The Ultraviolet Spectral Energy Distributions of Quiescent Black Holes and Neutron Stars

TL;DR

This study presents the first ultraviolet detections of quiescent black holes and neutron stars, revealing higher-than-expected UV emission likely from heated inner disks, and highlights differences in X-ray to UV flux ratios between the two types.

Contribution

First UV detections of quiescent black hole and neutron star systems, analyzing their spectral energy distributions and flux ratios to distinguish their accretion properties.

Findings

UV excesses are hotter than expected for outer disks.

No systematic difference in UV luminosity between black holes and neutron stars.

Neutron stars have about 10x higher X-ray to UV flux ratios than black holes.

Abstract

We present HST/ACS ultraviolet photometry of three quiescent black hole X-ray transients: X-ray Nova Muscae 1991 (GU Mus), GRO J0422+32 (V518 Per), and X-ray Nova Vel 1993 (MM Vel), and one neutron star system, Aql X-1. These are the first quiescent UV detections of these objects. All are detected at a much higher level than expected from their companion stars alone and are significant detections of the accretion flow. Three of the four UV excesses can be characterized by a black body of temperature 5000-13,000K, hotter than expected for the quiescent outer disk. A good fit could not be found for MM Vel. The source of the black-body-like emission is most likely a heated region of the inner disk. Contrary to initial indications from spectroscopy there does not appear to be a systematic difference in the UV luminosity or spectral shape between black holes and neutron star systems. However combining our new data with earlier spectroscopy and published X-ray luminosities there is a significant difference in the X-ray to UV flux ratios with the neutron stars exhibiting Lx/Luv about 10x higher than the black hole systems. Since both bandpasses are expected to be dominated by accretion light this suggests the difference in X-ray luminosities cannot simply reflect differences in quiescent accretion rates and so is a more robust discriminator between the black hole and neutron star populations than the comparison of X-ray luminosities alone.