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Extremely Hard X-ray Variation of WR140 through a Whole Binary Orbit

Presentation #107.20 in the session Stellar/Compact Objects - Poster Session.

Published onMay 03, 2024
Extremely Hard X-ray Variation of WR140 through a Whole Binary Orbit

The winds from the components of massive, hot stellar binaries collide with each other at speeds in the thousands of km/s, heating plasmas via shocks to temperatures in the millions of Kelvin in the X-ray domain. The observed X-rays from the shock heated plasmas vary gradually with the orbital motion due to the radial wind density profile and intervening wind absorption. With precise stellar wind and binary parameters determined from optical or radio observations, some binary systems provide precious laboratories for shock physics. The Wolf-Rayet binary system WR140 drives the most powerful wind-wind collision (WWC) activity within 3 kpc, only surpassed by the infamous supermassive binary η Carinae. NuSTAR and XMM-Newton have performed joint observations at key orbital phases of WR140’s long (P = 7.9 years), eccentric (e ~0.90) binary orbit between 2015 and 2020, covering one whole orbital cycle variation. NuSTAR’s hard X-ray sensitivity combined with XMM-Newton’s good emission line measurements enables the monitoring of high-energy X-ray activities without significant interference from wind absorption. The light curve at 8-12 keV, which originates from kT ~3.5 keV plasma emission, matches the WWC 1/D relation (D = stellar separation) within 10% at orbital phases 0.02 < Φorbit < 0.98. This result demonstrates that WWC is adiabatic outside periastron. The NuSTAR spectra show small but significant residuals between 15-25 keV, requiring an additional extremely hard X-ray component. This component does not vary strongly through the binary orbit except for a precipitous flux drop near periastron, coinciding with the thermal light curve decline at X-ray minimum. This behavior is similar to the non-thermal X-ray emission from η Carinae, suggesting further confirmation that WR140 also drives non-thermal high-energy particle acceleration. This result should help understand the high-energy cosmic rays acceleration mechanism.

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