Unveiling the reflection spectrum in the ultracompact LMXB 4U 1820–30

¹ Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu, KM 0.7, Monserrato 09042, Italy
² Dipartimento di Fisica e Chimica – Emilio Segrè, Università di Palermo, Via Archirafi 36, 90123 Palermo, Italy
³ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, I-00146 Roma, Italy
⁴ INAF/IASF Palermo, Via Ugo La Malfa 153, I-90146 Palermo, Italy
⁵ INFN, Sezione di Cagliari, Cittadella Universitaria, 09042 Monserrato, CA, Italy
⁶ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, E-08193 Barcelona, Spain
⁷ Institut d’Estudis Espacials de Catalunya (IEEC), Esteve Terradas 1, RDIT Building, Of. 212 Mediterranean Technology Park (PMT), 08860 Castelldefels, Spain
⁸ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy

^⋆ Corresponding author: alessio.anitra@inaf.it

Received: 10 February 2025
Accepted: 24 March 2025

Abstract

4U 1820–30 is a well-known ultracompact X-ray binary located in the globular cluster NGC 6624, consisting of a neutron star accreting material from a helium white dwarf companion characterized by the shortest known orbital period for this type of star (11.4 minutes). Despite extensive studies, the detection of the relativistic Fe K emission line, indicative of a Compton reflection component, has been inconsistently reported and no measurement of the system inclination has been achieved (although it is hypothesized to be low). In this work, we investigate the broadband spectral and polarimetric properties of 4U 1820–30, exploring the presence of a reflection component and its role in shaping the polarization signal. We analyzed simultaneous X-ray observations from NICER, NuSTAR, and IXPE. The spectral continuum was modeled with a disk blackbody, a power law, and a Comptonization component with seed photons originating from a boundary layer. We detected a strong reflection component, described, for the first time, with two self-consistent models (Relxillns and Rfxconv), allowing us to provide a measurement of the system inclination angle (${31}_{-5}^{+9}$ degrees), supporting the low-inclination hypothesis. Subsolar iron abundances were detected in the accretion disk and interstellar medium, probably related to the source location in a metal-poor globular cluster. The source exhibits an increase in polarization from an upper limit of 1.2% in the 2−4 keV band up to about 8% in the 7−8 keV range. The main contribution to the polarization comes from the Comptonization, with a possible significant contribution from the weak, but highly polarized reflection component. The disk is expected to be orthogonally polarized to these components, which may help to explain the decreasing of the observed polarization at low energies. However, the high polarization degree we found in the 7−8 keV band challenges the current models, also taking into consideration the relatively low inclination angle derived from the spectral analysis.