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. 1 - Life Cycle of a Star.
Fig. 2 presents an illustration of an X-ray binary and the physical processes involved.
Fig. 2 - Illustration of an X-ray Binary.
Historically, X-ray binaries have been divided into two distinct categories:
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.
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.
Fig. 3 - The Solar and Heliospheric Observatory (SOHO) used its coronagraph to capture this image of a coronal mass ejection (the circle of light to the left of the sun) leaving the sun and moving into space. (Image credit: ESA&NASA/SOHO/GSFC ).
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).
Fig. 4 - W. M. Keck Observatory.
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).
Fig. 5 - L'-band high-contrast image of RX J1744.7-2713. Source: Prasow-Émond et al. (to be submitted).Fig. 6 - Ks-band high-contrast image of RX J1744.7-2713. Source: Prasow-Émond et al. (to be submitted).
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.
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.
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.
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.
More observations. In order to study an object more in depth, we can ask for additional observations, e.g. spectoscopic analysis, other band, etc.
Fig. 7 - Absolute magnitude in L'-band versus Ks-L' color for 16 detected sources (circles) in RX J1744.7-2713. More detail in the audio. Source: Prasow-Émond et al. (to be submitted).
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.
