We report a study of the X-ray emission from the white dwarf/M-type star binary system AR Scorpii using archival data taken in 2016-2020. It has been known that the X-ray emission is dominated by the optically thin thermal plasma emission, and its fl
ux level varies significantly over the orbital phase. The X-ray emission also contains a component that modulates with the beat frequency between the white dwarfs spin frequency and orbital frequency. In this new analysis, the 2020 data taken by NICER shows that the X-ray emission is modulating with the spin frequency as well as the beat frequency, indicating that part of the X-ray emission is coming from the white dwarfs magnetosphere. It is found that the signal of the spin frequency appears only at a specific orbital phase, while the beat signal appears over the orbital phase. We interpret the X-ray emission modulating with the spin frequency and the beat frequency as a result of the synchrotron emission from electrons with a smaller and larger pitch angle, respectively. In a long-term evolution, the beat pulse profile averaged over the orbital phase changed from a single-peak structure in 2016/2018 to a double-peak structure in 2020. The observed X-ray flux levels measured in 2016/2017 are higher than those measured in 2018/2020. The plasma temperature and amplitude of the orbital waveform might vary with time too. These results indicate that the X-ray emission from AR Scorpii evolves on a timescale of years. This long-term evolution would be explained by a super-orbital modulation related to, for example, a precession of the white dwarf, or a fluctuation of the system related to activity of the companion star.
PSR J2021+4026 is a radio-quiet gamma-ray pulsar and the first pulsar that shows state change of the gamma-ray emission and spin-down rate. The state change of PSR J2021+4026 was first observed at 2011 October, at which the pulsar changes the state f
rom high gamma-ray flux/low spin-down rate state to low gamma-ray flux/high spin-down rate st ate. In December 2014, PSR J2021+4026 recovered the state before the 2011 state change over a timescale of a few months. We report that the long term evolution of the gamma-ray flux and timing behavior suggests that PSR J2021+4026 changed the state near 2018 February 1st and entered a new low gamma-ray flux/high spin-down rate state. At the 2018 state change, the averaged flux dropped from $(1.29pm 0.01)times 10^{-6} {rm cts~cm^{-2}s^{-1}}$ to $(1.12pm 0.01)times 10^{-6} {rm cts~cm^{-2}s^{-1 }}$, which has the similar behavior to the case of 2011 event. The spin-down rate has increased by $sim 3%$ in the new state since the 2018 state change. The shapes of pulse profile and spectrum in GeV bands also changed at the 2018 event, and they are consistent with behavior at the 2011 state change. Our results probably suggest that PSR J2021+4026 is switching between different states with a timescale of several years, like some radio pulsars (e.g. PSR~B1828-11). PSR J2021+4026 will provide a unique opportunity to study the mechanism of the state switching.
We study linear polarization of optical emission from white dwarf (WD) binary system AR~Scorpii. The optical emission from this binary is modulating with the beat frequency of the system, and it is highly polarized, with the degree of the polarizatio
n reaching $sim 40$%. The angle of the polarization monotonically increases with the spin phase, and the total swing angle can reach $360^{circ}$ over one spin phase. It is also observed that the morphology of the pulse profile and the degree of linear polarization evolve with the orbital phase. These polarization properties can constrain the scenario for nonthermal emission from AR Scorpii. In this paper, we study the polarization properties predicted by the emission model, in which (i) the pulsed optical emission is produced by the synchrotron emission from relativistic electrons trapped by magnetic field lines of the WD and (ii) the emission is mainly produced at magnetic mirror points of the electron motion. We find that this model can reproduce the large swing of the polarization angle, provided that the distribution of the initial pitch angle of the electrons that are leaving the M-type star is biased to a smaller angle rather than a uniform distribution. The observed direction of the swing suggests that the Earth viewing angle is less than $90^{circ}$ measured from the WD spin axis. The current model prefers an Earth viewing angle of $50^{circ}-60^{circ}$ and a magnetic inclination angle of $50^{circ}-60^{circ}$ (or $120^{circ}-130^{circ}$). We discuss that the different contribution of the emission from M-type star to total emission causes a large variation in the pulsed fraction and the degree of the linear polarization along the orbital phase.
We have discovered an extended X-ray feature which is apparently associated with millisecond pulsar (MSP) PSR J1911-1114 from a XMM-Newton observation, which extends for ~1 and the radio timing position of PSR J1911-1114 is in the mid point of the fe
ature. The orientation of the feature is similar to the proper motion direction of PSR J1911-1114. Its X-ray spectrum can be well-modeled by an absorbed power-law with a photon index of $Gamma=1.8^{+0.3}_{-0.2}$. If this feature is confirmed to be a pulsar wind nebula (PWN), this will be the third case that an X-ray PWN found to be powered by a MSP.
We have conducted a systematic survey for the X-ray properties of millisecond pulsars (MSPs). Currently, there are 47 MSPs with confirmed X-ray detections. We have also placed the upper limits for the X-ray emission from the other 36 MSPs by using th
e archival data. We have normalized their X-ray luminosities $L_{x}$ and their effective photon indices $Gamma$ into a homogeneous data set, which enable us to carry out a detailed statistical analysis. Based on our censored sample, we report a relation of $L_{x}simeq10^{31.05}left(dot{E}/10^{35}right)^{1.31}$ erg/s (2-10 keV) for the MSPs. The inferred X-ray conversion efficiency is found to be lower than previously reported estimate that could be affected by selection bias. $L_{x}$ also correlates/anti-correlates with the magnetic field strength at the light cylinder $B_{LC}$/characteristic age $tau$. On the other hand, there is no correlation between $L_{x}$ and their surface magnetic field strength $B_{s}$. We have further divided the sample into four classes: (i) black-widows, (ii) redbacks, (iii) isolated MSPs and (iv) other MSP binaries, and compare the properties among them. We noted that while the rotational parameters and the orbital periods of redbacks and black-widow are similar, $L_{x}$ of redbacks are significantly higher than those of black-widows in the 2-10 keV band. Also the $Gamma$ of redbacks are apparently smaller than those of black-widows, which indicates the X-ray emission of redbacks are harder than that of black-widows. This can be explained by the different contribution of intrabinary shocks in the X-ray emission of these two classes.
PSR~J2021+4026 showed a sudden decrease in the gamma-ray emission at the glitch that occurred around 2011, October 16, and a relaxation of the flux to the pre-glitch state at around 2014 December. We report X-ray analysis results of the data observed
by XMM-Newton on 2015 December 20 in the post-relaxation state. To examine any change in the X-ray emission, we compare the properties of the pulse profiles and spectra at the low gamma-ray flux state and at the post-relaxation state. The phase-averaged spectra for both states can be well described by a power-law component plus a blackbody component. The former is dominated by unpulsed emission and is probably originated from the pulsar wind nebula as reported by Hui et al (2015). The emission property of the blackbody component is consistent with the emission from the polar cap heated by the back-flow bombardment of the high-energy electrons or positrons that were accelerated in the magnetosphere. We found no significant change in the X-ray emission properties between two states. We suggest that the change of the X-ray luminosity is at an order of ~4%, which is difficult to measure with the current observations. We model the observed X-ray light curve with the heated polar cap emission and we speculate that the observed large pulsed fraction is owing to asymmetric magnetospheric structure.
We report the analysis result of UV/X-ray emission from AR~Scorpii, which is an intermediate polar (IP) composed of a magnetic white dwarf and a M-type star, with the XMM-Newton data. The X-ray/UV emission clearly shows a large variation over the orb
it, and their intensity maximum (or minimum) is located at the superior conjunction (or inferior conjunction) of the M-type star orbit. The hardness ratio of the X-ray emission shows a small variation over the orbital phase, and shows no indication of the absorption by an accretion column. These properties are naturally explained by the emission from the M-type star surface rather than from the accretion column on the WDs star similar to the usual IPs. Beside, the observed X-ray emission also modulates with WDs spin with a pulse fraction of $sim 14%$. The peak position is aligned in the optical/UV/X-ray band. This supports the hypothesis that the electrons in AR~Scorpii are accelerated to a relativistic speed, and emit non-thermal photons via the synchrotron radiation. In the X-ray bands, the evidence of the power-law spectrum is found in the pulsed component, although the observed emission is dominated by the optically thin thermal plasma emissions with several different temperatures. It is considered that the magnetic dissipation/reconnection process on the M-type star surface heats up the plasma to a temperature of several keV, and also accelerates the electrons to the relativistic speed. The relativistic electrons are trapped in the WDs closed magnetic field lines by the magnetic mirror effect. In this model, the observed pulsed component is explained by the emissions from the first magnetic mirror point.
AR~Scorpii is an intermediate polar system composed of a magnetic white dwarf (WD) and an M-type star, and shows non-thermal, pulsed, and highly linearly polarized emission. The radio/optical emission modulates with the WDs spin and show the double p
eak structure in the light curves. In this paper, we discuss a possible scenario for the radiation mechanism of AR~Scorpii. The magnetic interaction on the surface of the companion star produces an outflow from the companion star, the heating of the companion star surface, and the acceleration of electrons to a relativistic energy. The accelerated electrons, whose typical Lorentz factor is $sim 50-100$, from the companion star move along the magnetic field lines toward the WD surface. The electrons injected with the pitch angle of $sintheta_{p,0}>0.05$ are subject to the magnetic mirror effect and are trapped in the closed magnetic field line region.We find that the emission from the first magnetic mirror points mainly contributes to the observed pulsed emission and the formation of the double-peak structure in the light curve. For the inclined rotator, the pulse peak in the calculated light curve shifts the position in the spin phase, and a Fourier analysis exhibits a beat frequency feature, which are consistent with the optical/UV observations. The pulse profile also evolves with the orbital phase owing to the effect of the viewing geometry. The model also interprets the global features of the observed spectral energy distribution in radio to X-ray energy bands. We also discuss the curvature radiation and the inverse-Compton scattering process in the outer gap accelerator of the WD in AR Scorpii and discuss the possibility of the detection by future high-energy missions.
PSR J2032+4127 is a radio-loud gamma-ray-emitting pulsar; it is orbiting around a high-mass Be type star with a very long orbital period of 25-50years, and is approaching periastron, which will occur in late 2017/early 2018. This system comprises wit
h a young pulsar and a Be type star, which is similar to the so-called gamma-ray binary PSR~B1259-63/LS2883. It is expected therefore that PSR J2032+4127 shows an enhancement of high-energy emission caused by the interaction between the pulsar wind and Be wind/disk around periastron. Ho et al. recently reported a rapid increase in the X-ray flux from this system. In this paper, we also confirm a rapid increase in the X-ray flux along the orbit, while the GeV flux shows no significant change. We discuss the high-energy emissions from the shock caused by the pulsar wind and stellar wind interaction and examine the properties of the pulsar wind in this binary system. We argue that the rate of increase of the X-ray flux observed by Swift indicates (1) a variation of the momentum ratio of the two-wind interaction region along the orbit, or (2) an evolution of the magnetization parameter of the pulsar wind with the radial distance from the pulsar. We also discuss the pulsar wind/Be disk interaction at the periastron passage, and propose the possibility of formation of an accretion disk around the pulsar. We model high-energy emissions through the inverse-Compton scattering process of the cold-relativistic pulsar wind off soft photons from the accretion disk.
We discuss X-ray and gamma-ray emissions from Crab-like pulsars, PSRs~J0537-6910 and~J0540-6919, in Large Magellanic Cloud. Fermi-LAT observations have resolved the gamma-ray emissions from these two pulsars and found the pulsed emissions from PSR~J0
540-6919. The total pulsed radiation in the X-ray/gamma-ray energy bands of PSR~J0540-6919 is observed with the efficiency $eta_{J0540}sim 0.06$ (in 4$pi$ sr), which is about a factor of ten larger than $eta_{Crab}sim 0.006$ of the Crab pulsar. Although PSR~J0537-6910 has the highest spin-down power among currently known pulsars, the efficiency of the observed X-ray emissions is about two orders of magnitude smaller than that of PSR~J0540-6919. This paper mainly discusses what causes the difference in the radiation efficiencies of these three energetic Crab-like pulsars. We discuss electron/positron acceleration and high-energy emission processes within the outer gap model. By solving the outer gap structure with the dipole magnetic field, we show that the radiation efficiency decreases as the inclination angle between the magnetic axis and the rotation axis increases. To explain the difference in the pulse profile and in the radiation efficiency, our model suggests that PSR~J0540-6919 has an inclination angle much smaller than the that of Crab pulsar (here we assume the inclination angles of both pulsars are $alpha<90^{circ}$). On the other hand, we speculate that the difference in the radiation efficiencies between PSRs~J0537-6910 and J0549-6919 is mainly caused by the difference in the Earth viewing angle, and that we see PSR~J0537-6910 with an Earth viewing angle $zeta>>90^{circ}$ (or $<<90^{circ}$) measured from the spin axis, while we see PSR~J0540-6919 with $zetasim 90^{circ}$.
