Research Note

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

¹ Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, via Archirafi 36, 90123 Palermo, Italy e-mail: mcarama@astropa.unipa.it
² INAF Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy e-mail: [giusi;flaccomio]@astropa.unipa.it
³ Smithsonian Astrophysical Observatory, MS-3, 60 Garden Street, Cambridge, MA 02138, USA e-mail: jdrake@cfa.harvard.edu

Received: 13 November 2008
Accepted: 29 May 2009

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 ~210³¹-610³² erg s^-1 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.