The evolution of a jet ejection of the ultraluminous X-ray source Holmberg II X-1

• Published in: Monthly notices of the Royal Astronomical Society Vol. 452; no. 1; pp. 24 - 31
• Main Authors: Cseh, D., Miller-Jones, J. C. A., Jonker, P. G., Grisé, F., Paragi, Z., Corbel, S., Falcke, H., Frey, S., Kaaret, P., Körding, E.
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 01-09-2015; Oxford University Press (OUP): Policy P - Oxford Open Option A
• Subjects: Adiabatic flow; Arrays; Astrophysics; Ejection; High Energy Astrophysical Phenomena; Luminosity; Pollution sources; Radio; Radio astronomy; Radio telescopes; Sciences of the Universe; Solar and Stellar Astrophysics; Spectrum analysis; Star & galaxy formation; X-ray astronomy; X-ray telescopes; X-rays; black hole physics; X-rays: binaries; accretion, accretion discs; accretion discs – black hole physics – X-rays: binaries; accretion
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stv1308

We present quasi-simultaneous, multi-epoch radio and X-ray measurements of Holmberg II X-1 using the European VLBI Network (EVN), the Karl G. Jansky Very Large Array (VLA), and the Chandra and Swift X-ray telescopes. The X-ray data show apparently hard spectra with steady X-ray luminosities four months apart from each other. In the high-resolution EVN radio observations, we have detected an extended milliarcsecond scale source with unboosted radio emission. The source emits non-thermal, likely optically thin synchrotron emission, and its morphology is consistent with a jet ejection. The 9-GHz VLA data show an arcsecond-scale triple structure of Holmberg II X-1 similar to that seen at lower frequencies. However, we find that the central ejection has faded by at least a factor of 7.3 over 1.5 yr. We estimate the dynamical age of the ejection to be higher than 2.1 yr. We show that such a rapid cooling can be explained with simple adiabatic expansion losses. These properties of Holmberg II X-1 imply that ULX radio bubbles may be inflated by ejecta instead of self-absorbed compact jets.