The First High-contrast Images of X-Ray Binaries: Detection of Candidate Companions in the γ Cas Analog RX J1744.7-2713

Abstract

X-ray binaries provide exceptional laboratories for understanding the physics of matter under the most extreme conditions. Until recently, there were few, if any, observational constraints on the circumbinary environments of X-ray binaries at ∼100–5000 au scales. It remains unclear how the accretion onto the compact objects or the explosions giving rise to the compact objects interact with their immediate surroundings. Here, we present the first high-contrast adaptive optics images of X-ray binaries. These observations target all X-ray binaries within ∼3 kpc accessible with the Keck/NIRC2 vortex coronagraph. This paper focuses on one of the first key results from this campaign; our images reveal the presence of 21 sources potentially associated with the γ Cassiopeiae analog high-mass X-ray binary RX J1744.7−2713. By conducting different analyses—a preliminary proper motion analysis, a color–magnitude diagram, and a probability of chance alignment calculation—we found that three of these 21 sources have a high probability of being bound to the system. If confirmed, they would be in wide orbits (∼450 to 2500 au). While follow-up astrometric observations will be needed in ∼5–10 yr to confirm further the bound nature of these detections, these discoveries emphasize that such observations may provide a major breakthrough in the field. In fact, they would be useful not only for our understanding of stellar multiplicity, but also for our understanding of how planets, brown dwarfs, and stars can form even in the most extreme environments.

The First High-contrast Images of Near High-mass X-Ray Binaries with Keck/NIRC2

“…NGC 6231, located within the OB association Sco OB1, has been recently confirmed as the parent cluster of 4U 1700−377 (van der Meij et al 2021). Moreover, inferred an estimated age of ∼80 Myr for the system, a value we have regarded as an upper limit as in Prasow-Émond et al (2022). Its X-ray luminosity can reach up to ∼7 × 10 36 erg s −1 (Laurent et al 1992).…”

“…Similarly to Prasow-Émond et al (2022), we performed data reduction using the Vortex Image Processing (VIP) and NIRC2 Preprocessing packages (Gomez Gonzalez et al 2017). To obtain a preprocessed data cube, we proceeded as follows: (1) flat-fielding of the frames, (2) bad pixel masking using the dark frames, (3) determination of the vortex center for each frame followed by cropping the science cube around the mean center, (4) removal of sky contribution via a principal component analysis (PCA) based technique, and (5) image registration to align the quasi-static speckle pattern across frames.…”

“…Errors in the fit parameters (θ, ρ, and f 1 ) were estimated using an injection/recovery approach (see Prasow-Émond et al 2022). This approach relies on injecting synthetic sources with known parameters into the images.…”

“…It is also known as a γ Cas analog, due to the similarities in their X-ray properties (e.g., Shrader et al 2015). The first highcontrast images of this HMXB were presented in Prasow-Émond et al (2022), in which more comprehensive information on the system can be found.…”

“…In Prasow-Émond et al (2022), we presented the first set of observations from a pilot study aiming to survey all X-ray binaries amenable for direct imaging within ∼3 kpc. We first targeted a γ Cassiopeiae−like X-ray binary harboring a Be donor star, RX J1744.7−2713, for which we had observations from two different bands and two epochs.…”