Mechanical Luminosity Research Articles

Evaluation of Several Explanations of the Strong X-Ray Polarization of the Black Hole X-Ray Binary 4U 1630-47

The Imaging X-ray Polarimetry Explorer observations of the X-ray binary 4U 1630–47 in the high soft state revealed high linear polarization degrees (PDs) rising from 6% at 2 keV to 10% at 8 keV. We discuss in this Letter three different mechanisms that impact the polarization of the observed X-rays: the reflection of gravitationally lensed emission by the accretion disk, reprocessing of the emission in outflowing plasma, and electron and ion anisotropies in the accretion disk atmosphere. We conducted detailed ray-tracing studies to evaluate the impact of the reflection of strongly gravitationally lensed emission on the PDs. Although the reflected emission can produce high PDs in the high-energy tail of the thermal emission component, we do not find models that describe the PDs and are consistent with independent estimates of the source distance. We discuss the energetics of another proposed mechanism: the emission or scattering of the X-rays in mildly relativistically moving plasma outflows. We argue that these models are disfavored as they require large mechanical luminosities on the order of, or even exceeding, the Eddington luminosity. We investigated the impact of electron and ion anisotropies but find that their impact on the observed PDs are likely negligible. We conclude with a discussion of all three effects and avenues for future research.

CLusters in the Uv as EngineS (CLUES). II. Subkiloparsec-scale Outflows Driven by Stellar Feedback

We analyze the far-ultraviolet (1130−1770 Å rest frame) spectroscopy of 20 young (<50 Myr) and massive (>104 M ⊙) star clusters (YSCs) in 11 nearby star-forming galaxies. We probe the interstellar gas intervening along the line of sight, detecting several metal absorption lines of a wide range of ionization potentials, from 6.0 to 77.5 eV. Multiple-component Voigt fits to the absorption lines are used to study the kinematics of the gas. We find that nearly all targets in the sample feature gas outflowing from 30 up to 190 km s−1, often in both the neutral and ionized phases. The outflow velocities correlate with the underlying stellar population properties directly linked to the feedback: the mass of the YSCs, the photon production rate, and the instantaneous mechanical luminosity produced by stellar winds and supernovae. We detect a neutral inflow in four targets, which we interpret as likely not associated with the star cluster but tracing larger-scale gas kinematics. A comparison between the outflows’ energy and that produced by the associated young stellar populations suggests an average coupling efficiency of 10% with a broad scatter. Our results extend the relation found in previous works between galactic outflows and the host galaxy star formation rate to smaller scales, pointing toward the key role that clustered star formation and feedback play in regulating galaxy growth.

Delayed Massive-star Mechanical Feedback at Low Metallicity

The classical model of massive-star mechanical feedback is based on effects at solar metallicity (Z ⊙), yet feedback parameters are very different at low metallicity. Metal-poor stellar winds are much weaker, and more massive supernova progenitors likely collapse directly to black holes without exploding. Thus, for ∼0.4 Z ⊙ we find reductions in the total integrated mechanical energy and momentum of ∼40% and 75%, respectively, compared to values classically expected at solar metallicity. But in particular, these changes effectively delay the onset of mechanical feedback until ages of ∼10 Myr. Feedback from high-mass X-ray binaries could slightly increase mechanical luminosity between ages 5 and 10 Myr, but it is stochastic and unlikely to be significant on this timescale. Stellar dynamical mechanisms remove most massive stars from clusters well before 10 Myr, which would further promote this effect; this process is exacerbated by gas retention implied by weak feedback. Delayed mechanical feedback implies that radiation feedback therefore dominates at early ages, which is consistent with the observed absence of superwinds in some extreme starbursts. This scenario may lead to higher star formation efficiencies, multiple stellar populations in clusters, and higher Lyman continuum escape. This could explain the giant star-forming complexes in metal-poor galaxies and the small sizes of OB superbubble shells relative to their inferred ages. It could also drive modest effects on galactic chemical evolution, including on oxygen abundances. Thus, delayed low-metallicity mechanical feedback may have broad implications, including for early cosmic epochs.

Sensitivity of the Cherenkov Telescope Array to TeV photon emission from the Large Magellanic Cloud

ABSTRACT A deep survey of the Large Magellanic Cloud at ∼0.1–100 TeV photon energies with the Cherenkov Telescope Array is planned. We assess the detection prospects based on a model for the emission of the galaxy, comprising the four known TeV emitters, mock populations of sources, and interstellar emission on galactic scales. We also assess the detectability of 30 Doradus and SN 1987A, and the constraints that can be derived on the nature of dark matter. The survey will allow for fine spectral studies of N 157B, N 132D, LMC P3, and 30 Doradus C, and half a dozen other sources should be revealed, mainly pulsar-powered objects. The remnant from SN 1987A could be detected if it produces cosmic-ray nuclei with a flat power-law spectrum at high energies, or with a steeper index 2.3–2.4 pending a flux increase by a factor of &amp;gt;3–4 over ∼2015–2035. Large-scale interstellar emission remains mostly out of reach of the survey if its &amp;gt;10 GeV spectrum has a soft photon index ∼2.7, but degree-scale 0.1–10 TeV pion-decay emission could be detected if the cosmic-ray spectrum hardens above &amp;gt;100 GeV. The 30 Doradus star-forming region is detectable if acceleration efficiency is on the order of 1−10 per cent of the mechanical luminosity and diffusion is suppressed by two orders of magnitude within &amp;lt;100 pc. Finally, the survey could probe the canonical velocity-averaged cross-section for self-annihilation of weakly interacting massive particles for cuspy Navarro–Frenk–White profiles.

Heating of the intracluster medium by buoyant bubbles and sound waves

ABSTRACT Active galactic nuclei (AGN) powered by the central supermassive black holes (SMBHs) play a major role in modifying the thermal properties of the intracluster medium (ICM). In this work, we implement two AGN heating models: (i) by buoyant cavities rising through stratified ICM (effervescent model) and, (ii) by viscous and conductive dissipation of sound waves (acoustic model). Our aim is to determine whether these heating models are consistent with ICM observables and if one is preferred over the other. We assume an initial entropy profile of ICM that is expected from the purely gravitational infall of the gas in the potential of the dark matter halo. We then incorporate heating, radiative cooling, and thermal conduction to study the evolution of ICM over the age of the clusters. Our results are: (i) Both the heating processes can produce comparable thermal profiles of the ICM with some tuning of relevant parameters. (ii) Thermal conduction is crucially important, even at the level of 10 per cent of the Spitzer values, in transferring the injected energy beyond the central regions, and without which the temperature/entropy profiles are unrealistically high. (iii) The required injected AGN power scales with cluster mass as $M_{\rm vir}^{1.5}$ for both models. (iv) The required AGN luminosity is comparable with the observed radio jet power, reinforcing the idea that AGNs are the dominant heating source in clusters. (v) Finally, we estimate that the fraction of the total AGN luminosity available as the AGN mechanical luminosity at 0.02 r500 is less than 0.05 per cent.

Large-scale outflow structure and radiation properties of super-Eddington flow: Dependence on the accretion rates

Abstract In order to evaluate the impacts made by super-Eddington accretors on their environments precisely, it is essential to guarantee a large enough simulation box and long computational time to avoid any artefacts from numerical settings as much as possible. In this paper, we carry out axisymmetric two-dimensional radiation hydrodynamic simulations around a 10 M⊙ black hole in large simulation boxes and study the large-scale outflow structure and radiation properties of super-Eddington accretion flow for a variety of black hole accretion rates, ${\dot{M}}_{\,\,\rm BH} = (110\!-\!380)L_{\rm Edd}/c^{\,\,2}$ (with LEdd being the Eddington luminosity and c being the speed of light). The Keplerian radius of the inflow material, at which centrifugal force balances with gravitational force, is fixed to 2430 Schwarzschild radii. We find that the mechanical luminosity grows more rapidly than the radiation luminosity with an increase of ${\dot{M}}_{\,\,\rm BH}$. When seen from a nearly face-on direction, especially, the isotropic mechanical luminosity grows in proportion to ${\dot{M}}_{\,\,\rm BH}^{\,\,2.7}$, while the total mechanical luminosity is proportional to ${\dot{M}}_{\,\,\rm BH}^{\,\,1.7}$. The reason for the former is that the higher ${\dot{M}}_{\,\,\rm BH}$ is, the more vertically inflated the disk surface becomes, which makes radiation fields more confined in the region around the rotation axis, thereby strongly accelerating outflowing gas. The outflow is classified into pure outflow and failed outflow, depending on whether the outflowing gas can reach the outer boundary of the simulation box or not. The fraction of the failed outflow decreases with a decrease of ${\dot{M}}_{\,\,\rm BH}$. We analyze physical quantities along each outflow trajectory, finding that the Bernoulli parameter (Be) is not a good indicator to discriminate between pure and failed outflows, since it is never constant because of continuous acceleration by radiation-pressure force. Pure outflow can arise, even if Be &amp;lt; 0 at the launching point.

X-ray and optical emission from large superbubbles including the stellar photoionization

ABSTRACT N70 is a giant superbubble (SB) produced by the stellar cluster LH 114. The dynamic of the stellar SB can be explained with hydrodynamic simulations. However, the explanation of the observed X-ray luminosity is still a matter of debate. Observations show that this kind of superbubbles exhibits X-ray luminosities of the order of 1035 erg s−1, which cannot be explained by only considering the mechanical luminosity of the stellar components. Instead, theoretical models predict luminosities one order of magnitude lower. In this work, we attempt to answer this discrepancy by including a core-collapse supernova explosion (SN) and the photons emitted by each of the stars involved. We find that a core-collapse SN, with an initial mass of 10 M⊙, can explain the missing X-ray emission, lasting approximately 1000 yr. Also, the radiation produces an enhancement of the filamentary superbubble morphology and it increases the SB radius by $\sim 23{{\ \rm per\ cent}}$ with respect to the pure hydrodynamic case.

The time-dependent Rayleigh–Taylor instability in interstellar shells and supershells, including the eROSITA bubbles

ABSTRACT The Rayleigh–Taylor (RT) instability is omnipresent in the physics of inversely density-stratified fluids subject to effective gravitational acceleration. In astrophysics, a steep stratification of the ambient medium can fragment a bubble shell faster due to a strongly time-dependent RT instability, causing the classical constant gravity models to fail. We derive the time-dependent instability criteria analytically for the cases of constant, exponential, and power-law accelerations, verifying them through high-resolution numerical simulations. Our results show that (1) even in the linear phase there is a term opposing exponential growth, (2) non-linear growth approaches asymptotically the solution found by Fermi and von Neumann, (3) the interpenetrating spikes and bubbles promote a significant mixing, with the fractal dimension of the interface approaching 1.6, only limited by numerical diffusion, and (4) the probability density function for the passive scalar to study mixing becomes increasingly sharper peaked for power-law and exponential accelerations. Applying our solutions to stellar wind bubbles, young supernova remnants (SNRs), and superbubbles (SBs), we find that the growth rate of the RT instability is generally higher in the shells of wind-blown bubbles in a power-law stratified medium than in those with power-law rising stellar mechanical luminosities, Tycho-like than Cas A-like SNRs, and one-sided than symmetric SBs. The recently observed eROSITA bubbles indicate smooth rim surfaces, implying that the outer shell has not been affected by RT instabilities. Therefore, the dynamical evolution of the bubbles suggests maximum final ages that are significantly above their current age, which we estimate to be about 20 Myr.

The Unusual Weak-line Quasar PG1407+265 and Its Foreground z ∼ 0.7 X-Ray Cluster

Abstract We present new observations of the odd z = 0.96 weak-line quasar PG1407+265, and report the discovery of CXOU J140927.9+261813, a z = 0.68 X-ray cluster. Archival X-ray photometry spanning nearly four decades reveals that PG1407+265 is variable at the 1 dex level on a timescale of years. V-band variability is observed with an amplitude less than 0.1 mag. The emission-line properties of PG1407+265 also reveal clear evidence for a powerful inflow or outflow due to near- or super-Eddington accretion, having a mechanical luminosity of order 1048 erg s−1. Our follow-up Chandra exposure centered on this object reveals a foreground z = 0.68 cluster roughly 1′ × 1.′5 in extent, offset to the east of PG1407+265, roughly coincident with the z = 0.68 radio galaxy FIRST J140927.8+261818. This non-cool-core cluster contributes about 10% of the X-ray flux of PG1407+265, has a mass of (0.6–5.5) × 1014 M ⊙, and an X-ray gas temperature of 2.2−4.3 keV. Because the projected position of the quasar lies at about twice that of the cluster’s inferred Einstein radius, lensing by the cluster is unlikely to explain the quasar’s unusual properties. We also discuss the evidence for a second cluster centered on and at the redshift of the quasar.

The CALYPSO IRAM-PdBI survey of jets from Class 0 protostars

Aims. As a part of the CALYPSO Large Program, we aim to constrain the properties of protostellar jets and outflows by searching for corresponding emission in a sample of 21 Class 0 protostars covering a broad range of internal luminosities (Lint from 0.035 L⊙ to 47 L⊙). Methods. We analyzed high angular (~0.′′5–1.′′0) resolution IRAM Plateau de Bure Interferometer (PdBI) observations in three typical outflow and jet tracers, namely: CO (2−1), SO (56−45), and SiO (5−4). We obtained the detection rate, spatial distribution, kinematics, and collimation of the outflow and jet emission in the three lines. Molecular column densities and abundances, the jet mass-loss and momentum rates, and mechanical luminosity are estimated from the integrated line intensities. Results. Blue- and red-shifted emission in CO (2−1), which probes outflowing gas, was detected in all the sources in the sample and detected for the first time in the cases of SerpS-MM22 and SerpS-MM18b. Collimated high-velocity jets in SiO (5−4) were detected in 67% of the sources (for the first time in IRAS4B2, IRAS4B1, L1448-NB, SerpS-MM18a) and 77% of these also showed jet or outflow emission in SO (56−45). The detection rate of jets in SiO and SO increases with internal luminosity. In five sources (that is, 24% of the sample) SO (56−45) is elongated and reveals a velocity gradient perpendicular to the jet direction, hence, it probes the inner envelope or the disk, or both. The detected SiO jets are collimated (typical opening angle, α ~ 10°) and surrounded by wider angle SO (α ~ 15°) and CO (α ~ 25°) emission. The abundance of SO relative to H2 ranges from &lt;10−7 up to 10−6; whereas for SiO, the lower limits were found to range from &gt;2.4 × 10−7 to &gt;5 × 10−6, with the exception of the jets from IRAS4A1 and IRAS4A2, which show low SiO abundance (≤2−6 × 10−8). The mass-loss rates range from ~7 × 10−8 M⊙ yr−1 up to ~3 × 10−6 M⊙ yr−1 for Lint ~ 1−50 L⊙. Conclusions. The CALYPSO IRAM-PdBI survey shows that the outflow phenomenon is ubiquitous in our sample of protostars and that the detection rate of high-velocity collimated jets increases for increasing protostellar accretion, with at least 80% of the sources with Lint &gt; 1 L⊙ driving a jet. The detected protostellar flows exhibit an onion-like structure, where the SiO jet is nested into a wider angle SO and CO outflow. On scales &gt; 300 au the SiO jets are less collimated (4°−12°) than atomic jets from Class II sources (~3°), possibly dueto projection effects and contamination by SiO emission from the bow-shocks. On the other hand, velocity asymmetry between the two jet lobes are detected in one third of the sources, similarly to Class II atomic jets, suggesting that the same launching mechanism is at work. Most of the jets are SiO rich, which indicates very efficient release of &gt; 1− 10% of elemental silicon in gas phase likely in dust-free winds, launched from inside the dust sublimation radius. The estimated mass-loss rates are larger by up to five orders of magnitude than what was measured for Class II jets, indicating that the ejection decreases as the source evolves and accretion fades. Similarly to Class II sources, the mass-loss rates are ~ 1− 50% of the mass accretion rate, Ṁacc, suggesting that the correlation between mass ejection and mass accretion holds along the star-formation process from 104 yr up to a few Myr.

The origins and impact of outflow from super-Eddington flow

Abstract It is widely believed that super-Eddington accretion flow can produce powerful outflow, but where does this originate and how much mass and energy are carried away in which directions? To answer these questions, we perform a new large-box, two-dimensional radiation hydrodynamic simulation, paying special attention lest the results should depend on the adopted initial and boundary conditions. We achieve a quasi-steady state at an unprecedentedly large range, r = 2–600rS (with rS being the Schwarzschild radius), from the black hole. The accretion rate onto the central 10 M⊙ black hole is $\dot{M}_{\rm BH} \sim 180 L_{\rm Edd}/c^{2}$, whereas the mass outflow rate is ${\dot{M}}_{\rm outflow} \sim 24 L_{\rm Edd}/c^2$ (where LEdd and c are the Eddington luminosity and the speed of light, respectively). The ratio ${\dot{M}}_{\rm outflow}/{\dot{M}}_{\rm BH} \sim 0.14$ is much less than previously reported. By careful inspection we find that most of the outflowing gas reaching the outer boundary originates from the region at R ≲ 140rS, while gas at 140–230rS forms failed outflow. Therefore, significant outflow occurs inside the trapping radius ∼450rS. The mechanical energy flux (or mass flux) reaches its maximum in the direction of ∼15° (∼80°) from the rotation axis. The total mechanical luminosity is Lmec ∼ 0.16LEdd, while the isotropic X-ray luminosity varies from $L_{\rm X}^{\rm ISO}\sim 2.9 L_{\rm Edd}$ (for a face-on observer) to ∼2.1LEdd (for a nearly edge-on observer). The power ratio is $L_{\rm mec}/L_{\rm X}^{\rm ISO}\sim 0.05$–0.08, in good agreement with observations of ultra-luminous X-ray sources surrounded by optical nebulae.

High-sensitivity Millimeter Imaging of Molecular Outflows in Nine Nearby High-mass Star-forming Regions

Abstract We present a study of molecular outflows using six molecular lines (including 12CO/13CO/C18O/HCO+ (J = 1−0) and CS/SiO (J = 2−1)) toward nine nearby high-mass star-forming regions with accurate known distances. This work is based on the high-sensitivity observations obtained with the 14 m millimeter telescope of the Purple Mountain Observatory in Delingha. The detection rate of outflows (including 12CO, 13CO, HCO+, and CS) is 100%. However, the emission of SiO was not detected for all sources. The full line widths (ΔV) at 3σ above the baseline of these molecular lines have the relationship &gt; . 12CO and HCO+ can be used to trace relatively high-velocity outflows, while 13CO and CS can be employed to trace relatively low-velocity outflows. The dynamical timescales of the 13CO and CS outflows are longer than those of the 12CO and HCO+ outflows. The mechanical luminosities, masses, mass-loss rates and forces of all outflows (including 12CO, 13CO, HCO+, and CS) are correlated with the bolometric luminosities of their central IRAS sources.

A hadronic emission model for black hole-disc impacts in the blazar OJ 287

ABSTRACTA supermassive black hole (SMBH) binary in the core of the blazar OJ 287 has been invoked in previous works to explain its observed optical flare quasi-periodicity. Following this picture, we investigate a hadronic origin for the X-ray and γ-ray counterparts of the November 2015 major optical flare of this source. An impact outflow must result after the lighter SMBH (the secondary) crosses the accretion disc of the heavier one (the primary). We then consider acceleration of cosmic ray (CR) protons in the shock driven by the impact outflow as it expands and collides with the active galactic nucleus (AGN) wind of the primary SMBH. We show that the emission of these CRs can reproduce the X-ray and γ-ray flare data self-consistently with the optical component of the 2015 November major flare. The derived emission models are consistent with a magnetic field B ∼ 5 G in the emission region and a power-law index of q ∼ 2.2 for the energy distribution of the emitting CRs. The mechanical luminosity of the AGN wind represents $\lesssim 50{{\ \rm per\ cent}}$ of the mass accretion power of the primary SMBH in all the derived emission profiles.

Simulating stellar winds in AMUSE

Aims. We present STELLAR_WIND.PY, a module that provides multiple methods of simulating stellar winds using smoothed particle hydrodynamics codes (SPH) within the astrophysical multipurpose software environment (AMUSE) framework. Methods. The module currently includes three ways of simulating stellar winds: With the simple wind mode, we create SPH wind particles in a spherically symmetric shell for which the inner boundary is located at the radius of the star. We inject the wind particles with a velocity equal to their terminal velocity. The accelerating wind mode is similar, but with this method particles can be injected with a lower initial velocity than the terminal velocity and they are accelerated away from the star according to an acceleration function. With the heating wind mode, SPH particles are created with zero initial velocity with respect to the star, but instead wind particles are given an internal energy based on the integrated mechanical luminosity of the star. This mode is designed to be used on longer timescales and larger spatial scales compared to the other two modes and assumes that the star is embedded in a gas cloud. Results. We present a number of tests and compare the results and performance of the different methods. For fast winds, we find that both the simple and accelerating mode can reproduce the desired velocity, density and temperature profiles. For slow winds, the simple wind mode is insufficient due to dominant hydrodynamical effects that change the wind velocities. The accelerating mode, with additional options to account for these hydrodynamical effects, can still reproduce the desired wind profiles. We test the heating mode by simulating both a normal wind and a supernova explosion of a single star in a uniform density medium. The stellar wind simulation results matches the analytical solution for an expanding wind bubble. The supernova simulation gives qualitatively correct results, but the simulated bubble expands faster than the analytical solution predicts. We conclude with an example of a triple star system which includes the colliding winds of all three stars.

Possible connection between the asymmetry of the North Polar Spur and Loop I and Fermi bubbles

The origin of North Polar Spur (NPS) and Loop-I has been debated over almost half a century and is still unresolved. Most of the confusion is caused by the absence of any prominent counterparts of these structures in the southern Galactic hemisphere (SGH). This has also led to doubts over the claimed connection between the NPS and Fermi Bubbles (FBs). I show in this paper, that such asymmetries of NPS and Loop-I in both X-rays and $\gamma$-rays can be easily produced if the circumgalactic medium (CGM) density in the southern hemisphere is only smaller by $\approx 20\%$ than the northern counterpart in case of a star formation driven wind scenario. The required mechanical luminosity, $\mathcal{L} \approx 4-5\times 10^{40} $ erg s$^{-1}$ (reduces to $\approx 0.3$ M$_\odot$ yr$^{-1}$ including the non-thermal pressure) and the age of the FBs, $t_{\rm age} \approx 28$ Myr, are consistent with previous estimations in case of a star formation driven wind scenario. One of the main reasons for the asymmetry is the projection effects at the Solar location. Such a proposition is also consistent with the fact that the southern FB is $\approx 5^\circ$ bigger than the northern one. The results, therefore, indicate towards a possibility for a common origin of the NPS, Loop-I and FBs from the Galactic centre (GC). I also estimate the average sky brightness in X-ray towards the south Galactic pole and North Galactic pole in the ROSAT-R67 band and find that the error in average brightness is far too large to have any estimation of the deficiency in the southern hemisphere.

Parameters and models of the jets from Sanduleak’s star in the LMC

Context. Angeloni and collaborators have discovered a bipolar jet extending out to ~6 pc from “Sanduleak’s star” in the Large Magellanic Cloud (LMC). This is the first angularly resolved stellar jet system that has been observed outside our Galaxy. Aims. In this paper we use archival Hubble Space Telescope (HST) images of this jet in order to estimate the flow parameters, and use them to explore two possible theoretical scenarios for modelling this bipolar outflow. Methods. We have computed axisymmetric gasdynamic simulations of a conical outflow with an opening angle that increases with time and of a cylindrical jet interacting with a stratified circumstellar structure. The results of these models are then compared with the observed morphology of the jets from Sanduleak’s star. Results. From the observations, we obtain Ṁj ≈ 1.2 × 10−5M⊙ yr−1 and Lm ≈ 1000 L⊙ for the mass loss rate and the mechanical luminosity (respectively) of each of the outflow lobes. We also obtain a (deprojected) flow velocity vj ≈ 1000 km s−1 and a dynamical time tdyn ≈ 7300 yr. From the simulations that we have computed (with these flow parameters), we find that both the “opening out conical wind” and the “jet+stratified environment” scenarios have characteristics that resemble the observed morphologies. A more complete model for the jets from Sanduleak’s star might incorporate some of the features of these two scenarios.

Constraining cosmic ray acceleration in young star clusters using multi-wavelength observations

We use $1$D and $3$D two-fluid cosmic ray (CR) hydrodynamic simulations to investigate the role of CRs in the vicinity of a compact young star cluster. We model a self-gravitating cloud (density profile $\rho \propto r^{-1}$), include important thermal and non-thermal processes, and explore two different CR injection scenarios. We show that if internal shocks in the wind-driving region are the main site for CR acceleration, then the resulting $\gamma$-ray luminosity ($L_{\rm \gamma}$) can reach $\approx 5\%$ of the mechanical luminosity ($L_{\rm w}$), independent of the fraction of wind energy ($\sim 1-20\%$) injected into CRs. In contrast, if the forward/reverse shock of a bubble is the injection site then $L_{\rm \gamma}$ increases linearly with the CR injection fraction, as expected analytically. We find that the X-ray luminosity ($L_{\rm x}$) in the forward/reverse shock injection scenario is $\gtrsim 10^{-3} L_{\rm w}$, which is $\sim 10$ times larger than in the central wind-driving injection case. We predict the corresponding range of the synchrotron radio luminosity. We show how multi-wavelength observations can constrain the CR parameters. Comparing the predicted multi-wavelength luminosities with those of 30 Doradus we identify the reverse shock as the most probable CR injection site, and that thermal conduction is important. We do not find significant dynamical impact of CRs in our models.

Stellar population of the superbubble N 206 in the LMC

Context. Clusters or associations of early-type stars are often associated with a “superbubble” of hot gas. The formation of such superbubbles is caused by the feedback from massive stars. The complex N 206 in the Large Magellanic Cloud (LMC) exhibits a superbubble and a rich massive star population. Aims. Our goal is to perform quantitative spectral analyses of all massive stars associated with the N 206 superbubble in order to determine their stellar and wind parameters. We compare the superbubble energy budget to the stellar energy input and discuss the star formation history of the region. Methods. We observed the massive stars in the N 206 complex using the multi-object spectrograph FLAMES at ESO’s Very Large Telescope (VLT). Available ultra-violet (UV) spectra from archives are also used. The spectral analysis is performed with Potsdam Wolf–Rayet (PoWR) model atmospheres by reproducing the observations with the synthetic spectra. Results. We present the stellar and wind parameters of the OB stars and the two Wolf–Rayet (WR) binaries in the N 206 complex. Twelve percent of the sample show Oe/Be type emission lines, although most of them appear to rotate far below critical. We found eight runaway stars based on their radial velocity. The wind-momentum luminosity relation of our OB sample is consistent with the expectations. The Hertzsprung–Russell diagram (HRD) of the OB stars reveals a large age spread (1–30 Myr), suggesting different episodes of star formation in the complex. The youngest stars are concentrated in the inner part of the complex, while the older OB stars are scattered over outer regions. We derived the present day mass function for the entire N 206 complex as well as for the cluster NGC 2018. The total ionizing photon flux produced by all massive stars in the N 206 complex is Q0 ≈ 5 × 1050 s−1, and the mechanical luminosity of their stellar winds amounts to Lmec = 1.7 × 1038 erg s−1. Three very massive Of stars are found to dominate the feedback among 164 OB stars in the sample. The two WR winds alone release about as much mechanical luminosity as the whole OB star sample. The cumulative mechanical feedback from all massive stellar winds is comparable to the combined mechanical energy of the supernova explosions that likely occurred in the complex. Accounting also for the WR wind and supernovae, the mechanical input over the last five Myr is ≈ 2.3 × 1052 erg. Conclusions. The N206 complex in the LMC has undergone star formation episodes since more than 30 Myr ago. From the spectral analyses of its massive star population, we derive a current star formation rate of 2.2 × 10−3 M⊙ yr−1. From the combined input of mechanical energy from all stellar winds, only a minor fraction is emitted in the form of X-rays. The corresponding input accumulated over a long time also exceeds the current energy content of the complex by more than a factor of five. The morphology of the complex suggests a leakage of hot gas from the superbubble.

The environment of the wind–wind collision region of η Carinae

Context. η Carinae is a colliding wind binary hosting two of the most massive stars and featuring the strongest wind collision mechanical luminosity. The wind collision region of this system is detected in X-rays and γ-rays and offers a unique laboratory for the study of particle acceleration and wind magneto-hydrodynamics. Aim. Our main goal is to use X-ray observations of η Carinae around periastron to constrain the wind collision zone geometry and understand the reasons for its variability. Methods. We analysed 10 Nuclear Spectroscopic Telescope Array (NuSTAR) observations, which were obtained around the 2014 periastron. The NuSTAR array monitored the source from 3 to 30 keV, which allowed us to grasp the continuum and absorption parameters with very good accuracy. We were able to identify several physical components and probe their variability. Results. The X-ray flux varied in a similar way as observed during previous periastrons and largely as expected if generated in the wind collision region. The flux detected within ~10 days of periastron is lower than expected, suggesting a partial disruption of the central region of the wind collision zone. The Fe Kα line is likely broadened by the electrons heated along the complex shock fronts. The variability of its equivalent width indicates that the fluorescence region has a complex geometry and that the source obscuration varies quickly with the line of sight.

The star formation history of the Sextans dwarf spheroidal galaxy: a true fossil of the pre-reionization era

We present the star formation history (SFH) of the Sextans dwarf spheroidal galaxy based on deep archive $B$,$I$ photometry taken with Suprime-Cam at Subaru telescope focusing our analysis on the inner region of the galaxy, fully located within the core radius. Within the errors of our SFH we have not detected any metallicity gradient along the considered radial distance interval. As a main result of this work we can state that the Sextans dwarf spheroidal stopped forming stars less than $\sim1.3$ Gyr after Big Bang in correspondance to the end of the reionization epoch. We have been able to constrain the duration of the main burst of star formation to $\sim0.6$ Gyr. From the calculation of the mechanical luminosity released from supernovae (SNe) during the brief episode of star formation, there are strong indications that SNe could have played an important role in the fate of Sextans, by removing almost completely the gas component, so preventing a prolonged star formation.