ABSTRACT The low-mass companions of evaporating binary pulsars (black widows and similar) are strongly heated on the side facing the pulsar. However, in high-quality photometric and spectroscopic ...data, the heating pattern does not match that expected for direct pulsar illumination. Here we explore a model where the pulsar power is intercepted by an intra-binary shock (IBS) before heating the low-mass companion. We develop a simple analytic model and implement it in the popular "ICARUS" light curve code. The model is parameterized by the wind momentum ratio β and the companion wind speed , and assumes that the reprocessed pulsar wind emits prompt particles or radiation to heat the companion surface. We illustrate an interesting range of light curve asymmetries controlled by these parameters. The code also computes the IBS synchrotron emission pattern, and thus can model black widow X-ray light curves. As a test, we apply the results to the high-quality asymmetric optical light curves of PSR J2215+5135; the resulting fit gives a substantial improvement upon direct heating models and produces an X-ray light curve consistent with that seen. The IBS model parameters imply that at the present loss rate, the companion evaporation has a characteristic timescale of Myr. Still, the model is not fully satisfactory, indicating that there are additional unmodeled physical effects.
Abstract
The optical study of the heated substellar companions of
black widow
(BW) millisecond pulsars (MSPs) provides unique information on the MSP particle and radiation output and on the neutron ...star mass. Here we present an analysis of optical photometry and spectroscopy of a set of relatively bright BWs, many newly discovered in association with Fermi
γ
-ray sources. Interpreting the optical data requires sophisticated models of the companion heating. We provide a uniform analysis, selecting the preferred heating model and reporting on the companion masses and radii, the pulsar heating power, and neutron star mass. The substellar companions are substantially degenerate, with average densities 15–30× Solar, but are inflated above their zero temperature radii. We find evidence that the most extreme recycled BW pulsars have both large >0.8
M
⊙
accreted mass and low <10
8
G magnetic fields. Examining a set of heavy BWs, we infer that neutron star masses larger than 2.19
M
⊙
(1
σ
confidence) or 2.08
M
⊙
(3
σ
confidence) are required; these bounds exclude all but the stiffest equations of state in standard tabulations.
We present a model for atmospheric wind circulation in binary millisecond pulsar (MSP) companions, showing how the optical light curve (LC) and radial velocities are sensitive to the wind flow, ...causing LC orbital phase shifts and asymmetries, as observed for several "spider" MSPs. Velocity widths of spectral lines offer additional opportunities for measuring surface wind speed. As examples, we fit optical data for the black widow pulsar J1959+2048 and the redback pulsar J2215+5135; the wind heating models (WH) are statistically strongly preferred over direct heating (DH) for both objects, although the latter is even better fit with a heated spot. In general, WH effects tend to increase the inferred orbital inclination i and decrease the inferred companion center-of-mass radial velocity amplitude Kc; both effects decrease the inferred neutron star mass. Even with such a decrease, we find large masses for the two neutron stars: and , respectively (for the modest surface speeds fit from the bulk heat flow; supersonic photospheric winds can slightly change these values). These are among the highest masses known, and our improved modeling increases confidence that the results are important for understanding the dense matter equation of state.
Abstract
We describe Keck-telescope spectrophotometry and imaging of the companion of the “black widow” pulsar PSR J0952−0607, the fastest known spinning neutron star (NS) in the disk of the Milky ...Way. The companion is very faint at minimum brightness, presenting observational challenges, but we have measured multicolor light curves and obtained radial velocities over the illuminated “day” half of the orbit. The model fits indicate system inclination
i
= 59.°8 ± 1.°9 and a pulsar mass
M
NS
= 2.35 ± 0.17
M
⊙
, the largest well-measured mass found to date. Modeling uncertainties are small, since the heating is not extreme; the companion lies well within its Roche lobe and a simple direct-heating model provides the best fit. If the NS started at a typical pulsar birth mass, nearly 1
M
⊙
has been accreted; this may be connected with the especially low intrinsic dipole surface field, estimated at 6 × 10
7
G. Joined with reanalysis of other black widow and redback pulsars, we find that the minimum value for the maximum NS mass is
M
max
>
2.19
M
⊙
(2.09
M
⊙
) at 1
σ
(3
σ
) confidence. This is ∼ 0.15
M
⊙
heavier than the lower limit on
M
max
implied by the white dwarf–pulsar binaries measured via radio Shapiro-delay techniques.
The companions of evaporating binary pulsars (black widows and related systems) show optical emission suggesting strong heating. In a number of cases, large observed temperatures and asymmetries are ...inconsistent with direct radiative heating for the observed pulsar spindown power and expected distance. Here we describe a heating model in which the pulsar wind sets up an intrabinary shock (IBS) against the companion wind and magnetic field, and a portion of the shock particles duct along this field to the companion magnetic poles. We show that a variety of heating patterns, and improved fits to the observed light curves, can be obtained at expected pulsar distances and luminosities, at the expense of a handful of model parameters. We test this "IBS-B" model against three well-observed binaries and comment on the implications for system masses.
Abstract
Keck-telescope spectrophotometry of the companion of PSR J1810+1744 shows a flat, but asymmetric light-curve maximum and a deep, narrow minimum. The maximum indicates strong gravity ...darkening (GD) near the
L
1
point, along with a heated pole and surface winds. The minimum indicates a low underlying temperature and substantial limb darkening. The GD is a consequence of extreme pulsar heating and the near-filling of the Roche lobe. Light-curve modeling gives a binary inclination
i
= 65.°7 ± 0.°4. With the Keck-measured radial-velocity amplitude
K
c
= 462.3 ± 2.2 km s
−1
, this gives an accurate neutron star mass
M
NS
= 2.13 ± 0.04
M
⊙
, with important implications for the dense-matter equation of state. A classic direct-heating model, ignoring the
L
1
gravitational darkening, would predict an unphysical
M
NS
> 3
M
⊙
. A few other “spider” pulsar binaries have similar large heating and fill factor; thus, they should be checked for such effects.
We present broadband spectral energy distributions and light curves of the gamma-ray binary 1FGL J1018.6−5856 measured in the X-ray and the gamma-ray bands. We find that the orbital modulation in the ...low-energy gamma-ray band is similar to that in the X-ray band, suggesting a common spectral component. However, above a GeV the orbital light curve changes significantly. We suggest that the GeV band contains significant flux from a pulsar magnetosphere, while the X-ray to TeV light curves are dominated by synchrotron and Compton emission from an intrabinary shock (IBS). We find that a simple one-zone model is inadequate to explain the IBS emission, but that beamed Synchrotron-self Compton radiation from adiabatically accelerated plasma in the shocked pulsar wind can reproduce the complex multiband light curves, including the variable X-ray spike coincident with the gamma-ray maximum. The model requires an inclination of ∼50° and an orbital eccentricity of ∼0.35, consistent with the limited constraints from existing optical observations. This picture motivates searches for pulsations from the energetic young pulsar powering the wind shock.
We describe a geometric model for synchrotron and synchrotron self-Compton (SSC) radiation from blazar jets, involving multiple emission zones with turbulent magnetic fields and fully self-consistent ...seed photon mixing for SSC. Including the effects of jet divergence, particle cooling, and the relativistic PA rotation to the observer frame, we find that the multizone model recovers simple predictions for SSC polarization, but describes new dependencies on jet viewing geometry and zone multiplicity. Increasing the zone number decreases both synchrotron and SSC polarization, but with different scaling. A rise in synchrotron polarization fraction Sync at high energies is guaranteed by basic relativity considerations, and strengthened by jet nonuniformity. Finite light travel time effects can suppress the synchrotron polarization at energies well below the Sync peak. In general Sync and SSC are correlated with SSC/ Sync 0.3, but individual realizations can lie far from this trend. This study lets us estimate across the SED, leading to predictions in the X-ray band helpful for planning observations with IXPE and other upcoming X-ray polarization missions.
Abstract
We describe an optimal signal extraction process for imaging X-ray polarimetry using an ensemble of deep neural networks. The initial photoelectron angle, used to recover the polarization, ...has errors following a von Mises distribution. This is complicated by events converting outside of the fiducial gas volume, whose tracks have little polarization sensitivity. We train a deep ensemble of convolutional neural networks to select against these events and to measure event angles and errors for the desired gas-conversion tracks. We show how the expected modulation amplitude from each event gives an optimal weighting to maximize signal-to-noise ratio of the recovered polarization. Applying this weighted maximum likelihood event analysis yields sensitivity (MDP
99
) improvements of ∼10% over earlier heuristic weighting schemes and mitigates the need to adjust said weighting for the source spectrum. We apply our new technique to a selection of astrophysical spectra, including complex extreme examples, and compare the polarization recovery to the current state of the art.
Abstract By measuring photoelectron tracks, the gas pixel detectors of the Imaging X-ray Polarimetry Explorer satellite provide estimates of the photon detection location and its electric vector ...position angle (EVPA). However, imperfections in reconstructing event positions blur the image, and EVPA-position correlations result in artificial polarized halos around bright sources. We introduce a new model describing this “polarization leakage” and use it to recover the on-orbit telescope point-spread functions, useful for faint-source detection and image reconstruction. These point-spread functions are more accurate than previous approximations or ground-calibrated products (Δ χ 2 ≈ 3 × 10 4 and 4 × 10 4 respectively for a bright 10 6 -count source). We also define an algorithm for polarization leakage correction substantially more accurate than existing prescriptions (Δ χ 2 ≈ 1 × 10 3 ). These corrections depend on the reconstruction method, and we supply prescriptions for the mission-standard “Moments” methods as well as for “Neural Net” event reconstruction. Finally, we present a method to isolate leakage contributions to polarization observations of extended sources and show that an accurate PSF allows the extraction of sub-PSF-scale polarization patterns.
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