The puzzling symbiotic X-ray system 4U1700+24

Abstract

Context. Symbiotic X-ray binaries form a subclass of low-mass X-ray binary systems consisting of a neutron star accreting material from a red giant donor star via stellar wind or Roche lobe overflow. Only a few confirmed members are currently known; 4U 1700+24 is a good candidate as it is a relatively bright X-ray object, possibly associated with the late-type star V934 Her. Aims. We analysed the archive XMM-Newton and Swift/XRT observations of 4U 1700+24 in order to have a uniform high-energy (0.3−10 keV) view of the source. Apart from the 2003, 2010, and 2012 data, publicly available but still unpublished, we also took the opportunity to re-analyze a set of XMM-Newton data acquired in 2002. Methods. After reducing the XMM-Newton and Swift/XRT data with standard methods, we performed a detailed spectral and timing analysis. Results. We confirmed the existence of a red-shifted O VIII Ly-α transition (already observed in the 2002 XMM-Newton data) in the high-resolution spectra collected via the RGS instruments. The red-shift of the line is found in all the analysed observations and, on average, it was estimated to be 0.009. We also observed a modulation of the centroid energy of the line on short time scales (a few days) and discuss the observations in the framework of different scenarios. If the modulation is due to the gravitational red-shift of the neutron star, it might arise from a sudden re-organization of the emitting X-ray matter on the scale of a few hundreds of km. Alternatively, we are witnessing a uni-polar jet of matter (with typical velocity of 1000−4000 km s −1 ) possibly emitted by the neutron star in an almost face-on system. The second possibility seems to be required by the apparent lack of any modulation in the observed X-ray light curve. We also note also that the low-resolution spectra (both XMM-Newton and Swift/XRT in the 0.3−10 keV band) show the existence of a black-body radiation emitted by a region (possibly associated with the neutron star polar cap) with typical size from a few tens to hundreds of meters. The size of this spot-like region reduces as the overall luminosity of 4U 1700+24 decreases.

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

“…The FPM spectra could be equivalently well described using the addition of a black-body component to the cut-off powerlaw in place of the two absorption features. The presence of a similar thermal component would not be unexpected, as it was previously reported for this source by Nucita et al (2014) while analyzing XMM-Newton and SwiftXRT data outside the outbursts. If we compare the results from these authors with those in Table 3, we note that the black-body component required by the NuSTAR data is significantly hotter (1.4 keV vs. the previously reported 0.7-1.0 keV) and would be dominating a large portion of the XRT energy coverage.…”

“…By using the online tool webpimms 4 and the spectral model measured from the Chandra + NuSTAR data, we converted the above values into a 3 σ upper limit on the source 0.5-10 keV X-ray flux of 8.6×10 −13 erg cm −2 s −1 and 5.6×10 −13 erg cm −2 s −1 , respectively. 4 https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3pimms/w3pimms.pl 3 4U 1700+24 4U 1700+24 was discovered in the '70s and it is known since then to be a relatively bright persistent X-ray source (see, e.g., Garcia et al 1983;Nucita et al 2014;Hinkle et al 2019, and references therein). The source is known to host an accreting NS which pulsations have been so far elusive most likely due to the pole-on direction through which the system is being observed.…”

Infrared Spectroscopy of Symbiotic Stars. XII. The Neutron Star SyXB System 4U 1700+24 = V934 Herculis

“…Again the requirement is that the system is observed nearly pole on with the magnetic and rotation poles aligned. The O VIII line observation was confirmed by Nucita et al (2014). They endorsed both the small size of the X-ray emitting area and the nearly pole on aspect of the NS.…”

“…They found this is in agreement with the emitting gas being accreted by the NS at the magnetospheric radius. Nucita et al (2014) found that this is ∼1000 km above the NS. Again the requirement is that the system is observed nearly pole on with the magnetic and rotation poles aligned.…”

An Ultra-Low-Mass and Small-Radius Compact Object in 4u 1746-37?

“…Nevertheless, in the case of a quarkcluster star, an additional so-called strangeness barrier exists on the quark-cluster surface. Xu (2014) demonstrated that a quark-cluster star may be surrounded by a hot corona or an atmosphere, or even a crust for different accretion rates, which could be helpful to understand the O viii Lyα emission line in 4U 1700+24 (Nucita et al 2014). The mass of the corona or atmosphere or crust is much less than the conventional value ∼10 −5 M of a strange star; hence, the scale is negligible compared with the radius of NSs.…”