Astronomy & Astrophysics

Free access article

A&A 419, 653-665 (2004)
DOI: 10.1051/0004-6361:20034070

Chandra observation of an unusually long and intense X-ray flare from a young solar-like star in M 78

N. Grosso^{1, 2}, T. Montmerle², E. D. Feigelson³ and T. G. Forbes⁴

¹ Max-Planck-Institut für extraterrestrische Physik, PO Box 1312, 85741 Garching bei München, Germany
² Laboratoire d'Astrophysique de Grenoble, Université Joseph-Fourier, 38041 Grenoble Cedex 9, France
³ Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802, USA
⁴ Institute for the Study of Earth, Oceans, and Space, 39 College Road, University of New Hampshire, Durham, NH 03824, USA

(Received 11 July 2003 / Accepted 29 January 2004 )

Abstract
LkH $\alpha$ 312 has been observed serendipitously with the ACIS-I detector on board the Chandra X-ray Observatory with a 26 h continuous exposure. This H $\alpha$ emission line star belongs to M 78 ( NGC 2068 ), one of the star-forming regions of the Orion B giant molecular cloud at a distance of 400 pc. From the optical and the near-infrared (NIR) data, we show that LkH $\alpha$ 312 is a pre-main sequence (PMS) low-mass star with a weak NIR excess. This genuine T Tauri star displayed an X-ray flare with an unusually long rise phase (~8 h). The X-ray emission was nearly constant during the first 18 h of the observation, and then increased by a factor of 13 during a fast rise phase (~2 h), and reached a factor of 16 above the quiescent X-ray level at the end of a gradual phase (~6 h) showing a slower rise. To our knowledge this flare, with ~0.4-0.5 cts s ^-1, has the highest count rate observed so far with Chandra from a PMS low-mass star. By chance, the source position, $8.2\arcmin$ off-axis, protected this observation from pile-up. We make a spectral analysis of the X-ray emission versus time, showing that the plasma temperature of the quiescent phase and the flare peak reaches 29 MK and 88 MK, respectively. The quiescent and flare luminosities in the energy range 0.5-8 keV corrected from absorption ( $N_{\rm H} \approx 1.7\times 10^$ cm ^-2) are $6\times 10^$ erg s ^-1 and ~10 ³² erg s ^-1, respectively. The ratio of the quiescent X-ray luminosity on the LkH $\alpha$ 312 bolometric luminosity is very high with $\log (L_{\rm X}/L_{\rm bol})= -2.9$ , implying that the corona of LkH $\alpha$ 312 reached the "saturation" level. The X-ray luminosity of the flare peak reaches ~2% of the stellar bolometric luminosity. The different phases of this flare are finally discussed in the framework of solar flares, which leads to the magnetic loop height from $3.1\times 10^$ to 10¹¹ cm (0.2-0.5 $R_\star$ , i.e., 0.5-1.3 $R_\odot$ ).

Key words: Galaxy: open clusters and associations: individual: M 78 (NGC 2068) -- X-rays: stars -- stars: individual: LkH $\alpha$ 312 -- stars: flare -- stars: pre-main sequence -- infrared: stars

Offprint request: N. Grosso, Nicolas.Grosso@obs.ujf-grenoble.fr

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