ABSTRACT
Some ultraluminous X-ray sources (ULXs) are surrounded by collisionally ionized bubbles, larger and more energetic than supernova remnants: they are evidence of the powerful outflows ...associated with super-Eddington X-ray sources. We illustrate the most recent addition to this class: a huge (350 pc × 220 pc in diameter) bubble around a ULX in NGC 5585. We modelled the X-ray properties of the ULX (a broadened-disc source with LX ≈ 2–4 × 1039 erg s−1) from Chandra and XMM–Newton, and identified its likely optical counterpart in Hubble Space Telescope images. We used the Large Binocular Telescope to study the optical emission from the ionized bubble. We show that the line emission spectrum is indicative of collisional ionization. We refine the method for inferring the shock velocity from the width of the optical lines. We derive an average shock velocity ≈125 km s−1, which corresponds to a dynamical age of ∼600 000 yr for the bubble, and an average mechanical power Pw ∼ 1040 erg s−1; thus, the mechanical power is a few times higher than the current photon luminosity. With Very Large Array observations, we discovered and resolved a powerful radio bubble with the same size as the optical bubble, and a 1.4-GHz luminosity ∼1035 erg s−1, at the upper end of the luminosity range for this type of source. We explain why ULX bubbles tend to become more radio luminous as they expand while radio supernova remnants tend to fade.
Abstract
We present detailed timing and spectral analyses of the transient X-ray pulsar RX J0209.6−7427 in the Small Magellanic Cloud during its 2019 giant outburst. With a better known distance than ...most galactic X-ray pulsars, its peak luminosity is determined to be (1.11 ± 0.06) × 10
39
erg s
−1
; it is thus a bona fide pulsating ultraluminous X-ray source (PULX). Owing to the broad energy band of Insight-HXMT, its pulsed X-ray emission was detected from 1 keV up to the 130–180 keV band, which is the highest energy emission detected from any PULXs outside the Milky Way. This allows us to conclude that its main pulsed X-ray emission is from the
fan beam
of the accretion column, and its luminosity is thus intrinsic. We also estimate its magnetic field of (4.8–8.6) × 10
12
or (1.7–2.2) × 10
13
G, from its spin evolution or transition in the accretion column structure during the outburst; we suggest that the two values of the magnetic field strength correspond to the dipole and multipole magnetic fields of the neutron star, similar to the recent discovery in the Galactic PULX Swift J0243.6+6124. Therefore, the nature of the neutron star and its ULX emission can be understood within the current theoretical frame of accreting neutron stars. This may have implications for understanding the nature of those farther away extragalactic PULXs.
ABSTRACT
Ultraluminous X-ray sources (ULXs) are mainly powered by accretion in neutron stars or stellar-mass black holes. Accreting at rates exceeding the Eddington limit by factors of a few up to ...hundreds, radiation pressure is expected to inflate the accretion disc, and drive fast winds that have in fact been observed at significant fractions of the speed of light. Given the super-Eddington luminosity, the accretion disc will be thicker than in sub-Eddington accretors such as common active galactic nuclei and X-ray binaries, leading to a different spectral energy distribution and, possibly, a different thermal status of the wind. Here, we show the first attempt to calculate the photoionization balance of the winds driven by strong radiation pressure in thick discs with a focus on ULXs hosting black holes or non-magnetic neutron stars. We find that the winds are generally in thermally stable equilibrium, but long-term variations in the accretion rate and the inclination due to precession may have significant effects on the wind appearance and stability. Our model trends can explain the observed correlation between the spectral residuals around 1 keV and the ULX spectral state. We also find a possible correlation between the spectral hardness of the ULX, the wind velocity, and the ionization parameter in support of the general scenario.
ABSTRACT
We present results from six epochs of quasi-simultaneous radio, (sub-)millimetre, infrared, optical, and X-ray observations of the black hole X-ray binary MAXI J1535−571. These observations ...show that as the source transitioned through the hard–intermediate X-ray state towards the soft–intermediate X-ray state, the jet underwent dramatic and rapid changes. We observed the frequency of the jet spectral break, which corresponds to the most compact region in the jet where particle acceleration begins (higher frequencies indicate closer to the black hole), evolves from the infrared band into the radio band (decreasing by ≈3 orders of magnitude) in less than a day. During one observational epoch, we found evidence of the jet spectral break evolving in frequency through the radio band. Estimating the magnetic field and size of the particle acceleration region shows that the rapid fading of the high-energy jet emission was not consistent with radiative cooling; instead, the particle acceleration region seems to be moving away from the black hole on approximately dynamical time-scales. This result suggests that the compact jet quenching is not caused by local changes to the particle acceleration, rather we are observing the acceleration region of the jet travelling away from the black hole with the jet flow. Spectral analysis of the X-ray emission shows a gradual softening in the few days before the dramatic jet changes, followed by a more rapid softening ∼1–2 d after the onset of the jet quenching.
This paper examines the performance of orthogonal frequency division multiplexing (OFDM) systems for vehicle-to-vehicle (V2V) communication channels. More specifically, a doubly selective channel ...under high intercarrier interference (ICI) is considered. Current solutions involve complex detection and/or reduced spectral efficiency receivers. This paper proposes the use of virtual carriers (VC) in an OFDM system with a low-complexity maximal ratio combining (MRC) detector to improve the bit error rate (BER) performance. The results show that VC provides diversity in received data, resulting in a ≥5 dB gain compared to previous OFDM systems with conventional linear/nonlinear detectors used as a reference. The detector presented in this paper has linear complexity, making it a suitable solution for real-time V2V communication systems.
ABSTRACT
Most ultraluminous X-ray sources (ULXs) are thought to be powered by neutron stars and black holes accreting beyond the Eddington limit. If the compact object is a black hole or a neutron ...star with a magnetic field ≲1012 G, the accretion disc is expected to thicken and launch powerful winds driven by radiation pressure. Evidence of such winds has been found in ULXs through the high-resolution spectrometers onboardXMM–Newton, but several unknowns remain, such as the geometry and launching mechanism of these winds. In order to better understand ULX winds and their link to the accretion regime, we have undertaken a major campaign with XMM–Newton to study the ULX NGC 1313 X-1, which is known to exhibit strong emission and absorption features from a mildly relativistic wind. The new observations show clear changes in the wind with a significantly weakened fast component (0.2c) and the rise of a new wind phase which is cooler and slower (0.06–0.08c). We also detect for the first time variability in the emission lines which indicates an origin within the accretion disc or in the wind. We describe the variability of the wind in the framework of variable super-Eddington accretion rate and discuss a possible geometry for the accretion disc.
Applying organic amendments to recover physical, chemical, and biological qualities of soil may enable recovery of soils degraded by mining in semiarid climates. This study's aim was to investigate ...the development and changes in the composition of fungal communities in restored soils with five different types of organic amendments (two types of vegetable compost and sewage sludge compost, and a mixture of both) compared with unamended soils and surrounding natural soils and to examine the relationships between the fungal taxa, the new physico-chemical and biological soil properties of technosoils after 18 months of restoration, and natural soils. Restoration improved soil quality and fungal diversity, placing these soils in an intermediate position between unrestored soils (with no fungi present) and undisturbed reference soils, which were the most fungal diverse. Sewage-treated soils and their mixtures showed high nitrogen and carbohydrate content as well as high basal respiration and fatty acid content, suggesting that they provided readily biodegradable organic matter. In contrast, greenhouse compost-treated soils showed high total organic carbon and polyphenol content, whereas garden compost-treated soils showed intermediate values. The biological soil properties of both composts showed were similar to those of the reference soils, suggesting that composts contained more resilient organic matter. Organic amendments of dissimilar origin caused significantly different fungal soil communities at the genus level among the restored soils. Results indicated that soil pH, electrical conductivity, total nitrogen content, soil basal respiration, fungi/bacteria-PLFA ratio, and dehydrogenase and β-glucosidase activities, together with Pearson's correlations, revealed that these properties and nutrient content (total organic carbon, C/N ratio, carbohydrates, and polyphenols) influenced 40 soil fungal taxa. Therefore, the organic amendments led to changes in soil properties that favoured plant cover by promoting the soil fungal community growth beneficial to the carbon cycle and symbiotic with plants.
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•The biological properties of restored soil were similar to reference soils.•Fungal community was influenced by the chemical diversity of the organic matter.•Soil restoration favoured the regrowth of naturally present fungi.•Forty fungal taxa were significantly influenced by the types of soils analysed.
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•Start-up and cloud-pass are the main tests to assess the tuning of PV Irrigation Systems.•Tuning rules based on step-input response tuning method are not adequate for PVIS.•Frequency ...response tuning method provides essential information of PVIS performance.•Ziegler-Nichols, Cohen-Coon and C-H-R tuning rules did not pass the cloud-pass test.•AMIGO-frequency tuning rules passed all star-up and cloud-pass tests.
In this work, a systematic study of different tuning rules of proportional-integral (PI) control parameters applied to Photovoltaic Irrigation Systems (PVIS) based on Standard Frequency Converters (SFCs) has been carried out. A laboratory system has been implemented to obtain the PI control parameters by means of the most widely accepted tuning rules in control systems. Based on the step-input response modeling, we have been applied the tuning rules of Ziegler-Nichols (ZNs), Chien-Hrones-Reswick (CHR), Cohen-Coon (C-C), Lambda and AMIGOs. Based on frequency response modeling we have been applied the tuning rules of Ziegler-Nichols (ZNf) and AMIGOf. The PI controller parameters obtained from these tuning rules have been evaluated by performing two types of tests: start-up and sudden drop of the available PV power. The experimental results show that the parameter sets obtained by the ZNs, CHR and AMIGOf rules are the only ones that pass all the start-up tests and that the tests of sudden drop of the available PV power are only passed by the configuration obtained by the AMIGOf rules. Therefore, it has experimentally verified that the AMIGO tuning rules based on the frequency response modeling are the only ones that provide adequate values for the PI control parameters for PV irrigation systems.
Discovery of a radio transient in M81 Anderson, G E; Miller-Jones, J C A; Middleton, M J ...
Monthly notices of the Royal Astronomical Society,
10/2019, Letnik:
489, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
We report the discovery of a radio transient in the spiral galaxy M81. The transient was detected in early 2015 as part of a two-year survey of M81 made up of 12 epochs using the Karl G. ...Jansky Very Large Array. While undetected on 2014 September 12, the source was first detected on 2015 January 2, from which point it remained visible at an approximately constant luminosity of LR, ν = 1.5 ± 0.1 × 1024 erg s−1 Hz−1 at the observing frequency of 6 GHz for at least 2 months. Assuming this is a synchrotron event with a rise-time between 2.6 and 112 d, the peak luminosity (at equipartition) corresponds to a minimum energy of 1044 ≲ Emin ≲ 1046 erg and jet power of Pmin ∼ 1039 erg s−1, which are higher than most known X-ray binaries. Given its longevity, lack of short-term radio variability, and the absence of any multiwavelength counterpart (X-ray luminosity Lx ≲ 1036 erg s−1), it does not behave like known Galactic or extragalactic X-ray binaries. The M81 transient radio properties more closely resemble the unidentified radio transient 43.78+59.3 discovered in M82, which has been suggested to be a radio nebula associated with an accreting source similar to SS 433. One possibility is that both the new M81 transient and the M82 transient may be the birth of a short-lived radio bubble associated with a discrete accretion event similar to those observed from the ULX Holmberg II X-1. However, it is not possible to rule out other identifications including long-term supernova shockwave interactions with the surrounding medium from a faint supernova or a background active galaxy.
A 70 black hole (BH) was discovered in the Milky Way disk in a long-period detached binary system (LB-1) with a high-metallicity 8 B star companion. Current consensus on the formation of BHs from ...high-metallicity stars limits the BH mass to be below 20 due to strong mass loss in stellar winds. Using analytic evolutionary formulae, we show that the formation of a 70 BH in a high-metallicity environment is possible if wind mass-loss rates are reduced by factor of five. As observations indicate, a fraction of massive stars have surface magnetic fields that may quench the wind mass-loss, independently of stellar mass and metallicity. We confirm such a scenario with detailed stellar evolution models. A nonrotating 85 star model at Z = 0.014 with decreased winds ends up as a 71 star prior to core collapse with a 32 He core and a 28 CO core. Such a star avoids the pair-instability pulsation supernova mass loss that severely limits BH mass and may form a ∼70 BH in the direct collapse. Stars that can form 70 BHs at high Z expand to significant sizes, with radii of R 600 , however, exceeding the size of the LB-1 orbit. Therefore, we can explain the formation of BHs up to 70 at high metallicity and this result is valid whether or not LB-1 hosts a massive BH. However, if LB-1 hosts a massive BH we are unable to explain how such a binary star system could have formed without invoking some exotic scenarios.
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