The Local Leo Cold Cloud and New Limits on a Local Hot Bubble

TL;DR

This study investigates the local Leo cold cloud's properties and its implications for the local hot bubble, revealing that solar wind charge exchange significantly contributes to the soft X-ray background and challenging previous models.

Contribution

It provides the first detailed multi-wavelength analysis of the LLCC and constrains the local hot bubble's X-ray emissivity, highlighting the importance of solar wind charge exchange.

Findings

LLCC is the closest cold neutral medium cloud at 11.3-24.3 pc.

The LLCC has temperatures between 15 K and 30 K.

A local hot bubble alone cannot explain the soft X-ray background.

Abstract

We present a multi-wavelength study of the local Leo cold cloud (LLCC), a very nearby, very cold cloud in the interstellar medium. Through stellar absorption studies we find that the LLCC is between 11.3 pc and 24.3 pc away, making it the closest known cold neutral medium cloud and well within the boundaries of the local cavity. Observations of the cloud in the 21-cm HI line reveal that the LLCC is very cold, with temperatures ranging from 15 K to 30 K, and is best fit with a model composed of two colliding components. The cloud has associated 100 micron thermal dust emission, pointing to a somewhat low dust-to-gas ratio of 48 x 10^-22 MJy sr^-1 cm^2. We find that the LLCC is too far away to be generated by the collision among the nearby complex of local interstellar clouds, but that the small relative velocities indicate that the LLCC is somehow related to these clouds. We use the LLCC to conduct a shadowing experiment in 1/4 keV X-rays, allowing us to differentiate between different possible origins for the observed soft X-ray background. We find that a local hot bubble model alone cannot account for the low-latitude soft X-ray background, but that isotropic emission from solar wind charge exchange does reproduce our data. In a combined local hot bubble and solar wind charge exchange scenario, we rule out emission from a local hot bubble with an 1/4 keV emissivity greater than 1.1 Snowdens / pc at 3 sigma, 4 times lower than previous estimates. This result dramatically changes our perspective on our local interstellar medium.