Results from DROXO

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 We combine photometry from infrared to millimeter wavelengths from the literature with mid-IR Spitzer and unpublished submm Herschel photometry that we analysed for this work, and we describe the resulting spectral energy distribution (SED) with a set of precomputed models.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 +1.2 −0.5 × 10 23 cm −2 and kT = 1.15 +2.35 −0.65 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.

YOUNG STELLAR POPULATIONS IN MYStIX STAR-FORMING REGIONS: CANDIDATE PROTOSTARS

“…Candidate #23 has extraordinary X-ray properties. A single flare produces most of the detected 243 counts; with extremely heavy absorption (M E 5.3 keV), the inferred intrinsic peak X-ray luminosities is among the highest every seen in a stellar flare, similar to the X-ray flare recently reported from protostar ISO-Oph 85 in the ρ Ophiuchi cloud (Pizzocaro et al 2016). Several other candidates in RCW 38 showed X-ray flares though with ∼30 times lower luminosity than seen in #23.…”

“…This essentially confines the ultrahard X-ray sources to the upper tail of the protostellar X-ray luminosity function with log L X 31 − 32 erg s −1 . Protostars with X-ray flares at this level have been found (Hamaguchi et al 2005;Pizzocaro et al 2016) but are rare (Imanishi et al 2001;McCleary & Wolk 2011;Prisinzano et al 2008). NGC2264 27, 30-32).…”

The EXTraS project: Exploring the X-ray transient and variable sky

“…These two cases are selected as a clear demonstration of EXTraS STV potentialities for the search for and characterisation of peculiar objects and rare events, and for the study of the properties of classes of sources. Further examples are for instance the discovery of X-ray flaring activity from a young pre-main-sequence star (Pizzocaro et al 2016), the discovery of a puzzling flaring Xray source in the Galactic globular cluster NGC6540 (Mereghetti et al 2018), and the study of the X-ray activity-rotation connection in cool stars (Pizzocaro et al 2019). In the last part of this section, we examine a science project published before the release of EXTraS catalogues: the study of the X-ray variability in a large sample of stars by Pye et al (2015).…”

Approximating Photo-z PDFs for Large Surveys

“…One parametrization that has not previously been investigated in the context of photometric redshifts is that of quantiles, though they have appeared elsewhere in the astronomy literature (Sun et al 2015;Pizzocaro et al 2016;Laycock et al 2017). The quantiles are defined in terms of the cumulative distribution function (CDF), which is the antiderivative of the PDF.…”