# A Combined Chandra and LAMOST Study of Stellar Activity

## TL;DR

This study combines Chandra and LAMOST data to analyze stellar X-ray activity across various stellar parameters, revealing bimodal distributions, correlations with chromospheric activity, and diverse flare behaviors in main-sequence stars.

## Contribution

It provides new insights into the bimodal distribution of X-ray activity and its relation to stellar properties, using combined X-ray and spectroscopic data for a large stellar sample.

## Key findings

- Bimodal distribution of $R_X$ for G and K stars.
- No $R_X$ trend with age beyond 4 Gyr.
- Strong correlation between $R_X$ and $R_{H extalpha}$.

## Abstract

We probed stellar X-ray activity over a wide range of stellar parameters, using {\it Chandra} and LAMOST data. We measured the X-ray-to-bolometric luminosity ratio ($R_X = L_X/L_{\rm bol}$) for 484 main-sequence stars, and found a bimodal distribution for G and K types. We interpret this bimodality as evidence of two sub-populations with different coronal temperatures, which are caused by different coronal heating rates. Using the metallicity and velocity information, we find both of the two sub-populations are mostly located in the thin disk. We find no trend of $R_X$ with stellar age for stars older than $\sim$ 4 Gyr; there is a trough in the $R_X$ vs age distribution, with the lowest range of $R_X$ appearing at ages around 2 Gyr. We then examined the correlation between $R_X$ and $R_{\rm H\alpha}$ (proxy of chromospheric activity): we find that the two quantities are well correlated, as found in many earlier studies. Finally, we selected a sample of twelve stars with X-ray flares, and studied the light-curve morphology of the flares. The variety of flare profiles and timescales observed in our sample suggests the contribution of different processes of energy release.