Radio Interpulse Research Articles

Six faint gamma-ray pulsars seen with theFermiLarge Area Telescope

Context: GeV gamma-ray pulsations from over 140 pulsars have been characterized using the Fermi Large Area Telescope, enabling improved understanding of the emission regions within the neutron star magnetospheres, and the contributions of pulsars to high energy electrons and diffuse gamma rays in the Milky Way. The first gamma-ray pulsars to be detected were the most intense and/or those with narrow pulses. Aims: As the Fermi mission progresses, progressively fainter objects can be studied. In addition to more distant pulsars (thus probing a larger volume of the Galaxy), or ones in high background regions (thus improving the sampling uniformity across the Galactic plane), we detect pulsars with broader pulses or lower luminosity. Adding pulsars to our catalog with inclination angles that are rare in the observed sample, and/or with lower spindown power, will reduce the bias in the currently known gamma-ray pulsar population. Methods: We use rotation ephemerides derived from radio observations to phase-fold gamma rays recorded by the Fermi Large Area Telescope, to then determine the pulse profile properties. Spectral analysis provides the luminosities and, when the signal-to-noise ratio allows, the cutoff energies. We constrain the pulsar distances by different means in order to minimize the luminosity uncertainties. Results: We present six new gamma-ray pulsars with an eclectic mix of properties. Three are young, and three are recycled. They include the farthest, the lowest power, two of the highest duty-cycle pulsars seen, and only the fourth young gamma-ray pulsar with a radio interpulse. We discuss the biases existing in the current gamma-ray pulsar catalog, and steps to be taken to mitigate the bias.

RADIO AND GAMMA-RAY PULSED EMISSION FROM MILLISECOND PULSARS

Pulsed gamma-ray emission from millisecond pulsars (MSPs) has been detected by the sensitive Fermi, which sheds light on studies of the emission region and mechanism. In particular, the specific patterns of radio and gamma-ray emission from PSR J0101-6422 challenge the popular pulsar models, e.g. outer gap and two-pole caustic (TPC) models. Using the three dimension (3D) annular gap model, we have jointly simulated radio and gamma-ray light curves for three representative MSPs (PSR J0034-0534, PSR J0101-6422 and PSR J0437-4715) with distinct radio phase lags and present the best simulated results for these MSPs, particularly for PSR J0101-6422 with complex radio and gamma-ray pulse profiles and for PSR J0437-4715 with a radio interpulse. It is found that both the gamma-ray and radio emission originate from the annular gap region located in only one magnetic pole, and the radio emission region is not primarily lower than the gamma-ray region in most cases. In addition, the annular gap model with a small magnetic inclination angle instead of "orthogonal rotator" can account for MSPs' radio interpulse with a large phase separation from the main pulse. The annular gap model is a self-consistent model not only for young pulsars but also MSPs, and multi-wavelength light curves can be fundamentally explained by this model.

Mapping the magnetosphere of PSR B1055−52

We present a geometric study of the radio and γ-ray pulsar B1055−52 based on recent observations at the Parkes radio telescope. We conclude that the pulsar's magnetic axis is inclined at an angle of 75° to its rotation axis and that both its radio main pulse and interpulse are emitted at the same height above their respective poles. This height is unlikely to be higher or much lower than 700 km, a typical value for radio pulsars. It is argued that the radio interpulse arises from emission formed on open fieldlines close to the magnetic axis which do not pass through the magnetosphere's null (zero-charge) surface. However, the main pulse emission must originate from fieldlines lying well outside the polar cap boundary beyond the null surface, and farther away from the magnetic axis than those of the outer gap region where the single γ-ray peak is generated. This casts doubt on the common assumption that all pulsars have closed, quiescent, corotating regions stretching to the light cylinder.

Inner Annular Gap and Related Topics

Geometrical properties of the annular gap model, efforts of observational tests on it and applications of the model to explain the bi-drifting phenomenon are introduced in this paper. It was shown that in the frame of the annular gap model the observed γ-ray and radio radiation properties of pulsars can be well reproduced. The annular gap model can naturally explain the bi-drifting phenomenon, which is difficult to be understood in classical inner vacuum models. Recently, Wang et al. proposed an independent geometrical method to determine the location for the multi-band radiation regions of PSR B1055–52. They found that the γ-ray emission and radio main pulse emission come from the annular gap, while the radio inter-pulse originates from the core cap region. The results show that the constrained radio and γ-ray emission regions are generally consistent with the predictions of the annular gap model.

Observational constraints on the radio and γ-ray emission regions of PSR B1055−52

Observational constraints on the radio and γ-ray emission regions of PSR B1055-52 seen through our line of sight are presented by analysing the position angle curves of radio linear polarization and fitting the observed pulse widths of radio and γ-ray pulses and the phase offsets between them. Aberration, retardation and magnetic field sweep back effects that can cause additional phase offset between the emissions from different locations are taken into account. The following conclusions are obtained. (i) The radio main pulse and γ-ray pulses are emitted from the same pole, while the radio interpulse is emitted from the opposite pole. (ii) The interpulse emission region locates on the open field lines much closer to the magnetic axis than those of the main pulse, and the emission altitudes are higher than those of the main pulse. (iii) At each pole, there are probably two groups of field lines where radio emission is generated, of which the outer one consists of open field lines near (or including) the last open field lines and the inner one consists of open field lines from very near the magnetic pole to approximately the midway between the magnetic axis and the last open field lines. (iv) The γ-ray pulse comes from inner open field lines rather than from the last open field lines, and the emission altitudes are beyond the null charge surface.

Possible Evidence for Pulsed X‐Ray Emission from the Outer Gap in PSR B1937+21

The fastest millisecond pulsar PSR B1937+21 presents an interpulse separated from the main pulse by nearly 180° at radio frequencies. Recently, the ASCA observations detected pulsed X-ray emission from this pulsar. Only a single narrow X-ray pulse is observed, which is coincident with the radio interpulse in phase. We investigate the possible origin of the pulsed X-rays from the polar cap (PC) accelerators or the outer gap (OG) accelerators in the frame of a PC model and an OG model, respectively, by assuming a dipolar magnetic field structure and the same radio emission pattern from its poles for the pulsar. The OG model can naturally explain the main observational facts. For the PC model, the coincidence between the X-ray pulse and the radio interpulse cannot be reproduced in the assumed case. However, when considering possible deviation from our assumption, the PC model may still be valid for this pulsar in some cases.

Pulsed X‐Ray Emission from the Fastest Millisecond Pulsar: PSR B1937+21 withASCA

We have detected pulsed X-ray emission from the fastest millisecond pulsar known, PSR B1937+21 (P = 1.558 ms), with ASCA. The pulsar is detected as a point source above ~1.7 keV, with no indication of nebulosity. The source flux in the 2-10 keV band is found to be f = (3.7 ± 0.6) × 10-13 ergs s-1 cm-2, which implies an isotropic luminosity of LX = 4πD2f ~ (5.7 ± 1.0) × 1032(D/3.6 kpc)2 ergs s-1, where D is the distance, and an X-ray efficiency of ~5 × 10-4, relative to the spin-down power of the pulsar. The pulsation is found at the period predicted by the radio ephemeris with a very narrow primary peak, the width of which is about 1/16 phase (~100 μs), near the time resolution limit (61 μs) of the observation. The instantaneous flux in the primary peak (1/16 phase interval) is found to be (4.0 ± 0.8) × 10-12 ergs s-1 cm-2. Although there is an indication for the secondary peak, we consider its statistical significance too low to claim a definite detection. The narrow pulse profile and the detection in the 2-10 keV band imply that the X-ray emission is caused by the magnetospheric particle acceleration. Comparison of X-ray and radio arrival times of pulses indicates, within the timing errors, that the X-ray pulse is coincident with the radio interpulse.

The Radio Interpulse from PSR 0950+08

Two hypotheses exist to explain the radio interpulse of PSR 0950+08 within the context of polar cap emission models: 1)The emission originates at a single magnetic pole as for other two-component pulsars, except that the component spacing is much wider.2)The interpulse emission originates at the opposite magnetic pole. We discuss here some new observations and their implications for the single and double pole hypotheses.