X-ray flares in Orion low-mass stars

¹ Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, Via Archirafi 36, 90123 Palermo, Italy emailmcarama@astropa.unipa.it
² INAF Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA

Received: 30 January 2007
Accepted: 14 June 2007

Abstract

Context.X-ray flares are common phenomena in pre-main sequence stars. Their analysis gives insights into the physics at work in young stellar coronae. The Orion Nebula Cluster offers a unique opportunity to study large samples of young low mass stars. This work is part of the Chandra Orion Ultradeep project (COUP), an ~10 day long X-ray observation of the Orion Nebula Cluster (ONC).

Aims.Our main goal is to statistically characterize the flare-like variability of 165 low mass (0.1–0.3 ) ONC members in order to test and constrain the physical scenario in which flares explain all the observed emission.

Methods.We adopt a maximum likelihood piece-wise representation of the observed X-ray light curves and detect flares by taking into account both the amplitude and time derivative of the count-rate. We then derive the frequency and energy distribution of the flares.

Results.The high energy tail of the energy distribution of flares is well described by a power-law with index ~2.2. We test the hypothesis that light curves are built entirely by overlapping flares with a single power law energy distribution. We constrain the parameters of this simple model for every single light curve. The analysis of synthetic light curves obtained from the model indicates a good agreement with the observed data.
Comparing low mass stars with stars in the mass interval (0.9–1.2 ), we establish that, at ~1 Myr, low mass and solar mass stars of similar X-ray luminosity have very similar flare frequencies.

Conclusions.Our observational results are consistent with the following model/scenario: the light curves are entirely built by overlapping flares with a power-law intensity distribution; the intense flares are individually detected, while the weak ones merge and form a pseudo-quiescent level, which we indicate as the characteristic level.