# First Detection of Candidate Exoplanets and Brown Dwarfs Orbiting X-Ray Binaries via Direct Imaging

Myriam Prasow-Émond1, 2, Julie Hlavacek-Larrondo1, Kevin Fogarty3, 4, Julien Rameau5, Louis-Simon Guité1, Dimitri Mawet3, 6 et al.
1Université de Montréal, 2 iREx, 3 California Institute of Technology, 4 NASA Ames Research Center, 5 Institut de Planétologie et d’Astrophysique de Grenoble, 6 Jet Propulsion Laboratory

### Introduction

X-RAY BINARIES
• X-ray binaries are composed of a compact stellar remnant (white dwarf, neutron star or black hole) accreting material from a donor star, and their interaction releases strong X-ray radiation.
• Fig. 1 presents the life cycle of a Sun-like star and of a massive star. The nature of the compact stellar remnant depends on the initial star mass. More detail in the subsequent audio.
• Fig. 2 presents an illustration of an X-ray binary and the physical processes involved.
• Historically, X-ray binaries have been divided into two distinct categories:
1. High-mass X-ray binaries (HMXB) which harbour a massive O-B spectral type donor star that transfers mass onto the compact object via strong stellar winds.
2. Low-mass X-ray binaries (LMXB) which harbour a K-M spectral type star that overflows the Roche Lobe of the compact object, giving rise to strong accretion.
• SUB-STELLAR COMPANIONS
• A sub-stellar companion is an astronomical object not massive enough to allow hydrogen fusion. Exoplanets and brown dwarfs are included in that category.
• Some studies indicate that sub-stellar companions can exist in a variety of environments.
• It was recently argued that X-ray binaries could host planetary systems (Imara & Di Stefano 2018).
• Those systems are more likely to harbour wide orbit planets because of planet-star/planet-planet interactions that would push away the companions (e.g. Bonavita et al. 2016).
X-ray binaries are thus unique laboratories for studying astronomical objects and phenomena under extreme conditions.

### Methods

RX J1744.7-2713
• RX J1744.7-2713, the object of interest in this study, is an HMXB harboring a massive Be star.
• Although the origin of X-ray emission is still debated, it may point to accretion onto a white dwarf.
• It is located at 1.21 kpc (in our Galaxy).
• Coleiro & Chaty (2013) estimated the age of RX J1744.4-2713 to be $$\sim$$ 60 Myr.
• CORONAGRAPHY & HIGH-CONTRAST IMAGING
• Coronagraphy was historically invented and used by Bernard Lyot (1939) to study the Sun’s corona without having to wait for a solar eclipse.
• It is now used to study astronomical system’s environment more effectively.
• The coronagraph blocks the central part of the host star (or here, of the X-ray binary), allowing higher contrast to be achieved (see Fig. 3).
• For our observations, we chose high-mass (star with at least twice the mass of the Sun) & near (< 2-3 kpc) X-ray binaries to increase the chance of detecting exoplanets with this technique.
• Not all exoplanets can be directly imaged with our current telescope's sensibility: only young and bright planets (e.g. Hot Jupiters) can be detected.
• Other imaging techniques are also used to increase the contrast and the signal-to-noise ratio. An great example is Angular Differential Imaging (ADI; Marois et al. 2006); this technique is used in the present work.
• OBSERVATIONS
• In July 2020, we obtained 3 nights of observations on the W. M. Keck Observatory (Hawai’i; see Fig. 4) using NIRC2, an near-infrared imager. We used also the vortex coronagraph (Mawet et al. 2005).
• Near-Infrared (NIR) is the wavelength range where young exoplanets are the brightest.
• We observed 8 HMXBs for $$\sim$$ 2 hours each in L’-band ($$\lambda = 3.776 \mu m$$, $$\Delta \lambda = 0.700 \mu m$$; PI: Mawet), and additionally in Ks-band ($$\lambda = 2.146 \mu m$$, $$\Delta \lambda = 0.311 \mu m$$; PI: Fogarty) for RX J1744.7-2713.
• In 2017, we also obtained data for 7 X-ray binaries, including two sources also observed in 2020 (Gamma Cas and RX J1744.7-2713).
First high-contrast images of X-ray binaries ever obtained!

### Sub-Stellar Companion Candidates

HIGH-CONTRAST IMAGES
• The left panel of Fig. 5 presents the L'-band high-contrast image of RX J1744.7-2713, and Fig. 6 shows the Ks-band.
• L' and Ks-band signal-to-noise ratio (SNR) maps, we found at least 16 sources with a SNR greater than 5. Note that is a very large number of sources, i.e. RX J1744.7-2713 is incredibly crowded!
• The projected separations from RX J1744.7-2713 range from $$\sim$$ 450 to $$\sim$$ 6000 AU (astronomical units, 1 AU = distance between the Sun and the Earth). These are extremely wide orbits: to give you an idea of the order, Pluton is located at $$\sim$$ 40 AU from the Sun.
• However, the mass of RX J1744.7-2713 is largely greater than the Sun's, so wide orbits are possible and have been observed in other systems (e.g. Naud et al. 2014).

### Discussion

HOW CAN WE DETERMINE IF THE CANDIDATES ARE SUB-STELLAR COMPANIONS?

A detected source is not always necesserally bound to the system: it could be, for example, a bright background star. Some of the most common techniques to determine the nature of the sources are listed below.

1. Astrometry (follow-up observations). It is The most rigorous way to confirm that candidates are sub-stellar companions to the host system. By taking additional data several days/months/years, we can study the proper motion of the objects and therefore conclude if they are bound or not. In the case of RX J1744.7-2713, this kind of analysis will be available in a least 10 years.
2. Color-magnitude diagram. If observations in two different bands are available, we can construct a color-magnitude diagram to determine if their color and magnitude are more coherent with stars or exoplanets. For RX J1744.7-2713, see Fig. 7 and audio for more detail.
3. Background probability. Using 3D models of the sky, we can estimate the expected number of sources in a certain area. Depending on the number of sources, we can calculate the probability of finding a source with the same magnitude of a detected candidate.
4. More observations. In order to study an object more in depth, we can ask for additional observations, e.g. spectoscopic analysis, other band, etc.
There are between 4 and 8 sub-stellar companion candidates in RX J1744.7-2713.
CONCLUSIONS
• In this work, we have presented the first high-contrast images of RX J1744.7-2713, using data from the Keck/NIRC2 vortex coronagraph.
• We also reported the possible discovery of 4 to 8 sub-stellar companions with masses from 5 to 26 $$M_\mathrm{Jup}$$ and really wide orbits, which may be the first brown dwarfs and giant planets ever imaged around X-ray binaries.
• A paper in preparation (Prasow-Émond et al. in prep) will present the high-contrast images of 13 other near X-ray binaries. We aim to conduct a statistical study of the presence of sub-stellar companions in those extreme environments.