Revisiting the X-ray emission of the asynchronous polar V1432 Aql

As the only eclipsing asynchronous polar, V1432 Aql provides an excellent laboratory to study the interaction between the accreted matter and the magnetic field. Here, we report an analysis of the X-ray data from the contemporaneous NuSTAR and Swift-XRT observations. The X-ray data present a profile with a low-intensity state for almost half an orbital period, a dip at 0.6 phase, and a peak at 0.75 phase, which suggests that there was only one accretion region during the observation and the claim is supported by the spectral analysis. The comparison with the previous data indicates that the X-ray data have an orbital modulation, as the case in BeppoSAX, rather than a spin one observed in ROSAT. We attribute the orbit and spin modulations to the different accretion geometries at work. The spectral analysis of the wide-band data presents a significant reflection effect, a commonly observed soft X-ray temperature, and the energy balance in V1432 Aql. Additionally, we obtained a low total accretion rate of 1.3 × 10−10 M ⊙ yr−1 and a high specific accretion rate of 3.8 g cm−2 s−1 which explains the strong reflection from the surface of the white dwarf. However, due to its complex emission, a more physical understanding of its accretion geometry is still outstanding.

Gamma-Ray Orbital Modulation of the Transitioning Millisecond Pulsar Binary XSS J12270–4859

We report on gamma-ray orbital modulation of the transitioning MSP binary XSS J12270–4859 detected in the Fermi Large Area Telescope (LAT) data. We use long-term optical data taken with the XMM-Newton OM and the Swift UltraViolet Optical Telescope to inspect radio timing solutions that are limited to relatively short time intervals and find that extrapolation of the solutions aligns well with the phasing of the optical data over 15 yr. The Fermi-LAT data folded on the timing solutions exhibit significant modulation (p = 5 × 10−6) with a gamma-ray minimum at the inferior conjunction of the pulsar. Intriguingly, the source seems to show similar modulation in both the low-mass X-ray binary and the MSP states, implying that mechanisms for gamma-ray emission in the two states are similar. We discuss these findings and their implications using an intrabinary shock scenario.

Observations of Her X-1 in low states during SRG/eROSITA all-sky survey

eROSITA (extended ROentgen Survey with an Imaging Telescope Array) instrument onboard the Russian-German ‘Spectrum-Roentgen-Gamma’ (SRG) mission observed the Her X-1/HZ Her binary system in multiple scans over the source during the first and second SRG all-sky surveys. Both observations occurred during a low state of the X-ray source when the outer parts of the accretion disk blocked the neutron star from view. The orbital modulation of the X-ray flux was detected during the low states. We argue that the detected X-ray radiation results from scattering of the emission of the central source by three distinct regions: (a) an optically thin hot corona with temperature ~(2−4) × 106 K above the irradiated hemisphere of the optical star; (b) an optically thin hot halo above the accretion disk; and (c) the optically thick cold atmosphere of the optical star. The latter region effectively scatters photons with energies above 5–6 keV.

Evidence for periodic accretion–ejection in LS I +61°303

ABSTRACT The stellar binary system LS I +61°303, composed of a compact object in an eccentric orbit around a B0 Ve star, emits from radio up to γ-ray energies. The orbital modulation of radio spectral index, X-ray, and GeV γ-ray data suggests the presence of two peaks. This two-peaked profile is in line with the accretion theory predicting two accretion–ejection events for LS I +61°303 along the 26.5 d orbit. However, the existing multiwavelength data are not simultaneous. In this paper, we report the results of a campaign covering radio, X-ray, and γ-ray observations of the system along one single orbit. Our results confirm the two predicted events along the orbit and in addition show that the positions of radio and γ-ray peaks are coincident with X-ray dips as expected for radio and γ-ray emitting ejections depleting the X-ray emitting accretion flow. We discuss future observing strategies for a systematic study of the accretion–ejection physical processes in LS I +61°303.

X-ray and γ-ray orbital variability from the γ-ray binary HESS J1832−093

Context.γ-ray binaries are systems composed of a massive star and a compact object whose interaction leads to particle acceleration up to relativistic energies. In the last fifteen years, a few binaries have been found to emit at high energies, but their number is still low. The TeV source HESS J1832−093 has been proposed as a binary candidate, although its nature is unclear. Neither a GeV counterpart nor a period was detected. Aims. The purpose of this work is to search for a GeV counterpart to understand the origin of the TeV signal detected by H.E.S.S. For an unambiguous identification of its binary nature, finding an orbital modulation is crucial. Methods. We analysed data spanning more than 10 years from the Fermi Large Area Telescope (Fermi-LAT), together with Swift archival observations taken between 2015 and 2018, using both the X-Ray Telescope and UV/Optical Telescope. We searched for periodicities in both X-ray and GeV bands. Results. We find a periodic modulation of ∼ 86 days in the X-ray source candidate counterpart XMMU J183245−0921539, together with indications of γ-ray modulation with a compatible period in the GeV candidate counterpart 4FGL J1832.9−0913. Neither an optical nor a UV counterpart is found at the X-ray source location. The overall spectral energy distribution strongly resembles the known γ-ray binary HESS J0632+057. Conclusions. Both the spectral energy distribution and the discovery of an orbital period allow the identification of the TeV source HESS J1832−093 as a new member of the γ-ray binary class.

GeV emission of gamma-ray binary with pulsar scenario

ABSTRACT We study GeV emission from gamma-ray binaries by assuming that the compact object is a young pulsar. We assume that the relativistic unshocked pulsar wind with a Lorentz factor of 104–5 can produce the GeV emission by the inverse-Compton scattering process in the dense soft-photon field of the companion star. The travel distance of the unshocked pulsar wind that moves toward the observer depends on the orbital phase of the pulsar. We discuss that the orbital modulation of the GeV emission is a result of combination of the effects of the travel distance of the unshocked pulsar wind and of the anisotropic soft-photon field of the companion star. In this paper, we study how the effect of the travel distance of the unshocked pulsar wind affects to the orbital modulation of GeV emission. We apply our scenario to two gamma-ray binaries, LMC P3 and 4FGL J1405.1−6119. We find that with the suggested system parameters of LMC P3, the observed amplitude of the orbital modulation and the peak width are more consistent with the model light curve by taking into account the effect of the travel distance. For LMC P3, we analyse the GeV spectrum with 8-yr Fermi-LAT data and discuss the broadband emission process in X-ray to TeV energy bands. We predict a possible system geometry for 4FGL J1405.1−6119 by fitting the GeV light curve.

The long-term optical activity of the propellers AE Aquarii and AR Scorpii

This analysis of the long-term optical activity of the propellers AE Aqr and AR Sco uses data from the Catalina Real-time Transient Survey, DASCH, and AAVSO. The site and character of the emissions from the phenomena caused by the magnetic field of the white dwarf (WD) vary from system to system. The histogram of intensities of the ensemble of flares of AE Aqr suggests that the long-term activity consists of a large variety of the peak magnitudes of the flares, with the probability of their detection gradually decreasing with increasing intensity. Any increase of activity only leads to an increase of the number of blobs of the transferring matter. We also detected a season with a transient decrease or even a cessation of the mass outflow from the donor to the lobe of the WD. The very strong orbital modulation of AR Sco is most stable in the phases of the extrema of brightness for about a century; its minor changes suggest that the trailing side of the synchrotron-emitting region is more unstable than the leading side.

Hints of γ-ray orbital variability from γ2 Velorum

Context. Colliding wind binaries are massive systems featuring strong, interacting stellar winds which may act as particle accelerators. Therefore, such binaries are good candidates for detection at high energies. However, only the massive binary η Carinae has been firmly associated with a γ-ray signal. A second system, γ2 Velorum, is positionally coincident with a γ-ray source, but we lack unambiguous identification. Aims. Observing orbital modulation of the flux would establish an unambiguous identification of the binary γ2 Velorum as the γ-ray source detected by the Fermi Large Area Telescope (Fermi-LAT). Methods. We used more than ten years of observations with Fermi-LAT. Events are phase-folded with the orbital period of the binary to search for variability. We studied systematic errors that might arise from the strong emission of the nearby Vela pulsar with a more conservative pulse-gated analysis. Results. We find hints of orbital variability, indicating maximum flux from the binary during apastron passage. Conclusions. Our analysis strengthens the possibility that γ-rays are produced in γ2 Velorum, most likely as a result of particle acceleration in the wind collision region. The observed orbital variability is consistent with predictions from recent magnetohydrodynamic simulations, but contrasts with the orbital variability from η Carinae, where the peak of the light curve is found at periastron.

NuSTAR and Parkes observations of the transitional millisecond pulsar binary XSS J12270–4859 in the rotation-powered state

ABSTRACT We report on the first NuSTAR observation of the transitional millisecond pulsar binary XSS J12270–4859 during its current rotation-powered state, complemented with a 2.5 yr-long radio monitoring at Parkes telescope and archival XMM–Newton and Swift X-ray and optical data. The radio pulsar is mainly detected at 1.4 GHz displaying eclipses over $\sim 40{{\ \rm per\ cent}}$ of the 6.91 h orbital cycle. We derive a new updated radio ephemeris to study the 3–79 keV light curve that displays a significant orbital modulation with fractional amplitude of $28\pm 3{{\ \rm per\ cent}}$, a structured maximum centred at the inferior conjunction of the pulsar and no cycle-to-cycle or low–high-flaring mode variabilities. The average X-ray spectrum, extending up to ∼70 keV without a spectral break, is well described by a simple power law with photon index Γ = 1.17 ± 0.08 giving a 3–79 keV luminosity of $\rm 7.6_{-0.8}^{+3.8} \times 10^{32}\, erg\, s^{-1}$ for a distance of 1.37$_{-0.15}^{+0.69}$ kpc. Energy resolved orbital light curves reveal that the modulation is not energy dependent from 3 to 25 keV and is undetected with an upper limit of ${\sim} 10{{\ \rm per\ cent}}$ above 25 keV. Comparison with previous X-ray XMM–Newton observations in common energy ranges confirms that the modulation amplitudes vary on time-scales of a few months, indicative of a non-stationary contribution of the intrabinary shock (IBS) formed by the colliding winds of the pulsar and the companion. A more detailed inspection of energy resolved modulations than previously reported gives hints of a mild softening at superior conjunction of the pulsar below 3 keV, likely due to the contribution of the thermal emission from the neutron star. The IBS emission, if extending into the MeV range, would be energetically capable alone to irradiate the donor star.