The Symbiotic X-ray binaries Sct X-1, 4U 1700+24 and IGR J17329-2731

TL;DR

This study investigates the magnetic properties of symbiotic X-ray binaries Sct X-1, 4U 1700+24, and IGR J17329-2731 through broad-band X-ray spectroscopy, exploring the presence of cyclotron features and accretion behaviors.

Contribution

It provides the first potential detection of a cyclotron resonant scattering feature in 4U 1700+24, suggesting a highly magnetized neutron star in this system.

Findings

No cyclotron features in Sct X-1

Possible cyclotron feature at 16 keV in 4U 1700+24

IGR J17329-2731 shows variable X-ray activity with flares and obscuration events

Abstract

Symbiotic X-ray binaries are systems hosting a neutron star accreting form the wind of a late type companion. These are rare objects and so far only a handful of them are known. One of the most puzzling aspects of the symbiotic X-ray binaries is the possibility that they contain strongly magnetized neutron stars. These are expected to be evolutionary much younger compared to their evolved companions and could thus be formed through the (yet poorly known) accretion induced collapse of a white dwarf. In this paper, we perform a broad-band X-ray and soft $\gamma$-ray spectroscopy of two known symbiotic binaries, Sct X-1 and 4U 1700+24, looking for the presence of cyclotron scattering features that could confirm the presence of strongly magnetized NSs. We exploited available Chandra, Swift, and NuSTAR data. We find no evidence of cyclotron resonant scattering features (CRSFs) in the case of Sct X-1 but in the case of 4U 1700+24 we suggest the presence of a possible CRSF at $\sim$16 keV and its first harmonic at $\sim$31 keV, although we could not exclude alternative spectral models for the broad-band fit. If confirmed by future observations, 4U 1700+24 could be the second symbiotic X-ray binary with a highly magnetized accretor. We also report about our long-term monitoring of the last discovered symbiotic X-ray binary IGR J17329-2731 performed with Swift/XRT. The monitoring revealed that, as predicted, in 2017 this object became a persistent and variable source, showing X-ray flares lasting for a few days and intriguing obscuration events that are interpreted in the context of clumpy wind accretion.