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

TL;DR

This study presents a long-term X-ray monitoring of HL Tau, revealing consistently hot plasma temperatures over 20 years and identifying variability on a roughly 21-day timescale, contributing to understanding young stellar object behavior.

Contribution

It provides the first long-term X-ray variability analysis of HL Tau, combining XMM-Newton and Chandra data to characterize plasma temperatures and variability patterns in a Class I YSO.

Findings

X-ray spectrum remains consistently hot ($T rac{30}{MK}$) over 20 years.

Detected significant variability on a rac{21}{day}rac{timescale}.

High-resolution spectra indicate presence of cooler plasma components.

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.