1999/03/23 by A. Zezas, A. L. Zezas, I. Georgantopoulos +1 · 5 citations
Physics and Astronomy · #Astrophysical Phenomena and Observations #Astrophysics and Cosmic Phenomena #Galaxies: Formation, Evolution, Phenomena #astro-ph
paper · pdf · doi:10.1046/j.1365-8711.1999.02626.x
Accepted for publication in MNRAS
arxiv created 1999/03/23 · openalex publication_date 1999/09/19 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
We present ROSAT PSPC and HRI observations of the dwarf irregular galaxy Holmberg II (UGC 4305). This is one of the most luminous dwarf galaxies (LX∼1040 erg s−1) detected in the ROSAT All-Sky Survey. The X-ray emission comes from a single unresolved point source, coincident with a large H ii region that emits intense radio emission. The source is variable on both year and day time-scales, clearly favouring accretion into a compact object rather than a supernova remnant or a superbubble interpretation for the origin of the X-ray emission. However, its X-ray spectrum is well-fitted by a steep power law or alternatively by a Raymond—Smith spectrum with kT∼0.8 keV, lower than the temperature of X-ray binaries in nearby spiral galaxies.