X-rays from Magnetically Confined Wind Shocks: Effect of Cooling-Regulated Shock Retreat

Asif ud-Doula (1), Stanley Owocki (2), Richard Townsend (3), Veronique Petit (2), David Cohen (4)

(1)- Penn State Worthington Scranton, Dunmore, PA 18512, USA.
(2)- Department of Physics and Astronomy, Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
(3)- Department of Astronomy, University of Wisconsin-Madison, 5534 Sterling Hall, 475 N Charter Street, Madison, WI 53706, USA
(4)- Department of Physics and Astronomy, Swarthmore College, Swarthmore, PA 19081, USA

We use 2D MHD simulations to examine the effects of radiative cooling and
inverse Compton (IC) cooling on X-ray emission from magnetically confined wind
shocks (MCWS) in magnetic massive stars with radiatively driven stellar winds.
For the standard dependence of mass loss rate on luminosity $Mdot sim L^{1.7}
$, the scaling of IC cooling with $L$ and radiative cooling with $Mdot$ means
that IC cooling become formally more important for lower luminosity stars.
However, because the sense of the trends is similar, we find the overall effect
of including IC cooling is quite modest. More significantly, for stars with
high enough mass loss to keep the shocks radiative, the MHD simulations
indicate a linear scaling of X-ray luminosity with mass loss rate; but for
lower luminosity stars with weak winds, X-ray emission is reduced and softened
by a {em shock retreat} resulting from the larger post-shock cooling length,
which within the fixed length of a closed magnetic loop forces the shock back
to lower pre-shock wind speeds. A semi-analytic scaling analysis that accounts
both for the wind magnetic confinement and this shock retreat yields X-ray
luminosities that have a similar scaling trend, but a factor few higher values,
compared to time-averages computed from the MHD simulations. The simulation and
scaling results here thus provide a good basis for interpreting available X-ray
observations from the growing list of massive stars with confirmed large-scale
magnetic fields.

Reference: MNRAS
Status: Manuscript has been accepted

Weblink: https://psu.box.com/s/r5c7phknvklro4naro7w


Email: asif@psu.edu