A candidate to the long sought optical counterpart to the Rapid Burster in the bulge fossil fragment Liller 1

Abstract

We report on the possible identification of the optical counterpart of the Rapid Burster MXB 1730-335 in the stellar system Liller 1. The identification was performed by taking advantage of a set of images acquired with the Hubble Space Telescope/Advanced Camera for Surveys in the optical band, and with the Gemini South Telescope in the near-infrared. The analysis of these images revealed the presence of a star with a position possibly compatible with the X-ray and radio band coordinates of the Rapid Burster, and showing significant optical variability. According to its location in the color-magnitude diagram, the candidate companion appears to belong to the young (~ 1-2 Gyr old) super-solar metallicity ([M/H]= +0.3) sub-population recently discovered in Liller 1. We discuss the main characteristics of the candidate counterpart and the Rapid Burster binary system as derived from the available data, also highlighting the need for further coordinated observations to solidly confirm their association and better clarify their physical properties.

Figures (5)

• Figure 1: HST/ACS (left panel) and VLA (right panel; https://zenodo.org/records/12806382) images of the $4\arcsec \times 4 \arcsec$ region centered on the X-ray position of the RB quoted in Table \ref{['Tab:positions']}. The crosses mark the coordinates listed in the table, while the ellipses have axes equal to 3 times the quoted uncertainties, with red, blue and cyan colors corresponding, respectively, to the values obtained for the candidate optical counterpart (this work), and the X-ray and radio band positions of the RB homer01_chandraPosjetradio. North is up, East is to the left.
• Figure 2: Images of the $2\arcsec \times 2\arcsec$ region surrounding the candidate optical counterpart to the RB (white circle) acquired at different times. The top and bottom panels show HST/ACS images in the F814W filter, and Ks-band GeMS images, respectively. For visualization purposes, a 2 pixel smoothing has been applied to the F814W images. The images in the right panels were acquired approximately 1.5/2 hours after those in the left panels. The corresponding changes in magnitudes are about $\sim 0.4$ mags for the Ks-band (bottom row) and $\sim0.5$ mags for the F814W filter (top row).
• Figure 3: Magnitude scatter measured for all the stars located within $20\arcsec$ (gray dots) from the RB X-ray position, as a function of their magnitude. The black circles highlight all the sources detected within a distance of just $1.5\arcsec$ from the RB, with the large red star marking the candidate optical counterpart. The top, middle and bottom panels refer to the F606W, F814W, and Ks filters, respectively.
• Figure 4: Magnitude differences with respect to the mean value in the F606W, F814W, and Ks filters (top, middle, and bottom panels, respectively), as a function of time. The latter is calculated in hours with respect to the reference MJD marked in each panel. Error bars for the magnitudes are displayed and are comparable in size to the plotted symbols
• Figure 5: Differential reddening corrected and proper motion selected CMD of all the stars detected within $20 \arcsec$ from the RB X-ray position (gray dots). The black circles and the large red star highlight, respectively, the sources located within a distance of $1.5\arcsec$ and the candidate optical counterpart to the RB. The red and blue lines (see also legend) are, respectively, a 12 Gyr old isochrone Bressan+12 with [M/H]=$-0.3$ (well reproducing the old sub-population of Liller 1), and two 1-2 Gyr old isochrones with [M/H]=$+0.3$ (tracing the young and super-solar sub-population).