Search for Diffuse X-Rays from the Bow Shock Region of Runaway Star BD +43 3654 with Suzaku

Abstract

The bow shocks of runaway stars with strong stellar winds of over 2000 km s$ ^{-1}$ can serve as particle acceleration sites. The conversion from stellar wind luminosity into particle acceleration power has an efficiency of the same order of magnitude as those in supernova remnants, based on the radio emission from the bow shock region of runaway star BD $ +$ 43 3654 (Benaglia et al. 2010, A&A, 517, L10). If this object exhibits typical characteristics, then runaway star systems can contribute a non-negligible fraction of Galactic cosmic-ray electrons. To constrain the maximum energy of accelerated particles from measurements of possible non-thermal emissions in the X-ray band, Suzaku observed BD $ +$ 43 3654 in 2011 April with an exposure of 99 ks. Because the onboard instruments have a stable and low background level, Suzaku detected a possible enhancement over the background of 7.6 $ \pm$ 3.4 counts arcmin$ ^{-2}$ at the bow shock region, where the error represents the 3 $ \sigma$ statistics only. However, the excess is not significant within the systematic errors of non-X-ray and cosmic-ray backgrounds of the X-ray Imaging Spectrometer, which are $ \pm$ 6.0 and $ \pm$ 34 counts arcmin$ ^{-2}$ , respectively, and the 3 $ \sigma$ upper limit in the X-ray luminosity from the shock region, which is 1.1 $ \times$ 10$ ^{32}$ erg s$ ^{-1}$ per 41.2 arcmin$ ^2$ in the 0.5 to 10 keV band. This result leads to three conclusions: (1) a shock-heating process is inefficient on this system; (2) the maximum energy of electrons does not exceed $ \sim$ 10 TeV, corresponding to a Lorentz factor of less than $ 10^7$ ; and (3) the magnetic field in the shock acceleration site might not be as turbulent as those in pulsar wind nebulae and supernova remnants.

Modeling nonthermal emission from stellar bow shocks

“…Up to now, BD+34 3654 was observed with XMM during 47 ks, which is not enough to detect the bow shock X-ray emission, according to our simulations. Terada et al (2012) failed in detecting this bow shock with Suzaku. From their observation, they obtained an X-ray luminosity lower limit at 1.1 × 10 32 erg s −1 , which is much higher than our predictions.…”

“…Later, del Valle & Romero (2012) presented a nonthermal emission model that predicted the production of high-energy photons in these acceleration regions in sufficiently large numbers to be detected in X-rays. López-Santiago et al (2012) published the first detection of X-ray emission from a bow shock produced by a runaway star (AE Aurigae), while Terada et al (2012) reported the non-detection of X-ray emission in the bow shock region formed by BD+43 3654 from a long-duration X-ray observation with Suzaku.…”

“…Further details on the nonthermal processes are also found in Adams (1980), Kelner et al (2006), Longair (2011) or Rybicki & Lightman (1979). Other contributions to explain the high-energy emission from the bow shocks produced by runaway stars are those of Benaglia et al (2010), Terada et al (2012), and del Valle & Romero (2014). …”

Systematic search for very-high-energy gamma-ray emission from bow shocks of runaway stars

“…One reason for the non-detection could be that particle acceleration is in general less efficient in bow shocks than in known gamma-ray sources. Terada et al (2012) concluded that the magnetic fields in the bow shocks of runaway stars might be less turbulent compared to those of pulsar wind nebulae or supernova remnants, where gamma-ray emission is detected in many cases. A lower maximum energy of the accelerated particles or lower photon densities could also explain the non-detections.…”

Observations of bow shocks of runaway stars with H.E.S.S.

“…• 3654 by Terada et al [6], and X-ray observations of AE Aurigae by López-Santiago et al [7] and Pereira et al [8]. All resulted in non-detections and subsequently upper limits on the emission.…”