Compact radio emission in Ultraluminous X-ray sources

TL;DR

This study investigates radio emissions from selected ULX sources using GMRT and EVN data, identifying structures that suggest the presence of intermediate mass black holes or supernova remnants.

Contribution

It provides new high-resolution radio observations of ULXs, confirming some sources as supernova remnants and suggesting the presence of intermediate mass black holes in others.

Findings

No faint radio emission detected at low frequencies, setting upper flux limits.

Detection of compact radio emission in ULXs N4088-X1 and N4861-X2.

Identification of N4449-X1 as a supernova remnant.

Abstract

We present results from our studies of radio emission from selected Ultraluminous X-ray (ULX) sources, using archival Giant Metrewave Radio Telescope (GMRT) data and new European VLBI Network (EVN) observations. The GMRT data are used to find possible faint radio emission from ULX sources located in late-type galaxies in the Chandra Deep Fields. No detections are found at 235, 325 and 610 MHz, and upper limits on the radio flux densities at these frequencies are given. The EVN observations target milliarcsecond-scale structures in three ULXs with known radio counterparts (N4449- X1, N4088-X1, and N4861-X2). We confirm an earlier identification of the ULX N4449-X1 with a supernova remnant and obtain the most accurate estimates of its size and age. We detect compact radio emission for the ULX N4088-X1, which could harbour an intermediate mass black hole (IMBH) of 10^5 M\odot accreting at a sub-Eddington rate. We detect a compact radio component in the ULX N4861-X2, with a brightness temperature > 10^6 K and an indication for possible extended emission. If the extended structure is confirmed, this ULX could be an HII region with a diameter of 8.6 pc and surface brightness temperature \geq 10^5 K. The compact radio emission may be coming from a ~ 10^5 M\odot black hole accreting at 0.1M_Edd.