Hard X-ray flux from low-mass stars in the Cygnus OB2 Association

TL;DR

This paper estimates the potential hard X-ray emission from low-mass stars in Cygnus OB2, suggesting it could be detectable and provide insights into stellar flaring processes and hidden star-forming regions.

Contribution

It introduces a method to estimate hard X-ray flux from low-mass stars based on soft X-ray observations and flare scaling, highlighting the significance of hard X-ray detection in star formation studies.

Findings

Estimated hard X-ray flux range: ~2x10^31 to 6x10^32 erg/s.

Hard X-ray emission could be near current detection limits.

Detection would reveal high-energy processes in pre-main sequence stars.

Abstract

Context. The Cygnus OB2 association, the central engine of the Cygnus X star-forming region, is the subject of an extensive INTEGRAL Key Project that will accumulate 6Ms of observations. Analysis of 2Ms of observations by De Becker and co-workers provides the most sensitive limit yet obtained on hard X-ray emission from the cluster. Aims. We investigate the X-ray emission in the 20-40 keV band expected from the flaring low-mass stellar population in Cygnus OB2. We discuss whether such emission needs to be considered in the interpretation of existing and future X-ray observations of the region, and whether such observations might provide insight into the high-energy processes on low-mass pre-main sequence stars. Methods. The total hard X-ray flux from low-mass stars is estimated by assuming the observed soft X-ray emission stems from a superposition of flares. We further assume the ratio of hard X-ray to soft X-ray emission is described by a scaling found for solar flares by Isola and co-workers. Results. We estimate the low-mass stellar hard X-ray flux in the 20-40 keV band to lie in the range ~2x10^31-6x10^32 erg/s and discuss some potential biases that might affect this result. Conclusions. Hard X-ray emission could lie at a level not much below the current observed flux upper limits for Cygnus OB2. If this emission could be detected, it would provide insight into the hard X-ray production of large flares on pre-main sequence stars. We highlight the penetrating power of hard X-rays from low-mass stellar populations as a possible pointer to our Galaxy's hidden star-forming clusters and super-clusters using more sensitive observations from future missions.