Results from DROXO

IV. EXTraS discovery of an X-ray flare from the Class I protostar candidate ISO-Oph 85

¹ INAF–Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via E. Bassini 15, 20133 Milano, Italy
e-mail: pizzocar@lambrate.inaf.it
² Università degli Studi dell’Insubria, via Ravasi 2, 21100 Varese, Italy
³ INAF–Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁴ Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA
⁵ IUSS–Istituto Universitario di Studi Superiori, piazza della Vittoria 15, 27100 Pavia, Italy
⁶ INFN–Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, via A. Bassi 6, 27100 Pavia, Italy
⁷ W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305, USA
⁸ Smithsonian Astrophysical Observatory (SAO) – Harvard Center for Astrophysics, Cambridge MA, USA
⁹ DSFA–Dipartimento di Scienze Fisiche e Astronomiche, Università degli Studi di Palermo, Piazza del Parlamento 1, Palermo 90134, Italy
¹⁰ IMATI, Istituto di Matematica Applicata e Tecnologie Informatiche “Enrico Magenes”, via dei Marini 6, 16149 Genova, Italy
¹¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
¹² Department of Physics and Astronomy, University of Leicester, Leicester LEI 7RH, UK
¹³ Dr. Karl-Remeis Sternwarte and ECAP, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany

Received: 20 May 2015
Accepted: 15 October 2015

Abstract

X-ray emission from young stellar objects (YSOs) is a key ingredient in understanding star formation. For the early, protostellar (Class I) phase, a very limited (and controversial) quantity of X-ray results is available to date. Within the EXTraS (Exploring the X-ray Transient and variable Sky) project, we have discovered transient X-ray emission from a source whose counterpart is ISO-Oph 85, a strongly embedded YSO in the ρ Ophiuchi star-forming region. We extract an X-ray light curve for the flaring state, and determine the spectral parameters for the flare from XMM-Newton/EPIC data with a method based upon quantile analysis. We combine photometry from infrared to millimeter wavelengths from the literature with mid-IR Spitzer and unpublished submm Herschel photometry that we analysed for this work, and we describe the resulting spectral energy distribution (SED) with a set of precomputed models. The X-ray flare of ISO-Oph 85 lasted ~2500 s and is consistent with a highly-absorbed one-component thermal model (N_H = 1.0_-0.5^+1.2 × 10²³ cm^-2 and kT= 1.15_-0.65^+2.35 keV). The X-ray luminosity during the flare is log L_X [erg/s] = 31.1^+2.0_-1.2; during quiescence we set an upper limit of log L_X [erg/s] < 29.5. We do not detect other flares from this source. The submillimeter fluxes suggest that the object is a Class I protostar. We caution, however, that the offset between the Herschel and optical/infrared position is larger than that for other YSOs in the region, leaving some doubt on this association. To the best of our knowledge, this is the first X-ray flare from a YSO that has been recognised as a candidate Class I protostar via the analysis of its complete SED, including the submm bands that are crucial for detecting the protostellar envelope. This work shows how the analysis of the whole SED is fundamental to the classification of YSOs, and how the X-ray source detection techniques we have developed can open a new era in time-resolved analysis of the X-ray emission from stars.