X-ray study of bow shocks in runaway stars

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.

Possible Association of Two Stellar Bowshocks with Unidentified Fermi Sources

“…Firstly, our results are in agreement with the upper limit obtained for bowshocks in the H. E. S. S. Collaboration (Abdalla et al 2017), simply because there is almost no emission at E > 100 GeV. The sources studied in De Becker et al (2017) are not so powerful as those studied here. The mechanical power in the wind of λ Cep and LS 2355 is very high, and hence there is more power available to be converted into relativistic electrons.…”

“…where ρ a is the ambient density. Using the parameters for the wind given above and the measured value of R, together with an assumed mean ISM gas with µ = 2.3 × 10 −24 g, we estimate n a ∼ 1.5 × 10 3 cm −3 , which is reasonable for an HII region (e.g., Cox 2000). With this density and the proton cosmic-ray flux, assuming a cloud with the semi-major axis of the Fermi ellipse of 8.4 pc, we calculate this expected background component.…”

“…Similar searches were carried out in X-rays using data from XMM-Newton (Toalá et al 2016(Toalá et al , 2017De Becker et al 2017), with no positive identification. On the basis of energy considerations for all sources in the E-BOSS catalogue, De Becker et al (2017) anticipate that the detection of inverse Compton emission from the bowshock of massive runaways with current X-ray facilities is very unlikely.…”

MHD-shock structures of astrospheres: λ Cephei -like astrospheres

“…The interpretation of recent Hα observations of bow shocks around O-stars (Meyer et al 2016;Kobulnicky et al 2017) and their associated X-ray observations (e.g. De Becker et al 2017) require a detailed analysis of the large-scale shock structure around such stars. Simulations aiming at an improved understanding of such observations have been performed, e.g.…”

Nonthermal Emission from Stellar Bow Shocks

“…Several searches for high-energy emission from bow shocks of massive runaway stars have followed. At X-rays, these searches were done using both XMM-Newton archived observations (Toalá et al 2017(Toalá et al , 2016 and dedicated observations (De Becker et al 2017), where no nonthermal extended emission was found. De Becker et al (2017) used the derived upper limits at X-rays and those available at radio wavelength to fit general physical parameters of the sources with a simple model for the nonthermal emission.…”