An official website of the United States government
ABSTRACT Spider pulsars are compact binary systems composed of a millisecond pulsar and a low-mass companion. Their X-ray emission – modulated on the orbital period – is interpreted as synchrotron radiation from high-energy electrons accelerated at the intrabinary shock. We perform global two-dimensional particle-in-cell simulations of the intrabinary shock, assuming that the shock wraps around the companion star. When the pulsar spin axis is nearly aligned with the orbital angular momentum, we find that the magnetic energy of the relativistic pulsar wind – composed of magnetic stripes of alternating field polarity – efficiently converts to particle energy at the intrabinary shock, via shock-driven reconnection. The highest energy particles accelerated by reconnection can stream ahead of the shock and be further accelerated by the upstream motional electric field. In the downstream, further energization is governed by stochastic interactions with the plasmoids/magnetic islands generated by reconnection. We also extend our earlier work by performing simulations that have a larger (and more realistic) companion size and a more strongly magnetized pulsar wind. We confirm that our first-principles synchrotron spectra and light curves are in good agreement with X-ray observations.
more »
« less
Particle acceleration and non-thermal emission at the intrabinary shock of spider pulsars – II. Fast-cooling simulations
ABSTRACT Spider pulsars are binary systems composed of a millisecond pulsar and a low-mass companion. Their X-ray emission, varying with orbital phase, originates from synchrotron radiation produced by high-energy electrons accelerated at the intrabinary shock. For fast-spinning pulsars in compact binary systems, the intrabinary shock emission occurs in the fast cooling regime. Using global 2D particle-in-cell simulations, we investigate the effect of synchrotron losses on the shock structure and the resulting emission, assuming that the pulsar wind is stronger than the companion wind (so, the shock wraps around the companion), as expected in black widows. We find that the shock opening angle gets narrower for greater losses; the light curve shows a more prominent double-peaked signature (with two peaks just before and after the pulsar eclipse) for stronger cooling; below the cooling frequency, the synchrotron spectrum displays a hard power-law range, consistent with X-ray observations.
more »
« less
- PAR ID:
- 10629150
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 542
- Issue:
- 2
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 917-926
- Size(s):
- p. 917-926
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Spider pulsars are compact binary systems composed of a millisecond pulsar and a low-mass companion. The relativistic magnetically dominated pulsar wind impacts onto the companion, ablating it and slowly consuming its atmosphere. The interaction forms an intrabinary shock, a proposed site of particle acceleration. We perform global fully kinetic particle-in-cell simulations of the intrabinary shock, assuming that the pulsar wind consists of plane-parallel stripes of alternating polarity and that the shock wraps around the companion. We find that particles are efficiently accelerated via shock-driven reconnection. We extract first-principles synchrotron spectra and light curves, which are in good agreement with X-ray observations: (1) the synchrotron spectrum is nearly flat,Fν∝ const; (2) when the pulsar spin axis is nearly aligned with the orbital angular momentum, the light curve displays two peaks, just before and after the pulsar eclipse (pulsar superior conjunction), separated in phase by ∼0.8 rad; (3) the peak flux exceeds the one at the inferior conjunction by a factor of 10.more » « less
-
Abstract We present the discovery of a new optical/X-ray source likely associated with the Fermi γ -ray source 4FGL J1408.6–2917. Its high-amplitude periodic optical variability, large spectroscopic radial-velocity semiamplitude, evidence for optical emission lines and flaring, and X-ray properties together imply the source is probably a new black widow millisecond pulsar binary. We compile the properties of the 41 confirmed and suspected field black widows, finding a median secondary mass of 0.027 ± 0.003 M ⊙ . Considered jointly with the more massive redback millisecond pulsar binaries, we find that the “spider” companion mass distribution remains strongly bimodal, with essentially zero systems having companion masses of between ∼0.07 and 0.1 M ⊙ . X-ray emission from black widows is typically softer and less luminous than in redbacks, consistent with less efficient particle acceleration in the intrabinary shock in black widows, excepting a few systems that appear to have more efficient “redback-like” shocks. Together black widows and redbacks dominate the census of the fastest spinning field millisecond pulsars in binaries with known companion types, making up ≳80% of systems with P spin < 2 ms. Similar to redbacks, the neutron star masses in black widows appear on average significantly larger than the canonical 1.4 M ⊙ , and many of the highest-mass neutron stars claimed to date are black widows with M NS ≳ 2.1 M ⊙ . Both of these observations are consistent with an evolutionary picture where spider millisecond pulsars emerge from short orbital period progenitors that had a lengthy period of mass transfer initiated while the companion was on the main sequence, leading to fast spins and high masses.more » « less
-
Geminga is the first pulsar around which a remarkable gamma-ray halo extending over a few degrees was discovered at TeV energies by MILAGRO and HAWC and later by H.E.S.S., and byFermi-LAT in the GeV band. More middle-aged pulsars have exhibited gamma-ray halos, and they are now recognised as an emerging class of Galactic gamma-ray sources. The emission appears in the late evolution stage of pulsars, and is most plausibly explained by inverse Compton scattering of CMB and interstellar photons by relativistic electrons and positrons escaping from the pulsar wind nebulae. These observations pose a number of theoretical challenges, particularly the origin of the inferred, significantly lower effective diffusion coefficients around the pulsar when compared to typical Galactic values. Tackling these questions requires constraining the ambient magnetic field properties, which can be achieved through X-ray observations. If the gamma-ray halos originate from a distribution of highly energetic electrons, synchrotron losses in the ambient magnetic fields of the same particles are expected to produce a diffuse X-ray emission with a similar spatial extension. We present the most comprehensive X-ray study of the Geminga pulsar halo to date, utilising archival data fromXMM-NewtonandNuSTAR. Our X-ray analysis covers a broad bandwidth (0.5 − 79 keV) and large field of view (θ ∼ 4°) for the first time. This was achieved by accurately measuring the background over the entire field of view, and taking into account both focused and stray-light X-ray photons from the pulsar halo withNuSTAR. We find no significant emission and set robust constraints on the X-ray halo flux. These are translated to stringent constraints on the ambient magnetic field strength and the diffusion coefficient by using a physical model considering particle injection, diffusion, and cooling over the pulsar’s lifetime, which is tuned by fitting multi-wavelength data. Our novel methodology for modelling and searching for synchrotron X-ray halos can be applied to other pulsar halo candidates.more » « less
-
Abstract We present an analysis of new multiwavelength observations of the TeV gamma-ray binary HESS J0632+057, conducted using SALT, Swift, NuSTAR, and VERITAS in 2023–2024. By combining these new data with archival observations, we confirm previous suggestions of orbital variability in the source’s X-ray spectrum, including increased X-ray absorption at the orbital phase interval ofϕ ≈ 0.3–0.4. The source’s X-ray flux within this phase interval seems to have exhibited a significant change on an orbital timescale. Additionally, occasional short-term variations in the X-ray band on a timescale of less than 3 days have been observed. The measured duration of the increased absorbing column density and the flux variability timescales can provide clues about the interaction between the putative pulsar and the Be companion’s disk if, as previously suggested, the pulsar crosses the disk at this phase interval. Moreover, the new contemporaneous X-ray and TeV observations around the pulsar-crossing phases revealed independent variability in the X-ray and TeV fluxes, contrary to a previous observation of concurrent flux increases. While these observations alone cannot provide definitive conclusions, we discuss our results in the context of pulsar–disk interaction and intrabinary shock emission scenarios.more » « less
