Gamma-ray and X-ray constraints on non-thermal processes inηCarinae

Abstract

The binary system η Carinae is a unique laboratory in which to study particle acceleration to high energies under a wide range of conditions, including extremely high densities around periastron. To date, no consensus has emerged as to the origin of the GeV γ-ray emission in this important system. With a re-analysis of the full Fermi-LAT dataset for η Carinae we show that the spectrum is consistent with a pion decay origin. A single population leptonic model connecting the X-ray to γ-ray emission can be ruled out.Here, we revisit the physical model of Ohm et al. (2015), based on two acceleration zones associated to the termination shocks in the winds of both stars. We conclude that inverse-Compton emission from in-situ accelerated electrons dominates the hard X-ray emission detected with NuSTAR at all phases away from periastron, and pion-decay from shock accelerated protons is the source of the γ-ray emission. Very close to periastron there is a pronounced dip in the hard X-ray emission, concomitant with the repeated disappearance of the thermal X-ray emission, which we interpret as being due to the suppression of significant electron acceleration in the system. Within our model, the residual emission seen by NuSTAR at this phase can be accounted for with secondary electrons produced in interactions of accelerated protons, in agreement with the variation in pion-decay γ-ray emission. Future observations with H.E.S.S., CTA and NuSTAR should confirm or refute this scenario.

Particle acceleration and non-thermal emission in colliding-wind binary systems

“…The solution to the diffusion-advection equation depends on max , which generally depends on geometrical (Hillas 1984) or temporal (Lagage & Cesarsky 1983) conditions 3 . In exceptional circumstances max can be set by proton-proton losses (as occurs in Carinae; White et al 2020), but this is not important in the models in the current work where we find the geometrical condition dominates.…”

“…While secondary electrons do not appear to significantly affect the total non-thermal emission for the standard model parameters with ∼ > 10 13 cm, they may be more important in systems with higher stellar mass-loss rates and slower wind speeds, or if the primary protons are able to interact with dense, radiatively cooled, gas. White et al (2020) show that secondary electrons dominate the emission between ≈ 1−40 MeV in their "off-periastron" models of Carinae (see the top panel in their Fig. 3).…”

Particle acceleration at quasi-perpendicular non-relativistic high Mach number shocks

“…their gyroradii exceeding the shock transition width. Our results are relevant for understanding the non-thermal x-ray emission in several astrophysical objects such as Cassiopeia A, Tycho and -Carinae [16][17][18].…”

The high-energy emission from HD 93129A near periastron

“…We note that there is a low significance detection of emission near April 2018 at a 4-σ level if emission from η-Car is not considered. Despite η-Car being a known γ-ray source located at just ∼ 0.2 • from HD 93129A (i.e., within the same beam for AGILE), during this epoch η-Car was expected to be in a low γ-ray emission state (Balbo & Walter 2017;White et al 2019). Thus, it is not likely that the detected flux of 8 × 10 −7 cm −2 s −1 is due to η-Car.…”