected signal properties. Lower left: S/N vs. DM. Upper right: time--phase plot. Lower right: summed pulse profile.