A Multiwavelength View of $ρ$ Oph I: Resolving the X-ray Source Between A and B

TL;DR

This study resolves the central ambiguity in the ρ Oph AB system by using high-resolution Chandra imaging to show that ρ Oph B, not ρ Oph A, is the dominant X-ray source. Temporal and spectral analyses indicate coronal activity consistent with a cool GK-type companion in an Algol-like binary with a B-type primary, rather than X-ray emission from a single OB wind or hotspot. Supporting evidence from optical RLMT and VLA radio data reinforces the cool-star companion interpretation, though spectroscopic confirmation remains needed. The work highlights the necessity of high-resolution X-ray imaging for close, young stellar systems and outlines concrete paths for verification via deeper radio observations and optical radial-velocity monitoring.

Abstract

We present a multiwavelength analysis of the central stellar pair of $ρ$ Oph, components A and B. Using recent high-resolution \textit{Chandra X-ray Observatory} observations, we demonstrate with high confidence that the dominant X-ray source is $ρ$ Oph B, while $ρ$ Oph A is comparatively X-ray faint. This result contrasts with earlier \textit{XMM-Newton} observations, which, due to limited spatial resolutions, attributed the X-ray emission to $ρ$ Oph A. An analysis of $ρ$ Oph B's X-ray light curves and spectra reveals properties more consistent with a cool star than a hot star. We therefore propose that $ρ$ Oph B is an Algol-like binary system, consisting of a B-type primary and an active, X-ray-emitting GK-type companion.

Figures (5)

• Figure 1: Top panel:XMM MOS1 event file from ObsID 0760900101 showing the reference stars used for correcting the aspect solutions in the X-ray observations. Bottom panels: Images from Chandra ACIS-S, RLMT, XMM MOS1 (in order of observation date), and VLA. Images are centered to the same WCS coordinate with north up and east to the left but have different scales. $\rho$ Oph A's and B's positions are marked with $2"$ radius regions in each image, B being northwest of A. For Chandra and RLMT, the optical positions are used. For XMM, the presented regions are corrected source positions. Regions in the VLA use the uniform beam shape (shown as the solid ellipse) and have corrected source positions. See Table \ref{['tab:RegCor']} for corrective values.
• Figure 2: Top: Concatenated Chandra light curve in observation order with 2 ks time bins. Bottom left:NuSTAR FPMA (black) and XMM PN (magenta; Obs Id 0870920101) light curves with 1 ks bins. Bottom right:NuSTAR FPMA (black) and XMM PN (red; Obs Id 0720690101, blue; Obs Id 0760900101, and magenta; Obs Id 0870920101) light curves with 1 ks bins phased to the beginning of Obs Id 0720690101 assuming a 1.205 day period.
• Figure 3: Top panel: total Chandra HEG+MEG spectrum (black) and RGS1+RGS2 (blue) with best fitting two-temperature plasma model (red and green) during quiescence. Both spectra have been binned by a constant factor of ten. Middle panel:NuSTAR FPMA+FPMB (black) with best fitting one-temperature plasma model (red), binned by a constant factor of three. Bottom panel:XMM PN (magenta), MOS1+MOS2 (black), and RGS1+RGS2 (blue) with best fitting two-temperature plasma model (red, cyan, and green, respectively). The PN and MOS spectra are binned by a constant of three while RGS is binned by ten.
• Figure 4: Ninety percent line width contours of $\rho$ Oph (solid) compared to the known unresolved line widths of HR 1099 (dashed).
• Figure 5: Zeroth order Chandra ACIS-S spectrum of $\rho$ Oph A (black) with best fitting power law DEM model (red). The spectrum of the background region is shown in gray.