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Register a new userConfirming NGC 6231 as the parent cluster of the runaway high-mass X-ray binary HD 153919/4U 1700-37 with Gaia DR2
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A significant fraction of the most massive stars move through space with a high velocity. One of the possible physical explanations is that a supernova in a compact binary system results in a high recoil velocity of the system. If the system remains bound, it can be subsequently observed as a spectroscopic binary (SB1), a high-mass X-ray binary, a compact binary, and finally a gravitational-wave event. If such a system is traced back to its parent cluster, binary evolution models can be tested in great detail. The Gaia proper motions and parallaxes are used to demonstrate that the high-mass X-ray binary HD153919/4U 1700-37 originates from NGC6231, the nucleus of the OB association Sco OB1. The O supergiant and its compact companion, of which the physical nature (a neutron star or a black hole) is unknown, move with a space velocity of 63 km/s with respect to NGC6231. The kinematical age of the system is 2.2 Myr. The parallaxes and accurate proper motions in Gaia DR2 were used to perform a membership analysis of NGC 6231. The distance to NGC6231 is 1.63 kpc. Isochrone fitting results in an age of 4.7 Myr. With the identification of NGC6231 as the parent cluster, the upper limit on the age of the progenitor of 4U1700-37 at the moment of the supernova explosion is 3.0 Myr. With these constraints, the evolutionary history of the system can be reconstructed with an initial mass of the progenitor of the compact object >60 Msun. Given its current high space velocity and the derived evolutionary history, the compact object in the system is more likely to have received a large natal kick, which suggests that it is more likely a neutron star than a black hole. HD153919/4U1700-37 might be a prototype in the Milky Way for the progenitor of gravitational wave events such as GW190412
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We present an X-ray spectral analysis of the high-mass binary 4U~1700-37 during its hard-soft state evolution. We use the BeppoSAX, Suzaku and RXTE (Rossi X-ray Timing Explorer), Suzaku and BeppoSAX observations for this investigation. We argue that the X-ray broad-band spectra during all spectral states can be adequately reproduced by a model, consisting of a low-temperature Blackbody component, two Comptonized components both due to the presence of a Compton cloud (CC) that up-scatters seed photons of $T_{s1}$~< 1.4 keV, and $T_{s2}<$1 keV, and an iron-line component. We find using this model that the photon power-law index is almost constant, $Gamma_{1}sim 2$ for all spectral states. However, $Gamma_{2}$ shows a behavior depending on the spectral state. Namely, $Gamma_{2}$ is quasi-constant at the level of $Gamma_{2}sim 2$ while the CC plasma temperature $kT^{(2)}_e$ is less than 40 keV; on the other hand, $Gamma_{2}$ is in the range of $1.3<Gamma_{2}<2$, when $kT^{(2)}_e$ is greater than 40 keV. We explain this quasi-stability of $Gamma$ during most of hard-soft transitions of 4U~1700-37 in a framework of the model in which the resulting spectrum is described by two Comptonized components. We find that these Comptonized spectral components of the HMXB 4U~1700-37 are similar to those previously found in NS sources. This index dependence versus both mass accretion rate and $kT_e$ revealed in 4U~1700-37 is a universal observational evidence for the presence of a NS in 4U 1700-37.
Based on its Hipparcos proper motion, we propose that the high-mass X-ray binary HD153919/4U1700-37 originates in the OB association Sco OB1. At a distance of 1.9 kpc the space velocity of 4U1700-37 with respect to Sco OB1 is 75 km/s. This runaway velocity indicates that the progenitor of the compact X-ray source lost about 7 Msun during the (assumed symmetric) supernova explosion. The systems kinematical age is about 2 +/- 0.5 million years which marks the date of the supernova explosion forming the compact object. The present age of Sco OB1 is <8 Myr; its suggested core, NGC 6231, seems to be somewhat younger (~5 Myr). If HD153919/4U1700-37 was born as a member of Sco OB1, this implies that the initially most massive star in the system terminated its evolution within <6 million years, corresponding to an initial mass >30 Msun. With these parameters the evolution of the binary system can be constrained.
We present new Chandra observations of the outer halo of the giant elliptical galaxy NGC 4472 (M49) in the Virgo Cluster. The data extend to 130 kpc (28), and have a combined exposure time of 150 ks. After identifying optical counterparts using the Next Generation Virgo Cluster Survey to remove background active galactic nuclei and globular cluster (GC) sources, and correcting for completeness, we find that the number of field low-mass X-ray binaries (LMXBs) per unit stellar V-band light increases significantly with galactocentric radius. Because the flux limit of the complete sample corresponds to the Eddington limit for neutron stars in NGC 4472, many of the ~90 field LMXBs in this sample could host black holes. The excess of field LMXBs at large galactocentric radii may be partially caused by natal kicks on black holes and neutron stars in binary systems in the inner part of the galaxy. Furthermore, since the metallicity in the halo of NGC 4472 strongly decreases towards larger galactocentric radii, the number of field LMXBs per unit stellar mass is anti-correlated with metallicity, opposite to what is observed in GCs. Another way to explain the spatial distribution of field LMXBs is therefore a reversed metallicity effect, although we have not identified a mechanism to explain this in terms of stellar and binary evolution.
We present the results of new X-ray observations of XMMU 122939.7+075333, the black hole (BH) in the globular cluster RZ 2109 in the Virgo Cluster galaxy NGC 4472. A combination of non-detections and marginal detections in several recent Swift and Chandra observations show that the source has varied by at least a factor of 20 in the past 6 years, and that the variations seem not just to be flickering. This variation could be explained with changes in the absorption column intrinsic to the source no larger than those which were previously seen near the peak of the 1989 outburst of the Galactic BH X-ray binary V404 Cyg. The large amplitude variations are also a natural expectation from a hierarchical triple system with Kozai cycles -- the mechanism recently proposed to produce BH-white dwarf (WD) binaries in globular clusters. On the other hand, variation by such a large factor on timescales of years, rather than centuries, is very difficult to reconcile with the scenario in which the X-ray emission from XMMU 122939.7+075333 is due to fallback of material from a tidally destroyed or detonated WD.
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