PSR B0943+10: low-frequency study of subpulse periodicity in the Bright mode with LOFAR

Abstract

We use broadband sensitive LOFAR observations in the 25-80 MHz frequency range to study the single-pulse emission properties of the mode-switching pulsar B0943+10. We review the derivation of magnetospheric geometry, originally based on low-frequency radio data, and show that the geometry is less strongly constrained than previously thought. This may be used to help explain the large fractional amplitudes of the observed thermal X-ray pulsations from the polar cap, which contradicted the almost aligned rotator model of PSR B0943+10. We analyse the properties of drifting subpulses in the Bright mode and report on the short-scale (minutes) variations of the drift period. We searched for the periodic amplitude modulation of drifting subpulses, which is a vital argument for constraining several important system parameters: the degree of aliasing, the orientation of the line-of-sight vector with respect to magnetic and spin axes, the angular velocity of the carousel, and thus, the gradient of the accelerating potential in the polar gap. The periodic amplitude modulation was not detected, indicating that it may be a rare or narrow-band phenomenon. Based on our non-detection and review of the available literature, we chose to leave the aliasing order unconstrained and derived the number of sparks under different assumptions about the aliasing order and geometry angles. Contrary to the previous findings, we did not find a large (of the order of 10%) gradual variation of the separation between subpulses throughout Bright mode. We speculate that this large variation of subpulse separation may be due to the incorrect accounting for the curvature of the line of sight within the on-pulse window. Finally, we report on the frequency-dependent drift phase delay, which is similar to the delay reported previously for PSR B0809+74. We provide a quantitative explanation of the observed frequency-dependent drift phase delay within the carousel model.

Thermal Emission and Magnetic Beaming in the Radio and X-Ray Mode-switching PSR B0943+10

“…In conclusion, the angle between magnetic and spin axis derived by Bilous (2018) is constrained in the range: 5 • < ξ < 30 • . For each value of ξ only two well defined values of χ are possible and are in the ranges 3 • < χ < 22 • (inside traverse model) or 7 • < χ < 38 • (outside traverse model).…”

“…In addition, thanks to the increased counting statistics, we showed that the single blackbody fit to the B-mode emission, that was consistent with the 2014 data, is actually disfavored. However, if we adopt the pulsar geometrical configuration derived from recent radio studies (Bilous 2018), these simple models face two problems: a) they can not give rise to the significant and energy-dependent pulsed fraction observed in the Q-mode; b) they can not correctly reproduce the spectrum of the unpulsed flux, that should be dominated by the blackbody emission, while it can be fitted well by a single power law.…”

“…On the other hand, Bilous (2018) noted that the uncertainties on ρ 1 GHz are larger and, following Mitra & Deshpande (1999), adopted 2 • < ρ 1 GHz < 8 • . She found that η ≈ ρ 1 GHz and considered that both branches of solutions corresponding to the inside and the outside traverse should be retained because the amplitude modulation at 37P of the drifting subpulses was not detected in her new radio data.…”

“…Recently, Bilous (2018) reconsidered the problem of the geometry of PSR B0943+10. The angles ξ and χ are evaluated by analyzing the frequency dependence of the observed…”

Nulling and subpulse drifting in PSR J1727-2739

“…When the line of sight passes through the emission region, drifting subpulses can be seen. The carousel model was successful in interpreting subpulse drifting behaviour of many pulsars (Bhattacharyya et al 2007;Rankin et al 2013;Rankin & Rosen 2014;Bilous 2018). In the original carousel model, the drift rate remains constant, and thus it is difficult to explain the observed multiple drift modes in a pulsar based on the original carousel model.…”