Searching for the Donor Stars of ULX Pulsars

Abstract

We report on our search for the optical counterparts of two ultraluminous X-ray pulsars with known orbital periods, M82 X-2 and NGC 5907 X-1, in new and archival HST observations, in an effort to characterize the donor stars in these systems. We detect five near-infrared sources consistent with the position of M82 X-2 that are too bright to be single stars. We also detect seven sources in the WFC3/UVIS F336W image whose photometry matches that of 10-15 M${}_{\odot }$ stars turning off the main sequence. Such stars have densities consistent with the properties of the donor star of M82 X-2 as inferred from X-ray timing analysis, although it is also possible that the donor is a lower mass star below our detection limit or that there is a significant contribution from the accretion disc to the optical emission. We detect three candidate counterparts to NGC 5907 X-1 in the near-infrared. All of these are too bright to be the donor star of the ULX, which based on its orbital period is a red giant. The high background at the location of NGC 5907 X-1 precludes us from detecting this expected donor star. The recently discovered NGC 5907 ULX-2 also falls within the field of view of the near-infrared imaging; we detect four sources in the error circle, with photometry that matches AGB stars. The star suggested to be the counterpart of NGC 5907 ULX-2 by Pintore et al. (2018) falls outside our 2-$\sigma$ error circle.

10 pages, 3 figures, accepted for publication in ApJ

References in corpus (14)

• Modules for Experiments in Stellar Astrophysics (MESA)
• An Ultraluminous X-ray Source Powered by An Accreting Neutron Star
• An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907
• Discovery of a 0.42-s pulsar in the ultraluminous X-ray source NGC 7793 P13
• Discovery of coherent pulsations from the Ultraluminous X-ray Source NGC 7793 P13
• A mass of less than 15 solar masses for the black hole in an ultraluminous X-ray source
• Ultra-luminous X-ray sources and neutron-star-black-hole mergers from very massive close binaries at low metallicity
• Spectral Changes in the Hyperluminous Pulsar in NGC 5907 as a Function of Super-Orbital Phase
• An ultraluminous nascent millisecond pulsar
• Optical emission from massive donors in ULX binary systems
• NuSTAR and XMM-Newton observations of the extreme ultraluminous X-ray source NGC 5907 ULX1: A Vanishing Act
• Irradiation models for ULXs and fits to optical data
• On the Formation of Ultraluminous X-ray Sources with Neutron Star Accretors: the Case of M82 X-2
• Understanding the spectral and timing behavior of a newly discovered transient X-ray pulsar Swift J0243.6+6124

Cited by in corpus (13)

• It has to be cool: on supergiant progenitors of binary black hole mergers from common-envelope evolution
• Ultraluminous X-ray sources
• Pulsing and Non-Pulsing ULXs: the Iceberg Emerges
• Discovery of a red supergiant donor star in SN2010da/NGC 300 ULX-1
• Orbital decay in M82 X-2
• Orbital and super-orbital periods in ULX pulsars, disc-fed HMXBs, Be/X-ray binaries and double-periodic variables
• Formation and Evolution of Ultraluminous X-ray Pulsar Binaries to Pulsar-Neutron Star and Pulsar-White Dwarf Systems
• A Population of Neutron Star Ultraluminous X-ray Sources with A Helium Star Companion
• A multi-wavelength view of distinct accretion regimes in the pulsating ultraluminous X-ray source NGC 1313 X-2
• Modelling multiwavelength emission of Ultra-luminous X-ray Sources accreting above the Eddington limit
• The Orbit of NGC 5907 ULX-1
• Constraining soft and hard X-ray irradiation in ultraluminous X-ray sources
• Ultraluminous X-ray sources in Globular Clusters