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Studying the black widow pulsars PSR J0312$-$0921 and PSR J1627$+$3219 in the optical and X-rays

A. V. Bobakov, A. Kirichenko, S. V. Zharikov, D. A. Zyuzin, A. V. Karpova, Yu. A. Shibanov, T. Begari

TL;DR

This paper presents the first optical identifications and phase-resolved multi-band photometry of the black widow pulsars PSR J0312$-$0921 and PSR J1627$+$3219, obtained with the GTC and HiPERCAM, complemented by archival X-ray data. The light curves are well described by a direct heating model, yielding compact constraints on the system geometry and component masses, with pulsar masses around $M_{ m p} \approx 2.6$--$2.7\ M_\\odot$ and companion masses of a few 10$^{-2}$ solar masses. The optical fits place distances at $D \approx 2.5$ and $4.6$ kpc, and reveal very low night-side temperatures for J0312$-$0921, while J1627$+$3219 shows a relatively warm night side. X-ray analysis finds no detection for J0312 and a faint X-ray counterpart for J1627 with a photon index $\Gamma \approx 3.3$ and $L_X \approx 1.1\times10^{31}$ erg s$^{-1}$, consistent with other black widows at similar distances. Collectively, the results illustrate how optical phase-resolved photometry can tightly constrain neutron-star masses, irradiation effects, and binary geometry in BW systems, while highlighting the need for deeper X-ray and timing data to refine the intrinsic energetics and evolution of these binaries.

Abstract

Context. PSR J0312$-$0921 and PSR J1627$+$3219 are black widow pulsars with orbital periods of 2.34 and 3.98 hours. They were recently detected in the radio and $γ$-rays. Aims. Our goals are to estimate the fundamental parameters of both binary systems and their components. Methods. We performed first phase-resolved multi-band photometry of both objects with the 10.4-m Gran Telescopio Canarias and fitted the obtained light curves with a model assuming direct heating of the companion by the pulsar. Archival X-ray data obtained with the Swift and XMM-Newton observatories were also analysed. Results. For the first time, we firmly identified both systems in the optical. Their optical light curves show a rather symmetric single peak per orbital period and a peak-to-peak amplitude of $\gtrsim$2 mag. We also identified the X-ray counterpart to J1627$+$3219, while for J0312$-$0921 we set an upper limit on the X-ray flux. Conclusions. We estimated the masses of the pulsars, companion temperatures and masses, Roche lobe filling factors, orbital inclinations, and the distances to both systems. PSR J0312$-$0921 has a very light ($\approx$0.02 M$_\odot$) companion which possibly has one of the lowest ($\approx$1600 K) `night-side' temperatures among the known black widow systems. We found that the distances to J0312$-$0921 and J1627$+$3219 are about 2.5 and 4.6 kpc, respectively. This likely explains their faintness in X-rays. The X-ray spectrum of PSR J1627$+$3219 can be described by the power-law model, and its parameters are compatible with those obtained for other black widows.

Studying the black widow pulsars PSR J0312$-$0921 and PSR J1627$+$3219 in the optical and X-rays

TL;DR

This paper presents the first optical identifications and phase-resolved multi-band photometry of the black widow pulsars PSR J03120921 and PSR J16273219, obtained with the GTC and HiPERCAM, complemented by archival X-ray data. The light curves are well described by a direct heating model, yielding compact constraints on the system geometry and component masses, with pulsar masses around -- and companion masses of a few 10 solar masses. The optical fits place distances at and kpc, and reveal very low night-side temperatures for J03120921, while J16273219 shows a relatively warm night side. X-ray analysis finds no detection for J0312 and a faint X-ray counterpart for J1627 with a photon index and erg s, consistent with other black widows at similar distances. Collectively, the results illustrate how optical phase-resolved photometry can tightly constrain neutron-star masses, irradiation effects, and binary geometry in BW systems, while highlighting the need for deeper X-ray and timing data to refine the intrinsic energetics and evolution of these binaries.

Abstract

Context. PSR J03120921 and PSR J16273219 are black widow pulsars with orbital periods of 2.34 and 3.98 hours. They were recently detected in the radio and -rays. Aims. Our goals are to estimate the fundamental parameters of both binary systems and their components. Methods. We performed first phase-resolved multi-band photometry of both objects with the 10.4-m Gran Telescopio Canarias and fitted the obtained light curves with a model assuming direct heating of the companion by the pulsar. Archival X-ray data obtained with the Swift and XMM-Newton observatories were also analysed. Results. For the first time, we firmly identified both systems in the optical. Their optical light curves show a rather symmetric single peak per orbital period and a peak-to-peak amplitude of 2 mag. We also identified the X-ray counterpart to J16273219, while for J03120921 we set an upper limit on the X-ray flux. Conclusions. We estimated the masses of the pulsars, companion temperatures and masses, Roche lobe filling factors, orbital inclinations, and the distances to both systems. PSR J03120921 has a very light (0.02 M) companion which possibly has one of the lowest (1600 K) `night-side' temperatures among the known black widow systems. We found that the distances to J03120921 and J16273219 are about 2.5 and 4.6 kpc, respectively. This likely explains their faintness in X-rays. The X-ray spectrum of PSR J16273219 can be described by the power-law model, and its parameters are compatible with those obtained for other black widows.

Paper Structure

This paper contains 7 sections, 3 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Optical observations and data reduction
  3. J0312
  4. J1627
  5. Optical light curves and the system parameters
  6. X-ray data
  7. Discussion and conclusions

Figures (3)

  • Figure 1: Optical images of the fields of the two pulsars. Top panel: 305 $\times$ 305 image of the J0312 field obtained with the GTC/OSIRIS in the $r'$ band. Bottom panel: 28 $\times$ 14 FoV of the GTC/HiPERCAM in the $g_s$ band containing J1627. The pulsars' vicinities are shown with the boxes and enlarged in the insets to demonstrate the maximum (top) and minimum (bottom) brightness phases of their companions, whose positions are marked by blue circles. The positions of the stars used for the photometric calibration are marked by red circles. The star used to account for the changing weather conditions is shown with the green circle in the top panel. For J1627, for this purpose we used the same stars as for the photometric calibration.
  • Figure 2: Light curves of J0312 (left) and J1627 (right) folded with the orbital periods and the best-fitting models (solid lines). Two periods are shown for clarity. The orbital phases $\phi=0.0$ correspond to the minima of the models' brightness. Panels show residuals calculated as the difference between the observed ($O$) and calculated ($C$) magnitudes for each data point in terms of the magnitude error, $\sigma$. Dashed lines correspond to 3$\sigma$ levels.
  • Figure 3: 6$^{\prime}$$\times$ 6$^{\prime}$XMM-Newton/PN image of the J1627 field in the 0.3--2 keV band. The 'X' symbol marks the pulsar timing position from Table \ref{['tab:pars']}. The dashed circle shows the region chosen for the background extraction.