The very faint hard state of the persistent neutron star X-ray binary SLX 1737–282 near the Galactic Centre

Monthly Notices of the Royal Astronomical Society

et al. 2018

MAXI J1807+132 is an X-ray transient discovered during the decay of an outburst in 2017. We present optical and X-ray monitoring of the source over more than 125 days, from outburst to quiescence. The outburst decay is characterized by the presence of several re-flares with a quasi-periodic recurrence time of ∼ 6.5 days. We detect broad H and He emission lines during outburst, characteristic of transient low mass X-ray binaries. These emission lines show strong variability from epoch to epoch and, in particular, during the early stages are found embedded into deep and very broad absorption features. The quiescent spectrum shows Hα in emission and no obvious signatures of the donor star. XMM-Newton and Swift spectra can be fitted with standard X-ray models for accreting black-holes and neutron stars, although the obtained spectral parameters favour the latter scenario. Conversely, other observables such as the optical/X-ray flux ratio, the likely systemic velocity (γ ∼ −150 km s −1 ) and the re-flares recurrence time suggest a black hole nature. We discuss all the above possibilities with emphasis on the strong similarities of MAXI J1807+132 with short orbital period systems.

Section: On the Nature Of The Compact Objectmentioning

confidence: 82%

The complex evolution of the X-ray binary transient MAXI J1807+132 along the decay of its discovery outburst

Jiménez-Ibarra

Muñoz-Darias

Monthly Notices of the Royal Astronomical Society

et al. 2018

“…The resulting blackbody temperature was kT bb =0.18 ± 0.01 keV with an inferred emission radius of R bb =43 +19 −14 km. We also tried the three-component model (DISKBB+BBODYRAD+NTHCOMP+GAUSS) proposed for NS hard state spectra when high-quality coverage at low energies is available (Armas Padilla et al 2017Padilla et al , 2018; see also Lin et al 2007). The model adequately fits the spectra ( χ 2 540 for 503 dof), but the extra thermal component is not statistically required (F-test probability > 0.1).…”

Section: Xmm-newton Spectral Analysismentioning

confidence: 99%

Multiwavelength spectroscopy of the black hole candidate MAXI J1813-095 during its discovery outburst

Monthly Notices of the Royal Astronomical Society

Muñoz-Darias

Sánchez-Sierras

et al. 2019

MAXI J1813-095 is an X-ray transient discovered during an outburst in 2018. We report on X-ray and optical observations obtained during this event, which indicate that the source is a new low-mass X-ray binary. The outburst lasted ∼70 d and peaked at L X (0.5-10keV)∼ 7.6 × 10 36 erg s −1 , assuming a distance of 8 kpc. Swift/XRT follow-up covering the whole activity period shows that the X-ray emission was always dominated by a hard power-law component with a photon index in the range of 1.4-1.7. These values are consistent with MAXI J1813-095 being in the hard state, in agreement with the ∼30 per cent fractional root-mean-square amplitude of the fast variability (0.1-50 Hz) inferred from the only XMM-Newton observation available. The X-ray spectra are well described by a Comptonization emission component plus a soft, thermal component (kT ∼0.2 keV), which barely contributes to the total flux ( 8 per cent). The Comptonization y-parameter (∼1.5), together with the low temperature and small contribution of the soft component supports a black hole accretor. We also performed optical spectroscopy using the VLT and GTC telescopes during outburst and quiescence, respectively. In both cases the spectrum lack emission lines typical of X-ray binaries in outburst. Instead, we detect the Ca ii triplet and Hα in absorption. The absence of velocity shifts between the two epochs, as well as the evolution of the Hα equivalent width, strongly suggest that the optical emission is dominated by an interloper, likely a G-K star. This favours a distance 3 kpc for the X-ray transient.

“…The 0.5-10 keV X-ray luminosity during these observations was ∼ 1 × 10 36 erg s −1 , which is consistent with previous measurements reinforcing the persistent nature of the system (e.g., Forman et al 1978;Markert et al 1979;Revnivtsev et al 2004;in 't Zand et al 2005a). Our spectra are well described by a thermally Comptonized model with Γ 1.9 and a distribution of seed photons characterised by kT seed 0.2 keV arising from either the accretion disc or NS surface/boundary layer (possibly from both regions; see Armas Padilla et al 2018). We do not detect direct emission from the accretion disc or NS surface as no softer thermal component was required in our analysis.…”

Section: Discussionmentioning

confidence: 77%

“…The corona electron temperature parameter (kT e ) typically adopts values above 10 keV in the low-hard state of LMXBs, which is beyond our 0.5-10 keV spectral coverage. Therefore, we fixed kT e to 25 keV, which is the typical value displayed by NS systems at this low-luminous states (Burke et al 2017 Padilla et al 2017Padilla et al , 2018. However, the extra thermal component does not account for the soft residuals and an evident structure remains below 2 keV.…”

Section: Analysis and Resultsmentioning

confidence: 99%

“…We added a thermal component (DISKBB or BBODYRAD) assuming that the soft residuals might be produced by emission from the accretion disc or NS surface/boundary layer, as commonly observed in NSs accreting a low luminosities (e.g. Armas Padilla et al 2017Padilla et al , 2018. However, the extra thermal component does not account for the soft residuals and an evident structure remains below 2 keV.…”

Section: Analysis and Resultsmentioning

confidence: 99%

See 1 more Smart Citation

On the ultra-compact nature of the neutron star system 1RXS J170854.4−321857: insights from X-ray spectroscopy

Monthly Notices of the Royal Astronomical Society

López-Navas

2019

The relatively small family of ultra-compact X-ray binary systems is of great interest for many areas of astrophysics. We report on a detailed X-ray spectral study of the persistent neutron star low mass X-ray binary 1RXS J170854.4-321857. We analysed two XMM-Newton observations obtained in late 2004 and early 2005 when, in agreement with previous studies, the system displayed an X-ray luminosity (0.5-10 keV) of ∼ 1 × 10 36 erg s −1 . The spectrum can be described by a Comptonized emission component with Γ ∼1.9 and a distribution of seed photons with a temperature of ∼ 0.23 keV. A prominent residual feature is present at soft energies, which is reproduced by the absorption model if over-abundances of Ne and Fe are allowed. We discuss how similar observables, that might be attributed to the peculiar (non-solar) composition of the plasma donated by the companion star, are a common feature in confirmed and candidate ultra-compact systems. Although this interpretation is still under debate, we conclude that the detection of these features along with the persistent nature of the source at such low luminosity and the intermediate-long burst that it displayed in the past confirms 1RXS J170854.4-321857as a solid ultra-compact X-ray binary candidate.