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In this paper, we focus on understanding the physical processes that lead to stable or unstable ionization fronts (I-fronts) observed in simulations of moving black holes (BHs). The front instability may trigger bursts of gas accretion, rendering the BH significantly more luminous than at steady state. We perform a series of idealized three-dimensional radiation hydrodynamics simulations resolving the I-fronts around BHs of mass MBH and velocity $v$ ∞ accreting from a medium of density nH. The I-front, with radius RI, transitions from D-type to R-type as the BH velocity becomes larger than a critical value $v_\mathrm{R}\sim 40\, \mathrm{km\,s}^{-1}$. The D-type front is preceded by a bow-shock of thickness ΔRI that decreases as $v$ ∞ approaches $v$ R. We find that both D-type and R-type fronts can be unstable given the following two conditions: (i) for D-type fronts the shell thickness must be ΔRI/RI < 0.05 (i.e. $v_\infty \gtrsim 20\, \mathrm{km\,s}^{ -1}$), while no similar restriction holds for R-type fronts; (ii) the temperature jump across the I-front must be TII/TI > 3. This second condition is satisfied if $T_\mathrm{I}\lt 5000\, \mathrm{K}$ or if $n_\mathrm{H}\, M_\mathrm{BH} \gtrsim 10^6\, M_\odot \, \mathrm{cm^{-3}}$. Due to X-ray pre-heating typically $T_\mathrm{I} \sim 10^4\, \mathrm{K}$ , unless the D-type shell is optically thick to X-rays, which also happens when $n_\mathrm{H}\, M_\mathrm{BH}$ is greater than a metallicity-dependent critical value. We thus conclude that I-fronts around BHs are unstable only for relatively massive BHs moving trough very dense molecular clouds. We briefly discuss the observational consequences of the X-ray luminosity bursts likely associated with this instability. [ABSTRACT FROM AUTHOR]

In 2005 May/June, after 10 yr of inactivity, the Be/X-ray binary system A 0535+26 underwent a major X-ray outburst. In this paper, data are presented from 10 yr of optical, infrared and X-ray monitoring showing the behaviour of the system during the quiescent epoch and the lead up to the new outburst. The results show the system going through a period when the Be star in the system had a minimal circumstellar disc and then a dramatic disc recovery leading, presumably, to the latest flare up of X-ray emission. The data are interpreted in terms of the state of the disc and its interaction with the neutron star companion. [ABSTRACT FROM AUTHOR]

Ice formation and propagation in the xylem of plants is a complex process. During freezing of xylem sap, gases dissolved in liquid sap are forced out of the ice lattice due to their low solubility in ice, and supersaturation of xylem sap as well as low water potential (Ѱ) are induced at the ice–liquid interface. Supersaturation of gases near the ice front may lead to bubble formation and potentially to cavitation and/or to burst of gases driven out from the branch. In this study, we investigated the origin and dynamics of freezing-related gas bursts and ultrasonic acoustic emissions (AEs), which are suggested to indicate cavitation. Picea abies (L.) H. Karst. and Salix caprea L. branch segments were exposed to frost cycles in a temperature test chamber, and CO2 efflux (indicating gas bursts) and AEs were recorded. On freezing, two-thirds of the observed gas bursts originated from the xylem and only one-third from the bark. Simultaneously with gas bursts, AEs were detected. Branch Ѱ affected both gas bursts and AEs, with high gas burst in saturated and dry samples but relevant AEs only in the latter. Repeated frost cycles led to decreasing gas burst volumes and AE activity. Experiments revealed that the expanding ice front in freezing xylem was responsible for observed gas bursts and AEs, and that branch Ѱ influenced both processes. Results also indicated that gas bursts and cavitation are independently induced by ice formation, though both may be relevant for bubble dynamics during freezing. [ABSTRACT FROM AUTHOR]

MAXI/GSC (Monitor of All-sky X-ray Image/Gas Slit Camera) observed 21 outbursts from Circinus X-1 between 2009 August and 2013 December. Although 14 outbursts showed ordinary gradual decays, in seven others we found a sudden drop in luminosity on a time scale of a few hours around the periastron, and then the outbursts terminated. These sudden drops started at the estimated luminosity of a few times 1037 erg s−1 and their luminosities dropped to ≲3 × 1036 erg s−1. We offer three interpretations of the sharp luminosity decrease: (1) the end of the outburst during its dip, (2) the propeller effect, and (3) the stripping effect by the stellar wind of the companion star. It is difficult to explain the phenomenon by any of these interpretations alone. The interpretation of (1) is possible for only two outbursts on the assumption of rapid decay. The propeller effect (2) is expected to occur at a constant luminosity, which is incompatible with the observed facts. In the stripping effect (3), the ram pressure of a typical stellar wind is not sufficient to blow most of the accretion disk off. In this paper, we discuss the possibility of a modified stripping effect by assuming other additional conditions such as wind clumping and disk instability. [ABSTRACT FROM PUBLISHER]

ABSTRACT Fermi-LAT has recently detected two gamma-ray bubbles disposed symmetrically with respect to the Galactic plane. The bubbles have been suggested to be in a quasi-steady state, inflated by ongoing star formation over the age of the Galaxy. Here we propose an alternative picture where the bubbles are the remnants of a large-scale wide-angle outflow from Sgr A*, the supermassive black hole (SMBH) of our Galaxy. Such an outflow would be a natural consequence of a short but bright accretion event on to Sgr A* if it happened concurrently with the well-known star formation event in the inner 0.5 pc of the Milky Way ∼6 Myr ago. We find that the hypothesized near-spherical outflow is focused into a pair of symmetrical lobes by the greater gas pressure along the Galactic plane. The outflow shocks against the interstellar gas in the Galaxy bulge. Gamma-ray emission could be powered by cosmic rays created by either Sgr A* directly or accelerated in the shocks with the external medium. The Galaxy disc remains unaffected, agreeing with recent observational evidence that SMBHs do not correlate with galaxy disc properties. We estimate that an accreted mass ∼2 × 103 M⊙ is needed for the accretion event to power the observed Fermi-LAT lobes. Within a factor of a few, this agrees with the mass of the young stars born during the star formation event. This estimate suggests that roughly 50 per cent of the gas was turned into stars, while the rest accreted on to Sgr A*. One interpretation of this is a reduced star formation efficiency inside the Sgr A* accretion disc due to stellar feedback, and the other a peculiar mass deposition geometry that resulted in a significant amount of gas falling directly inside the inner ∼0.03 pc of the Galaxy. [ABSTRACT FROM AUTHOR]

We present an analysis of the short time-scale variations in the properties of the strong (type 'C') quasi-periodic oscillation (QPO) observed in XTE J1550−564 during its 1998 outburst. In particular, the QPO shows a correlation between absolute rms amplitude and mean source flux over time-scales shorter than ks. A linear rms-flux relation has been observed to be a common property of broad-band noise, but here we report the first detection of rms-flux dependence in a QPO. The gradient of the rms-flux relation is correlated with the QPO peak frequency: from a strong positive correlation when the QPO peak frequency is below Hz, through no correlation, to a strong negative correlation when the peak frequency is above Hz. This is the first time a negative short term rms-flux relation has been observed in any component of the power spectrum. Previous work on both the broad-band noise and QPOs in a range of sources have suggested the presence of a filter reducing the amplitude of QPOs with increasing frequency. We attempt to remove the possible effects of this filter and find that the previously negative rms-flux relations above Hz become constant. [ABSTRACT FROM AUTHOR]

The fast recurrent nova V745 Sco was observed in the 3–79 keV X-rays band with NuSTAR 10 d after the optical discovery. The measured X-ray emission is consistent with a collisionally ionized optically thin plasma at temperature of about 2.7 keV. A prominent iron line observed at 6.7 keV does not require enhanced iron in the ejecta. We attribute the X-ray flux to shocked circumstellar material. No X-ray emission was observed at energies above 20 keV, and the flux in the 3–20 keV range was about 1.6 × 10−11 erg cm−2 s−1. The emission measure indicates an average electron density of the order of 107 cm−2. The X-ray flux in the 0.3–10 keV band almost simultaneously measured with Swift was about 40 times larger, mainly due to the luminous central supersoft source emitting at energy below 1 keV. The fact that the NuSTAR spectrum cannot be fitted with a power law, and the lack of hard X-ray emission, allow us to rule out Comptonized gamma-rays, and to place an upper limit of the order of 10−11 erg cm−2 s−1 on the gamma-ray flux of the nova on the tenth day of the outburst. [ABSTRACT FROM PUBLISHER]

Recent progress on the nature of short-duration gamma-ray bursts has shown that a fraction of them originate in the local Universe. These systems may well be the result of giant flares from soft gamma-repeaters (highly magnetized neutron stars commonly known as magnetars). However, if these neutron stars are formed via the core collapse of massive stars then it would be expected that the bursts should originate from predominantly young stellar populations, while correlating the positions of BATSE short bursts with structure in the local Universe reveals a correlation with all galaxy types, including those with little or no ongoing star formation. This is a natural outcome if, in addition to magnetars formed via the core collapse of massive stars, they also form via accretion-induced collapse following the merger of two white dwarfs, one of which is magnetic. We investigate this possibility and find that the rate of magnetar production via white dwarf–white dwarf (WD–WD) mergers in the Milky Way is comparable to the rate of production via core collapse. However, while the rate of production of magnetars by core collapse is proportional to the star formation rate, the rate of production via WD–WD mergers (which have long lifetimes) is proportional to the stellar mass density, which is concentrated in early-type systems. Therefore magnetars produced via WD–WD mergers may produce soft gamma-repeater giant flares which can be identified with early-type galaxies. We also comment on the possibility that this mechanism could produce a fraction of the observed short-duration burst population at higher redshift. [ABSTRACT FROM AUTHOR]

The Galactic microquasar GRS 1915+105 has been in a continuous state of outburst since 1992, over 20 times longer than any other black hole X-ray transient. Assuming that the outburst is powered via accretion of an irradiated gaseous disc, we calculate how the predicted outburst duration varies according to the efficiency of the self-irradiation mechanism. At least one current model leads to the conclusion that the end of the outburst is imminent. The timing of the decline of GRS 1915+105, whenever it arrives, will be an excellent discriminator of the self-irradiation mechanism in X-ray transients, allowing us to infer the fraction of the disc that is heated by the incident X-rays and the magnitude of the mass loss rate in the form of a wind. [ABSTRACT FROM AUTHOR]

We cross-correlate gamma-ray bursts (GRBs) and X-ray selected clusters of galaxies at . We find a positive 2σ signal for the angular cross-correlation function on scales between short GRBs and clusters. Conversely, no correlation is found between clusters and the population of long GRBs. The comparison with the cluster autocorrelation function shows that short GRBs do not trace the cluster distribution, as not all short GRBs are found in clusters. A higher signal in is found if we only consider the cluster population up to . By comparing the short-burst autocorrelation function with model predictions, we then constrain short bursts to mostly originate within ∼270 Mpc (i.e. ). Our analysis also reveals that short GRBs are better correlated with ‘normal’ galaxies. The double compact-object merger model for short GRBs would associate them preferentially with early-type galaxies, but the present statistics do not allow us to exclude that at least a fraction of these events might also take place in late-type galaxies, in agreement with recent evidence. [ABSTRACT FROM AUTHOR]