A gigantic X-ray flare from the star Trumpler 14 Y442 in the Carina star forming complex
Kenji Hamaguchi (CRESST NASA/GSFC & UMBC), Stephen A. Drake, Michael F. Corcoran (CRESST NASA/GSFC & USRA), Noel Richardson (Universite de Montreal), Mairan Teodoro (NASA/GSFC & Western Michigan University)
XMM-Newton, NuSTAR and Swift observations of the Carina star forming complex on 2015 July 16 (ObsID - XMM:0762910401, NuSTAR:30101005002, Swift:00081578001) detected an X-ray flare at the coordinates (R.A., Dec)[J2000] = (10 44 02.80, -59 39 46.7). This position is within 0.3" of the star Trumpler 14 Y442 and there is no other 2MASS nor SIMBAD object within ~13" of the detected coordinates. The maximum observed X-ray flux was 4.7e-12 ergs cm-2 s-1 (0.5-10 keV) which gradually decreased by a factor of 3 in half a day. Though these observations missed the flare onset, the observed maximum flux was 50-100 times higher than its typical flux (0.5-1e-13 ergs cm-2 s-1, Broos et al. 2011, ApJS, 194, 2: Lin et al. 2012, ApJ, 756, 27). The proper motion (Hur et al. 2012, AJ, 143, 41) and the measured X-ray absorption (~1.9e21 H cm-2) of Tr 14 Y442 suggest its membership in the Carina star forming complex. If, as suggested by its proper motions, Y442 is at the distance to the Carina nebula (2.3 kpc), the observed maximum X-ray luminosity of this event would make it one of the most powerful (~3e33 ergs s-1) stellar X-ray flares ever recorded. Follow-up Swift observations of this star between 2015 July 28-31 (ObsID - 00031097045-7) showed a moderately elevated X-ray flux of ~1.5-3e-13 ergs cm-2 s-1, assuming a plasma temperature between 1-8 keV. The count rate declined by 60% at ~4.0e-3 cts s-1 during another follow-up Swift observation on 2015 August 21 (ObsID - 00031097049), which is marginally higher than the average Swift count rate of the star before the flare (~3.0e-3 cts s-1).

The slow decay and the high plasma temperature (kT ~8 keV) when bright are characteristic of the largest X-ray flares from active low-mass stars, pre-main sequence stars and RS CVn systems (cf., Drake et al. 2014, ATel #6121; Drake et al. 2015, ATel #6940). Though the near-IR photometric properties classify this star as an early type (OB) star (e.g., Povich et al. 2011, ApJS, 194, 6), a stellar atmosphere model (Castelli F., Kurucz R. L., 2004, astro-ph/0405087) with an effective temperature of ~4500-5000K and Av of ~0.7-1 fits well to its archival photometric data (UBVI: Johnson, GRI: Sloan SDSS, JHK: 2MASS, [3.6, 4.5, 5.8, 8.0] um: Spitzer IRAC, [3.4, 4.6] um: WISE) . This extinction is consistent with the measured X-ray absorption for the intervening interstellar medium without any need for additional local absorption. Optical spectroscopy is needed to more fully characterize this flare source.

The X-ray image, light curve and spectrum can be found at http://asd.gsfc.nasa.gov/Kenji.Hamaguchi/research/Tr14Y442/index.html
infra-red,optical,ultra-violet,x-ray,pre-main-sequence star,star,transient,variables,young stellar object