AX J1754.2−2754, 1RXS J171824.2−402934 and 1RXH J173523.7−354013 are three persistent neutron star low-mass X-ray binaries that display a 2-10 keV accretion luminosity L
X of only (1-10) × 1034 erg ...s−1 (i.e. only 0.005-0.05 per cent of the Eddington limit). The phenomenology of accreting neutron stars which accrete at such low accretion rates is not yet well known and the reason why they have such low accretion rates is also not clear. Therefore, we have obtained XMM-Newton data of these three sources and here we report our analysis of the high-quality X-ray spectra we have obtained for them. We find that AX J1754.2−2754 has L
X ∼ 1035 erg s−1, while the other two have X-ray luminosities about an order of magnitude lower. However, all sources have a similar, relatively soft, spectrum with a photon index of 2.3-2.5, when the spectrum is fitted with an absorbed power-law model. This model fits the data of AX J1754.2−2754 adequately, but it cannot fit the data obtained for 1RXS J171824.2−402934 and 1RXH J173523.7−354013. For those sources, a clear soft thermal component is needed to fit their spectra. This soft component contributes 40-50 per cent to the 0.5-10 keV flux of the sources. The presence of this soft component might be the reason why the spectra of these two sources are soft. When including this additional spectral component, the power-law photon indices are significantly lower. It can be excluded that a similar component with similar contributions to the 2-10 keV X-ray flux is present for AX J1754.2−2754, indicating that the soft spectrum of this source is mostly due to the fact that the power-law component itself is not hard. We note that we cannot exclude that a weaker soft component is present in the spectrum of this source which only contributes up to ∼25 per cent to the 0.5-10 keV X-ray flux. We discuss our results in the context of what is known of accreting neutron stars at very low accretion rate.
We present time-resolved 10.4 m Gran Telescopio Canarias and 4.2 m William Herschel Telescope intermediate resolution spectroscopy of the X-ray transient MAXI J1820+070 (=ASASSN-18ey) obtained during ...its decline to the quiescent state. Cross-correlation of the 21 individual spectra against late-type templates reveals a sinusoidal velocity modulation with a period of 0.68549 0.00001 day and semi-amplitude of 417.7 3.9 km s−1. We derive a mass function f(M) = 5.18 0.15 M , dynamically confirming the black hole (BH) nature of the compact object. Our analysis of the stellar absorption features supports a K3-5 spectral classification for the donor star, which contributes 20% of the total flux at 5200-6800 . The photometric 0.703 0.003 day periodicity observed during outburst is 2.6% longer than the orbital period supporting the presence of a superhump modulation in the outburst light curves. In line with this interpretation, we constrain the binary mass ratio to be q 0.12. In addition, we observe a sharp increase in the H emission line equivalent width during inferior conjunction of the donor star that we interpret as a grazing eclipse of the accretion disk and allows us to constrain the binary inclination to i 69°. On the other hand, the absence of X-ray eclipses during outburst imply i 77°. These inclination limits, together with our dynamical solution, lead to a BH mass in the range 7-8 M . We also measure a systemic velocity γ = −21.6 2.3 km s−1, which, combined with the Gaia second data release proper motion and parallax, implies a large peculiar velocity ∼100 km s−1.
We report our multiwavelength study of the 2011 outburst evolution of the newly discovered black hole candidate X-ray binary Swift J1357.2−0933. We analysed the Swift X-ray Telescope and ...Ultraviolet/Optical Telescope (UVOT) data taken during the ∼7 months duration of the outburst. It displayed a 2-10 keV X-ray peak luminosity of ∼1035(D/1.5 kpc)2 erg s−1, which classifies the source as a very faint X-ray transient. We found that the X-ray spectrum at the peak was consistent with the source being in the hard state, but it softened with decreasing luminosity, a common behaviour of black holes returning to quiescence from the hard state. The correlations between the simultaneous X-ray and ultraviolet/optical data suggest a system with a black hole accreting from a viscous disc, and we do not detect X-ray reprocessing on the disc surface. The UVOT filters provide the opportunity to study these correlations up to ultraviolet wavelengths, a regime so far unexplored. If the black hole nature is confirmed, Swift J1357.2−0933 would be one of the very few established black hole very-faint X-ray transients.
A new radio census of neutron star X-ray binaries van den Eijnden, J; Degenaar, N; Russell, T D ...
Monthly notices of the Royal Astronomical Society,
11/2021, Volume:
507, Issue:
3
Journal Article
Peer reviewed
Open access
ABSTRACT
We report new radio observations of a sample of 36 neutron star (NS) X-ray binaries, more than doubling the sample in the literature observed at current-day sensitivities. These sources ...include 13 weakly magnetized (B < 1010 G) and 23 strongly magnetized (B ≥ 1010 G) NSs. 16 of the latter category reside in high-mass X-ray binaries, of which only two systems were radio-detected previously. We detect four weakly and nine strongly magnetized NSs; the latter are systematically radio fainter than the former and do not exceed LR ≈ 3 × 1028 erg s−1. In turn, we confirm the earlier finding that the weakly magnetized NSs are typically radio fainter than accreting stellar-mass black holes. While an unambiguous identification of the origin of radio emission in high-mass X-ray binaries is challenging, we find that in all but two detected sources (Vela X-1 and 4U 1700-37) the radio emission appears more likely attributable to a jet than the donor star wind. The strongly magnetized NS sample does not reveal a global correlation between X-ray and radio luminosity, which may be a result of sensitivity limits. Furthermore, we discuss the effect of NS spin and magnetic field on radio luminosity and jet power in our sample. No current model can account for all observed properties, necessitating the development and refinement of NS jet models to include magnetic field strengths up to 1013 G. Finally, we discuss jet quenching in soft states of NS low-mass X-ray binaries, the radio non-detections of all observed very-faint X-ray binaries in our sample, and future radio campaigns of accreting NSs.
We report on the X-ray spectral (using XMM-Newton data) and timing behaviour using XMM-Newton and Rossi X-ray Timing Explorer (RXTE) data of the very faint X-ray transient and black hole system Swift ...J1357.2-0933 during its 2011 outburst. The XMM-Newton X-ray spectrum of this source can be adequately fitted with a soft thermal component with a temperature of ∼0.22 keV (using a disc model) and a hard, non-thermal component with a photon index of Γ ∼ 1.6 when using a simple power-law model. In addition, an edge at ∼0.73 keV is needed likely due to interstellar absorption. During the first RXTE observation, we find a 6 mHz quasi-periodic oscillation (QPO) which is not present during any of the later RXTE observations or during the XMM-Newton observation which was taken 3 d after the first RXTE observation. The nature of this QPO is not clear, but it could be related to a similar QPO seen in the black hole system H1743-322 and to the so-called 1 Hz QPO seen in the dipping neutron-star X-ray binaries (although this latter identification is quite speculative). The observed QPO has similar frequencies as the optical dips seen previously in this source during its 2011 outburst, but we cannot conclusively determine that they are due to the same underlying physical mechanism. Besides the QPO, we detect strong band-limited noise in the power-density spectra of the source (as calculated from both the RXTE and the XMM-Newton data) with characteristic frequencies and strengths very similar to other black hole X-ray transients when they are at low X-ray luminosities. We discuss the spectral and timing properties of the source in the context of the proposed very high inclination of this source. We conclude that all the phenomena seen from the source cannot, as yet, be straightforwardly explained neither by an edge-on configuration nor by any other inclination configuration of the orbit.
We report on a detailed optical spectroscopic follow-up of the black hole (BH) transient MAXI J1820+070 (ASASSN-18ey). The observations cover the main part of the X-ray binary outburst, when the ...source alternated between hard and soft states following the classical pattern widely seen in other systems. We focus the analysis on the He i emission lines at 5876 and 6678 , as well as on H . We detect clear accretion disk wind features (P-Cyg profiles and broad emission line wings) in the hard state, both during outburst rise and decay. These are not witnessed during the several months long soft state. However, our data suggest that the visibility of the outflow might be significantly affected by the ionization state of the accretion disk. The terminal velocity of the wind is above ∼1200 km s−1, which is similar to outflow velocities derived from (hard-state) optical winds and (soft-state) X-ray winds in other systems. The wind signatures, in particular the P-Cyg profiles, are very shallow, and their detection has only been possible thanks to a combination of source brightness and intense monitoring at very high signal-to-noise. This study indicates that cold, optical winds are most likely a common feature of BH accretion, and therefore, that wind-like outflows are a general mechanism of mass and angular momentum removal operating throughout the entire X-ray binary outburst.
We search the literature for reports on the spectral properties of neutron star low-mass X-ray binaries when they have accretion luminosities between 1034 and 1036 erg s−1, corresponding to roughly ...0.01–1 per cent of the Eddington accretion rate for a neutron star. We found that in this luminosity range the photon index (obtained from fitting a simple absorbed power law in the 0.5–10 keV range) increases with decreasing 0.5–10 keV X-ray luminosity (i.e. the spectrum softens). Such behaviour has been reported before for individual sources, but here we demonstrate that very likely most (if not all) neutron star systems behave in a similar manner and possibly even follow a universal relation. When comparing the neutron star systems with black hole systems, it is clear that most black hole binaries have significantly harder spectra at luminosities of 1034–1035 erg s−1. Despite a limited number of data points, there are indications that these spectral differences also extend to the 1035–1036 erg s−1 range, but above a luminosity of 1035 erg s−1 the separation between neutron star and black hole systems is not as clear as below. In addition, the black hole spectra only become softer below luminosities of 1034 erg s−1 compared to 1036 erg s−1 for the neutron star systems. This observed difference between the neutron star binaries and black hole ones suggests that the spectral properties (between 0.5 and 10 keV) at 1034–1035 erg s−1 can be used to tentatively determine the nature of the accretor in unclassified X-ray binaries. More observations in this luminosity range are needed to determine how robust this diagnostic tool is and whether or not there are (many) systems that do not follow the general trend. We discuss our results in the context of properties of the accretion flow at low luminosities and we suggest that the observed spectral differences likely arise from the neutron star surface becoming dominantly visible in the X-ray spectra. We also suggest that both the thermal component and the non-thermal component might be caused by low-level accretion on to the neutron star surface for luminosities below a few times 1034 erg s−1.
ABSTRACT
We present high time resolution optical spectroscopy and imaging of the black hole transient Swift J1357.2−0933 during its 2017 outburst. The light curves show recurrent dips resembling ...those discovered during the 2011 outburst. The dip properties (e.g. duration and depth) as well as the evolution of their recurrence time are similar to those seen in 2011. Spectra obtained during the dips are characterized by broad and blueshifted absorptions in Balmer and He ii. The absorptions show core velocities of ∼−800 km s−1 and terminal velocities approaching ∼3000 km s−1 i.e. in the upper end of wind velocities measured in other black hole transients (both at optical and X-ray wavelengths). Our observations suggest that the dips are formed in a dense and clumpy outflow, produced near the disc equatorial plane and seen at high inclination. We also study the colour evolution and observe that, as it has been previously reported, the source turns bluer during dips. We show that this is due to a gradual change in the slope of the optical continuum and discuss possible implications of this behaviour.
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