X-ray flaring from the young stars in CygnusOB2

TL;DR

This study analyzes X-ray flares from young stars in CygnusOB2 and Orion, revealing similar flare activity levels and energy distributions, with detailed statistical methods and implications for understanding stellar magnetic activity.

Contribution

It introduces a binning-free maximum likelihood method for flare detection and compares flare properties between two star-forming regions, accounting for observational biases.

Findings

Flare energies follow a power-law distribution with index ~-2.1.

Flare frequency depends on the star's X-ray luminosity.

CygOB2 and ONC stars exhibit similar flare activity levels.

Abstract

Aims: We characterize individual and ensemble properties of X-ray flares from stars in the CygOB2 and ONC star-forming regions. Method: We analyzed X-ray lightcurves of 1003 CygOB2 sources observed with Chandra for 100 ksec and of 1616 ONC sources detected in the ``Chandra Orion Ultra-deep Project'' 850 ksec observation. We employed a binning-free maximum likelihood method to segment the light-curves into intervals of constants signal and identified flares on the basis of both the amplitude and the time-derivative of the source luminosity. We then derived and compared the flare frequency and energy distribution of CygOB2 and ONC sources. The effect of the length of the observation on these results was investigated by repeating the statistical analysis on five 100 ksec-long segments extracted from the ONC data. Results: We detected 147 and 954 flares from the CygOB2 and ONC sources, respectively. The flares in CygOB2 have decay times ranging from ~0.5 to about 10 hours. The flare energy distributions of all considered flare samples are described at high energies well by a power law with index alpha=-(2.1+-0.1). At low energies, the distributions flatten, probably because of detection incompleteness. We derived average flare frequencies as a function of flare energy. The flare frequency is seen to depend on the source's intrinsic X-ray luminosity, but its determination is affected by the length of the observation. The slope of the high-energy tail of the energy distribution is, however, affected little. A comparison of CygOB2 and ONC sources, accounting for observational biases, shows that the two populations, known to have similar X-ray emission levels, have very similar flare activity.