Long-Term X-ray Variability on the Benchmark YSO HL Tau

Abstract

HL Tau is one of the most well-studied Class I young stellar objects, including frequent observations at near- and mid-infrared, (sub-) millimeter, and X-ray wavelengths. We present the results of an X-ray variability monitoring campaign with XMM-Newton in 2020 and X-ray gratings spectroscopy from Chandra/HETGS in 2018. We find that the X-ray spectrum of HL Tau is consistently hot (with characteristic plasma temperatures $T \gtrsim 30$ MK) over 31 epochs spanning 20 years, which is consistent in temperature with most Class I YSOs. The high-resolution HETG spectrum indicates the presence of some cooler plasma. We characterize the variability of the star across the 31 observations and find a subset of observations with significant variability on a $\sim$21-day timescale in the observed count rate and flux. We discuss the possible origins of this variability, and identify further observations that would better constrain the nature of the changes.