Starlight, Star Bright, X-rays Tonight

Dwarf novae shine brighter than thought in X-rays, hinting at cosmic glow.

A new study reveals that faint, flickering stars called dwarf novae pump out more X-rays than previously believed, potentially contributing to a mysterious glow in our Milky Way galaxy.

Astronomers wanted to map the X-ray glow from these stellar systems. Dwarf novae are tiny, explosive binary stars—two stars orbiting each other—where a white dwarf (a dead star) siphons gas from a companion star. This gas forms a disk around the white dwarf, heating up and blasting out X-rays.

Research Methodology

Researchers focused on a select group of 13 dwarf novae located within about 200 parsecs (a cosmic distance unit, about 650 light-years) from Earth.

The team utilized data from powerful X-ray telescopes to peer into these systems, including:

Suzaku
XMM-Newton
ASCA

The X-ray light was then rigorously analyzed using models to understand its properties.

Key Findings

They discovered these dwarf novae emit a wide range of X-ray light, from 4 × 10^28 to 1.5 × 10^32 erg s^−1 (a unit for measuring energy output).

Most of the observed dwarf novae clustered around 10^30 erg s^−1, suggesting they are powerful X-ray emitters, especially the brighter ones. This strong emission could be a significant source of the Galactic Ridge X-ray Emission (GRXE), a diffuse X-ray glow observed throughout our galaxy.

The study found their contribution to the GRXE could be around 5 percent.

According to the authors, "the sample contains only a few sources below ∼10^30 erg s^−1." This indicates that the brighter dwarf novae dominated the sample. The team also determined a 'threshold luminosity' of 3 × 10^30 erg s^−1, below which their current detection methods struggled.

Future Implications

This study utilized a small number of dwarf novae. Future research with more cosmic data points will be crucial to clarify the X-ray map.

As the authors stated: "more dwarf novae need to be included in the sample to better constrain the integrated luminosity and the slope of the X-ray luminosity function."

This new work brings scientists closer to understanding the subtle X-ray chorus humming across our galaxy.

Reference

Byckling, K., Mukai, K., Thorstensen, J.R., Osborne, J.P. (2018) Deriving an X-ray luminosity function of dwarf novae based on parallax measurements. Mon. Not. R. Astron. Soc., 000, 1–16. doi: 10.1093/mnras/sty1234.