Discovery of the Goat Horn complex: a ∼1000 deg² diffuse X-ray source connected to radio loop XII

¹ Max-Planck-Institut für Extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching bei München, Germany
e-mail: nicola.locatelli@inaf.it
² INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
³ Dr. Karl Remeis Observatory, Erlangen Centre for Astroparticle Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg, Sternwartstraße 7, 96049 Bamberg, Germany

Received: 26 January 2024
Accepted: 4 May 2024

Abstract

A dozen patches of polarized radio emission spanning tens of degrees in the form of coherent and stationary arcs and loops are observed at radio frequencies across the sky. Their origin is usually associated with nearby shocks, possibly arising from nearby supernova explosions. The origin of radio loop XII remains unknown. We report an anticorrelation of the radio-polarized emission of loop XII with a large patch of soft X-ray emission found with SRG/eROSITA in excess of the background surface brightness in the same region. This seemingly coherent patch of soft X-ray emission, which we call the Goat Horn complex, extends over a remarkable area of ∼1000 deg² and includes an arc-shaped enhancement that might trace a cold front. An anticorrelation of the X-ray intensity with the temperature of the plasma that causes the X-ray emission is also observed. The X-ray bright arc seems to anticipate radio loop XII by some degrees on the sky. This behavior can be recast in terms of a correlation between X-ray surface brightness and radio depolarization. We explore and discuss different possible scenarios for the source of the diffuse emission in the Goat Horn complex: a large supernova remnant, an outflow from active star-forming regions in nearby Galactic spiral arms, and a hot atmosphere around the Large Magellanic Cloud. In order to probe these scenarios further, a more detailed characterization of the velocity of the hot gas is required.