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Register a new userThe variability plane of accreting compact objects
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E. G. Koerding
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Recently, it has been shown that soft-state black hole X-ray binaries and active galactic nuclei populate a plane in the space defined by the black hole mass, accretion rate and characteristic frequency. We show that this plane can be extended to hard-state objects if one allows a constant offset for the frequencies in the soft and the hard state. During a state transition the frequencies rapidly move from one scaling to the other depending on an additional parameter, possibly the disk-fraction. The relationship between frequency, mass and accretion rate can be further extended by including weakly accreting neutron stars. We explore if the lower kHz QPOs of neutron stars and the dwarf nova oscillations of white dwarfs can be included as well and discuss the physical implications of the found correlation.
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We present 15 high mass X-ray binary (HMXB) candidates in the disk of M31 for which we are able to infer compact object type, spectral type of the donor star, and age using multiwavelength observations from NuSTAR, Chandra, and the Hubble Space Telescope (HST). The hard X-ray colors and luminosities from NuSTAR permit the tentative classification of accreting X-ray binary systems by compact object type, distinguishing black hole from neutron star systems. We find hard state black holes, pulsars, and non-magnetized neutron stars associated with optical point source counterparts with similar frequency. We also find nine non-magnetized neutron stars coincident with globular clusters and an equal number of pulsars with and without point source optical counterparts. We perform spectral energy distribution (SED) fitting for the most likely optical counterparts to the HMXB candidates, finding 7 likely high mass stars and 1 possible red Helium burning star. The remaining 7 HMXB optical counterparts have poor SED fits, so their companion stars remain unclassified. Using published star formation histories, we find that the majority of HMXB candidates --- X-ray sources with UV-bright point source optical counterpart candidates --- are found in regions with star formation bursts less than 50 Myr ago, with 3 associated with young stellar ages (<10 Myr). This is consistent with similar studies of HMXB populations in the Magellanic Clouds, M33, NGC 300, and NGC 2403.
We report results on multi-epoch Very Large Array (VLA) and pc-scale Very Long Baseline Array (VLBA) observations of candidate compact symmetric objects (CSOs) from the faint sample of high frequency peakers. New VLBA observations could resolve the radio structure in about 42 per cent of the observed sources, showing double components that may be either mini-lobes or core-jet structures. Almost all the sources monitored by the VLA show some variability on time scale of a decade, and only 1 source does not show any significant variation. In 17 sources the flux density changes randomly as it is expected in blazars, and in 4 sources the spectrum becomes flat in the last observing epoch, confirming that samples selected in the GHz regime are highly contaminated by beamed objects. In 16 objects, the pc-scale and variability properties are consistent with a young radio source in adiabatic expansion, with a steady decrease of the flux density in the optically-thin part of the spectrum, and a flux density increase in the optically-thick part. For these sources we estimate dynamical ages between a few tens to a few hundreds years. The corresponding expansion velocity is generally between 0.1c and 0.7c, similar to values found in CSOs with different approaches. The fast evolution that we observe in some CSO candidates suggests that not all the objects would become classical Fanaroff-Riley radio sources.
In studies of accreting black holes in binary systems, empirical relations have been proposed to quantify the coupling between accretion processes and ejection mechanisms. These processes are probed respectively by means of X-ray and radio/optical-infrared observations. The relations predict, given certain accretion conditions, the expected energy output in the form of a jet. We investigated this coupling by studying the black hole candidate Swift J1753.5-0127, via multiwavelength coordinated observations over a period of ~4 years. We present the results of our campaign showing that, all along the outburst, the source features a jet that is fainter than expected from the empirical correlation between the radio and the X-ray luminosities in hard spectral state. Because the jet is so weak in this system the near-infrared emission is, unusually for this state and luminosity, dominated by thermal emission from the accretion disc. We briefly discuss the importance and the implications of a precise determination of both the slope and the normalisation of the correlations, listing some possible parameters that broadband jet models should take into account to explain the population of sources characterized by a dim jet. We also investigate whether our data can give any hint about the nature of the compact object in the system, since its mass has not been dynamically measured.
Current observations of double neutron stars provide us with a wealth of information that we can use to investigate their evolutionary history and the physical conditions of neutron star formation. Understanding this history and formation conditions further allow us to make theoretical predictions for the formation of other double compact objects with one or two black hole components and assess the detectability of such systems by ground-based gravitational-wave interferometers. In this paper we summarize our groups body of work in the past few years and we place our conclusions and current understanding in the framework of other work in this area of astrophysical research.
We present the first results of our VLT observation campaign of the Central Compact Objects (CCOs) in SNRs RX J085201.4-461753 (Vela Jr), 1E 1648-5051 (RCW 103) and RX J171328.4-394955 (G347.3-0.5). For Vela Jr., we found that the source is embedded in a compact optical nebulosity, possibly a bow-shock or a photo-ionization nebula, and we identified a candidate IR counterpart to the CCO. For RCW 103, we found no convincing evidence neither for 6 hrs IR modulation nor for variability on any time scale from the proposed counterpart, as well as for the other candidates close to the revised Chandra position. For G347.3-0.5, we identified few possible IR counterparts but none of them is apparently associated with the CCO.
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