X-ray study of bow shocks in runaway stars

TL;DR

This study used XMM-Newton to observe five runaway star bow shocks, finding no X-ray emission related to the shocks but setting upper limits that constrain physical parameters and highlight the need for more sensitive future observations.

Contribution

First X-ray observational study of a significant sample of runaway star bow shocks, providing constraints on non-thermal emission and informing future detection prospects.

Findings

No X-ray emission detected from bow shocks.

Established upper limits on bow shock X-ray emission.

Constraints on magnetic field and infrared photon density.

Abstract

Massive runaway stars produce bow shocks through the interaction of their winds with the interstellar medium, with the prospect for particle acceleration by the shocks. These objects are consequently candidates for non-thermal emission. Our aim is to investigate the X-ray emission from these sources. We observed with XMM-Newton a sample of 5 bow shock runaways, which constitutes a significant improvement of the sample of bow shock runaways studied in X-rays so far. A careful analysis of the data did not reveal any X-ray emission related to the bow shocks. However, X-ray emission from the stars is detected, in agreement with the expected thermal emission from stellar winds. On the basis of background measurements we derive conservative upper limits between 0.3 and 10 keV on the bow shocks emission. Using a simple radiation model, these limits together with radio upper limits allow us to constrain some of the main physical quantities involved in the non-thermal emission processes, such as the magnetic field strength and the amount of incident infrared photons. The reasons likely responsible for the non-detection of non-thermal radiation are discussed. Finally, using energy budget arguments, we investigate the detectability of inverse Compton X-rays in a more extended sample of catalogued runaway star bow shocks. From our analysis we conclude that a clear identification of non-thermal X-rays from massive runaway bow shocks requires one order of magnitude (or higher) sensitivity improvement with respect to present observatories.