Chandra and Kepler monitoring of Ultracool Dwarfs

Abstract

We present the results of Chandra observation of L1 dwarf WISEP J190648.47+401106.8 (W1906+40 hereafter) and Kepler monitoring of L8 dwarf WISEP J060738.65+242953.4 (W0607+24 hereafter). W1906+40 was observed by Chandra for 49 ks on August 14, 2015. The net count of X-ray photons from this L dwarf is estimated to be ≈2 with the lower and upper limits equal to 0.44 and 6.07 respectively at 90 % confidence level. The time averaged X-ray luminosity over the duration of observation is found to be 1.87 × 10^25 erg s^−1 with the lower and upper limits equal to 4.03 × 10^24 erg s^−1 and 5.19 × 10^25 erg s^−1 respectively at 90 % confidence level. The corresponding X-ray activity level is log L_X ≈ -4.64. This dwarf does not follow the rotation ac- tivity relation as seen in earlier-type stars despite being a rapid rotator with period of 8.9 hours. In addition, total time averaged energy (UBVRI ) of white light flares observed in the same dwarf is calculated to be 5.25 × 10^25 which is 30% of the total time averaged flare energy (XUBVRI). W0607+24 was monitored by Kepler for 36 days during K2 Campaign 0. Kepler photometry rules out any possibility of stable periodic signals in the optical with amplitudes greater than 1.5% and periods between 1.5 hours and 2 days. The results are consistent with those obtained by Spitzer observations which show no variabiltiy at the 0.2% level over 10 hours each in the 3.6 and 4.5 micron bands.