Atmospheric Circulation on Black Widow Companions

Kavli Affiliate: Roger W. Romani

| First 5 Authors: D. Kandel, Roger W. Romani, , ,

| Summary:

We present a model for atmospheric wind circulation in binary millisecond
pulsar (MSP) companions, showing how the optical light curve (LC) and radial
velocities (RV) are sensitive to the wind flow, causing LC orbital phase shifts
and asymmetries, as observed for several `spider’ MSP. Velocity widths of
spectral lines offer additional opportunities for measuring surface wind speed.
As examples, we fit optical data for a black widow pulsar J1959+2048 and a
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 $K_mathrm{c}$; both effects
decrease the inferred neutron star mass. Even with such a decrease, we find
large masses for the two neutron stars: $2.18pm0.09M_odot$ and
$2.28^{+0.10}_{-0.09}M_odot$, 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.

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