Observed X-ray Emission Research Articles

Symbiotic stars in X-rays

White dwarf symbiotic binaries are detected in X-rays with luminosities in the range of 1030–1034 ergs s−1. Their X-ray emission arises either from the accretion disk boundary layer, from a region where the winds from both components collide, or from nuclear burning on the surface of the white dwarf (WD). In our continuous effort to identify X-ray-emitting symbiotic stars, we studied four systems using observations from the Neil Gehrels Swift Observatory and XMM-Newton satellites in X-rays and from Transiting Exoplanet Survey Satellite (TESS) in the optical. The X-ray spectra were fit with absorbed optically thin thermal plasma models that are either single- or multitemperature with kT < 8 keV for all targets. Based on the characteristics of their X-ray spectra, we classified BD Cam as possible β-type, V1261 Ori and CD −27 8661 as δ-type, and confirmed NQ Gem as β/δ-type. The δ-type X-ray emission most likely arises from the boundary layer of the accretion disk, while in the case of BD Cam, its mostly soft emission originates from shocks, possibly between the red giant and WD and disk winds. In general, we find that the observed X-ray emission is powered by accretion at a low accretion rate of about 10−11 M⊙ yr−1. The low ratio of X-ray to optical luminosities, however indicates that the accretion-disk boundary layer is mostly optically thick and tends to emit in the far or extreme UV. The detection of flickering in optical data provides evidence of the existence of an accretion disk.

Study of X-ray emission from the S147 nebula by SRG/eROSITA: Supernova-in-the-cavity scenario

The Simeis 147 nebula (S147) is particularly well known for a spectacular net of Hα-emitting filaments. It is often considered one of the largest and oldest (∼105 yr) cataloged supernova remnants in the Milky Way, although the kinematics of the pulsar PSR J0538+2817 suggests that this supernova remnant might be a factor of three younger. The former case is considered in a companion paper, while here we pursue the latter. Both studies are based on the data of SRG/eROSITA All-Sky Survey observations. Here, we confront the inferred properties of the X-ray emitting gas data with the scenario of a supernova explosion in a low-density cavity, such as a wind-blown-bubble. This scenario assumes that a ∼20 M⊙ progenitor star has had a low velocity with respect to the ambient interstellar medium, and so stayed close to the center of a dense shell created during its main-sequence evolution till the moment of the core-collapse explosion. The ejecta first propagate through the low-density cavity until they collide with the dense shell, and only then does the reverse shock go deeper into the ejecta and power the observed X-ray emission of the nebula. The part of the remnant inside the dense shell remains non-radiative till this point, plausibly in a state with Te < Ti and nonequilibrium ionization. On the contrary, the forward shock becomes radiative immediately after entering the dense shell, and, being subject to instabilities, gives the nebula its characteristic “foamy” appearance in Hα and radio emission.

A spectropolarimetric study of the radio galaxy Pictor A using MeerKAT

ABSTRACT We present the results of a polarimetric study from our new high-sensitivity L-band (0.8–1.7 GHz) observation of Pictor A using the MeerKAT radio telescope. We confirm the presence of the radio jet extending from the nucleus to the western hotspot of this source. Additionally, we show the radio emission expected to coincide with previously observed X-ray emission in the radio lobes, confirming that the emission mechanism is of inverse Compton origin, as suggested by a previous study. Our spectropolarimetric analysis using the rotation measure synthesis technique reveals a relatively uniform mean rotation measure distribution across the lobes of Pictor A, with most lines of sight exhibiting single-peaked Faraday spectra. However, a number of the lines of sight exhibit single peaked spectra with a wide base or multiple peaks, suggesting the presence of multiple Faraday components or a Faraday thick structure along Pictor A’s lines of sight. We also confirm the asymmetry in rotation measure variability and depolarization between the two lobes of this source, as reported in a previous study.

Simulated non-thermal emission of the supernova remnant G1.9 + 0.3

ABSTRACT Supernova remnants are the nebular leftover of defunct stellar environments, resulting from the interaction between a supernova blastwave and the circumstellar medium shaped by the progenitor throughout its life. They display a large variety of non-spherical morphologies such as ears that shine non-thermally. We have modelled the structure and the non-thermal emission of the supernova remnant G1.9 + 0.3 through 3D magnetohydrodynamic numerical simulations. We propose that the peculiar ear-shaped morphology of this supernova remnant results from the interaction of its blast wave with a magnetized circumstellar medium, which was previously asymmetrically shaped by the past stellar wind emanating from the progenitor star or its stellar companion. We created synthetic non-thermal radio and X-ray maps from our simulated remnant structure, which are in qualitative agreement with observations, forming ears on the polar directions. Our synthetic map study explains the discrepancies between the measured non-thermal radio and X-ray surface brightness distributions assuming that the inverse Compton process produces the observed X-ray emission.

Complete X-ray census of M dwarfs in the solar neighborhood

Context. X-ray emission is the most sensitive diagnostic of magnetic activity in M dwarfs and, hence, of the dynamo in low-mass stars. Moreover it is crucial for quantifying the influence of the stellar irradiation on the evolution of planet atmospheres. Aims. We have embarked in a systematic study of the X-ray emission in a volume-limited sample of M dwarf stars to explore the full range of activity levels present in their coronae. We look to obtain a better understanding of the conditions in their outer atmospheres and their possible impact on the circumstellar environment. Methods. Based on a recent catalog of Gaia objects within 10 pc from the Sun, we identified all its stars with spectral types between M0 and M4 and we carried out a systematic search for X-ray measurements of this sample. To this end, we used both archival data from ROSAT, XMM-Newton, and the ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Russian Spektrum-Roentgen-Gamma mission, as well as our own dedicated XMM-Newton observations. To make inferences on the properties of the M dwarf corona, we compared the range of their observed X-ray emission levels to the flux radiated by the Sun from different types of magnetic structures: coronal holes, background corona, active regions, and cores of active regions. In this work, we focus on the properties of stars with the faintest X-ray emission. Results. At the current state of our project, with more than 90% of the 10 pc M dwarf sample observed in the X-ray, there is only one star that has had no detections: GJ 745 A. With an upper limit luminosity of log Lx (erg s−1) < 25.4 and an X-ray surface flux of log FX,SURF (erg cm−2 s−1) < 3.6, GJ 745 A defines the lower boundary of the X-ray emission level for M dwarfs. Together with its proper motion companion (GJ 745 B), it is the only star in this volume-complete sample located in the range of X-ray surface flux that corresponds to the faintest solar coronal structures, namely: coronal holes. The fact that the ultra-low X-ray emission level of GJ 745 B (log Lx (erg s−1) = 25.6 and log FX,SURF (erg cm−2 s−1) = 3.8) is entirely attributed to flaring activity indicates that while its corona is dominated by “holes,” at least one magnetically active structure is present. This structure determines the total X-ray brightness and the coronal temperature of the star.

The Core Starbursts of the Galaxy NGC 3628: Radio Very Long Baseline Interferometry and X-Ray Studies

We present radio very long baseline interferometry (VLBI) and X-ray studies of the starburst galaxy NGC 3628. The VLBI observation at 1.5 GHz reveals seven compact (0.7−7 pc) radio sources in the central ∼250 pc region of NGC 3628. Based on their morphology, high radio-brightness temperatures (105–107 K), and steep radio spectra, none of these seven sources can be associated with active galactic nuclei (AGNs); instead, they can be identified as supernova remnants (SNRs), with three of them appearing consistent with partial shells. Notably, one of them (W2) is likely a nascent radio supernova (SN) and appears to be consistent with the star formation rate of NGC 3628 when assuming a canonical initial mass function. The VLBI observation provides the first precise measurement of the diameter of the radio sources in NGC 3628, which allow us to fit a well-constrained radio surface brightness—diameter (Σ–D) correlation by including the detected SNRs. Furthermore, future VLBI observations can be conducted to measure the expansion velocity of the detected SNRs. In addition to our radio VLBI study, we analyze Chandra and XMM-Newton spectra of NGC 3628. The spectral fitting indicates that the SNR activities could well account for the observed X-ray emissions. Along with the Chandra X-ray image, it further reveals that the X-ray emission is likely maintained by the galactic-scale outflow triggered by SN activities. These results provide strong evidence that SN-triggered activities play a critical role in generating both radio and X-ray emissions in NGC 3628.

A different view of wind in X-ray binaries: the accretion disc corona source 2S 0921-630

ABSTRACT Accretion disc coronae (ADC) sources are very high inclination neutron star or black hole binaries, where the outer accretion flow blocks a direct view of the central source. The weak observed X-ray emission is instead produced mainly by scattering of the intrinsic radiation from highly ionized gas surrounding the source, the ADC. However, the origin of this scattering material is still under debate. We use the ADC source 2S 0921-630 (V395 Car) to test whether it is consistent with a thermal-radiative wind produced by the central X-ray source illuminating and puffing up the outer disc. This wind is clearly visible in blueshifted absorption lines in less highly inclined systems, where the source is seen directly through this material. Using the phenomenological photoionized plasma model, we first characterize the parameter that drives emission lines observed in 2S0921 in XMM–Newton and Chandra data. Following this, we run the Monte Carlo radiation transfer simulation to get scattered/reprocessed emissions in the wind, with the density and velocity structure obtained from the previous work. Our model agrees with all the wind emission lines in the Chandra high and medium energy grating spectra for an intrinsic source luminosity of L > 0.2 LEdd. This result strongly favours thermal-radiative winds as the origin of the ADC. We also show how high-resolution spectra via microcalorimeters can provide a definitive test by detecting blueshifted absorption lines.

Multiwavelength variability of γ-ray emitting narrow-line Seyfert 1 galaxies

ABSTRACT As one of the drivers of feedback in active galactic nuclei (AGNs), the jets launched from supermassive black holes (SMBHs) are important for understanding the co-evolution of SMBHs and their host galaxies. However, the formation of AGN jets is far from clear. The discovery of γ-ray narrow-line Seyfert 1 (NLS1) galaxies during the past two decades has provided us with a new means of studying the link between jets and accretion processes and the formation of jets. Here, we explore the coupling of jet and accretion discs in seven bright γ-ray NLS1 galaxies by studying simultaneous optical/ultraviolet and X-ray observations of these systems taken by Swift. The results show that, except for 1H 0323+342 in which the X-rays are significantly contributed from the accretion disc, the observed X-ray emission of the other sources is dominated by the jet, and accretion process makes little contribution if not absent. Although the origin of the X-ray emission is different, the broad-band spectral shape characterized by αox and the X-ray flux is found to follow the same evolutionary trend in 1H 0323+342, PMN J0948+0022, and PKS 1502+036. For the remaining sources, the trend is not observed or the sampling is not dense enough.

Thermal emission from bow shocks

The nearby, massive, runaway star ζ Ophiuchi has a large bow shock detected in optical and infrared light and, uniquely among runaway O stars, diffuse X-ray emission detected from the shocked stellar wind. Here we make the first detailed computational investigation of the bow shock of ζ Ophiuchi, to test whether a simple model of the bow shock can explain the observed nebula, and to compare the detected X-ray emission with simulated emission maps. We reanalysed archival Chandra observations of the thermal diffuse X-ray emission from the shocked wind region of the bow shock, finding total unabsorbed X-ray flux in the 0.3–2keV band corresponding to a diffuse X-ray luminosity of LX = 2.33−1.54+1.12 × 1029 erg s−1, consistent with previous work. The diffuse X-ray emission arises from the region between the star and the bow shock. Three-dimensional magnetohydrodyanmic simulations were used to model the interaction of the star’s wind with a uniform interstellar medium (ISM) using a range of stellar and ISM parameters motivated by observational constraints. Synthetic infrared, Hα, soft X-ray, emission measure, and radio 6 GHz emission maps were generated from three simulations, for comparison with the relevant observations. Simulations where the space velocity of ζ Ophiuchi has a significant radial velocity produce infrared emission maps with the opening angle of the bow shock in better agreement with observations than for the case where motion is fully in the plane of the sky. All three simulations presented here have X-ray emission fainter than observed, in contrast to results for NGC 7635. The simulation with the highest pressure has the closest match to X-ray observations, with a flux level within a factor of two of the observational lower limit, and emission weighted temperature of log10(TA/K) = 6.4, although the morphology of the diffuse emission appears somewhat different. The observed X-ray emission is from a filled bubble that is brightest near the star, whereas simulations predict brightening towards the contact discontinuity as density increases. This first numerical study of the bow shock and wind bubble around ζ Ophiuchi uses a relatively simple model of a uniform ISM and ideal-magnetohydrodynamics, and can be used as a basis for comparing results from models incorporating more physical processes, or higher resolution simulations that may show more turbulent mixing.

Flux and spectral variability of Mrk 421 during its moderate activity state usingNuSTAR: Possible accretion disc contribution?

Context.The X-ray emission in BL Lac objects is believed to be dominated by synchrotron emission from their relativistic jets. However, when the jet emission is not strong, one could expect signatures of X-ray emission from inverse Compton scattering of accretion disc photons by hot and energetic electrons in the corona. Moreover, the observed X-ray variability can also originate in the disc, and get propagated and amplified by the jet.Aims.Here, we present results on the BL Lac object Mrk 421 using the Nuclear Spectroscopic Telescope Array data acquired during 2017 when the source was in a moderate X-ray brightness state. For comparison with high jet activity state, we also considered one epoch of data in April 2013 when the source was in a very high X-ray brightness state. Our aim is to explore the possibility of the signature of accretion disc emission in the overall X-ray emission from Mrk 421 and also examine changes in accretion parameters considering their contribution to spectral variations.Methods.We divided each epoch of data into different segments in order to find small-scale variability. Data for all segments were fitted using a simple power-law model. We also fitted the full epoch data using the two component advective flow (TCAF) model to extract the accretion flow parameters. Furthermore, we estimated the X-ray flux coming from the different components of the flow using the lowest normalisation method and analysed the relations between them. For consistency, we performed the spectral analysis using models available in the literature.Results.The simple power-law function does not fit the spectra well, and a cutoff needs to be added. The spectral fitting of the data using the TCAF model shows that the data can be explained with a model where (a) the size of the dynamic corona at the base of the jet is from ∼28 to 10 rs, (b) the disc mass accretion rate is from 0.021 to 0.051ṀEdd, (c) the halo mass accretion rate is from 0.22 to 0.35ṀEdd, and (d) the viscosity parameter of the Keplerian accretion disc from 0.18 to 0.25. In the assumed model, the total flux, disc and jet flux correlate with the radio flux observed during these epochs.Conclusions.From the spectral analysis, we conclude that the spectra of all the epochs of Mrk 421 in 2017 are well described by the accretion-disc-based TCAF model. The estimated disc and jet flux relations with radio flux show that accretion disc can contribute to the observed X-ray emission, when X-ray data (that cover a small portion of the broad band spectral energy distribution of Mrk 421) are considered in isolation. However, the present disc-based models are disfavoured with respect to the relativistic jet models when considering the X-ray data in conjunction with data at other wavelengths.

NICER X-Ray Observations of Eta Carinae during Its Most Recent Periastron Passage

We report high-precision X-ray monitoring observations in the 0.4–10 keV band of the luminous, long-period colliding wind binary Eta Carinae, up to and through its most recent X-ray minimum/periastron passage in 2020 February. Eta Carinae reached its observed maximum X-ray flux on 2020 January 7, at a flux level of 3.30 ×10−10 ergs s−1 cm−2, followed by a rapid plunge to its observed minimum flux, 0.03 × 10−10 ergs s−1 cm−2, near 2020 February 17. The NICER observations show an X-ray recovery from the minimum of only ∼16 days, the shortest X-ray minimum observed so far. We provide new constraints for the “deep” and “shallow” minimum intervals. Variations in the characteristic X-ray temperatures of the hottest observed X-ray emission indicate that the apex of the wind–wind “bow shock” enters the companion’s wind acceleration zone about 81 days before the start of the X-ray minimum. There is a steplike increase in column density just before the X-ray minimum, probably associated with the presence of dense clumps near the shock apex. During the recovery and after, the column density shows a smooth decline, which agrees with previous N H measurements made by Swift at the same orbital phase, indicating that the changes in the mass-loss rate are only a few percent over the two cycles. Finally, we use the variations in the X-ray flux of the outer ejecta seen by NICER to derive a kinetic X-ray luminosity of the ejecta of ∼1041 ergs s−1 near the time of the “Great Eruption.”

Discovery of a bright extended X-ray jet in RGB J1512+020A

ABSTRACTWe report the discovery of a bright, extended X-ray jet in the quasar RGB J1512+020A (z = 0.22). Chandra observations show the X-ray core and 13 arcsec (∼45 kpc projected) extended emission coincident with the radio jet. The jet stands out as one of brightest X-ray jets (LX ∼ 7 × 1043 erg s−1) at low redshift (z < 0.5) discovered so far, with remarkably large X-ray to radio luminosity ratios (LX/Lr up to ∼50). We identified four main components, two unresolved knots and two extended structures, one being the jet brightest feature (JBF). All jet features are also detected in ALMA archival observations. The radio, sub-mm to X-ray spectra of the two knots can be modelled with a single synchrotron component. For the two resolved structures, the ALMA data unveil a turnover of the low-energy continuum at ∼460 GHz. External Comptonization of cosmic microwave background photons can account for the observed X-ray emission if the jet speed remains highly relativistic, with bulk motion Γbulk > 15, up to tens of kiloparsec from the core. However, the comparison with the spectral energy distribution of similar X-ray detected jets shows that the alternative hypothesis of synchrotron emission from an additional population of ultra-high-energy electrons is also possible. We report a tentative (≳2σ) optical detection of the JBF in images of the Dark Energy Camera Legacy Survey. If confirmed, the optical emission should be either the low-energy tail of the radiative component responsible for the X-ray emission or a third, separate component.

Chandra revisits WR 48a: testing colliding wind models in massive binaries

ABSTRACT We present results of new Chandra High-Energy Transmission Grating (HETG) observations (2019 November–December) of the massive Wolf–Rayet binary WR 48a. Analysis of these high-quality data showed that the spectral lines in this massive binary are broadened (full width at half-maximum, FWHM = 1400 km s−1) and marginally blueshifted (∼−100 km s−1). A direct modelling of these high-resolution spectra in the framework of the standard colliding stellar wind (CSW) picture provided a very good correspondence between the shape of the theoretical and observed spectra. Also, the theoretical line profiles are in most cases an acceptable representation of the observed ones. We applied the CSW model to the X-ray spectra of WR 48a from previous observations: Chandra-HETG (2012 October) and XMM–Newton (2008 January). From this expanded analysis, we find that the observed X-ray emission from WR 48a is variable on the long time-scale (years) and the same is valid for its intrinsic X-ray emission. This requires variable mass-loss rates over the binary orbital period. The X-ray absorption (in excess of that from the stellar winds in the binary) is variable as well. We note that lower intrinsic X-ray emission is accompanied by higher X-ray absorption. A qualitative explanation could be that the presence of clumpy and non-spherically symmetric stellar winds may play a role.

Long-term multiband correlation study and spectral energy distribution modeling of blazar 3C 454.3

Abstract We analyzed the multiband and long-term light curves of blazar 3C 454.3 over a time span of more than 10 yr. The discrete correlation functions (DCFs) between the γ-rays (Fermi-LAT) and the U and V bands (Swift) show simultaneous variations above the 99% confidence level. The DCFs between the X-ray band and the ultraviolet/U bands (Swift-UVW2, Swift-UVW1) are correlated over the 95% confidence level, with the X-ray variations leading by 98 d. The observed X-ray emission might not come from the same region as the γ-rays. For the studied time range, we identify four flare states and one quiescent state in the γ-ray light curve. With a synchrotron self-Compton + external Compton (EC) model, the spectral energy distributions (SEDs) of the five quasi-simultaneous observed states are modeled to discuss the underlying reasons for the flares. The EC soft photons could originate from the broad-line region (BLR) rather than from the dusty torus for the quiescent and four activity states. This indicates that the γ-ray radiation area may be located inside the BLR. From the quiescent state to the flare states, the fitting results show that N0, γmax, γbr, and δ increase, while B and R decrease. The reproduction of the SEDs implies that the increase in the bulk Lorentz factor and the emitted electron power might be the primary reason for the γ-ray flares.

The impact of the CMB on the evolution of high-z blazars

ABSTRACT Different works have recently found an increase of the average X-ray-to-radio luminosity ratio with redshift in the blazar population. We evaluate here whether the inverse Compton interaction between the relativistic electrons within the jet and the photons of the cosmic microwave background (IC/CMB) can explain this trend. Moreover, we test whether the IC/CMB model can also be at the origin of the different space density evolutions found in X-ray and radio-selected blazar samples. By considering the best statistically complete samples of blazars selected in the radio or in the X-ray band and covering a large range of redshift (0.5 ≲ z ≲ 5.5), we evaluate the expected impact of the CMB on the observed X-ray emission on each sample and then we compare these predictions with the observations. We find that this model can satisfactorily explain both the observed trend of the X-ray-to-radio luminosity ratios with redshift and the different cosmological evolutions derived from the radio and X-ray band. Finally, we discuss how currently on-going X-ray missions, like extended ROentgen Survey with an Imaging Telescope Array, could help to further constrain the observed evolution at even higher redshifts (up to z ∼ 6–7).

Discovery of Candidate X-Ray Jets in High-redshift Quasars

Abstract We present Chandra X-ray observations of 14 radio-loud quasars at redshifts 3 < z < 4, selected from a well-defined sample. All quasars are detected in the 0.5–7.0 keV energy band, and resolved X-ray features are detected in five of the objects at distances of 1″–12″ from the quasar core. The X-ray features are spatially coincident with known radio features for four of the five quasars. This indicates that these systems contain X-ray jets. X-ray fluxes and luminosities are measured, and jet-to-core X-ray flux ratios are estimated. The flux ratios are consistent with those observed for nearby jet systems, suggesting that the observed X-ray emission mechanism is independent of redshift. For quasars with undetected jets, an upper limit on the average X-ray jet intensity is estimated using a stacked image analysis. Emission spectra of the quasar cores are extracted and modeled to obtain best-fit photon indices, and an Fe K emission line is detected from one quasar in our sample. We compare X-ray spectral properties with optical and radio emission in the context of both our sample and other quasar surveys.

The Merger Dynamics of the Galaxy Cluster A1775: New Insights from Chandra and XMM-Newton for a Cluster Simultaneously Hosting a Wide-angle Tail and a Narrow-angle Tail Radio Source

Abstract We present a new study of the merger dynamics of A1775 by analyzing the high-quality Chandra and XMM-Newton archival data. We confirm/identify an arc-shaped edge (i.e., the head) at ∼48 kpc west of the X-ray peak, a split cold gas tail that extends eastward to ∼163 kpc, and a plume of spiral-like X-ray excess (within about 81–324 kpc northeast of the cluster core) that connects to the end of the tail. The head, across which the projected gas temperature rises outward from to keV, is found to be a cold front with a Mach number of . Along the surfaces of the cold front and tail, typical Kelvin–Helmholtz instability features (noses and wings, etc.) are found and are used to constrain the upper limit of the magnetic field (∼11.2 μG) and the viscosity suppression factor (∼0.01). Combining optical and radio evidence, we propose a two-body merger (instead of systematic motion in a large-scale gas environment) scenario and have carried out idealized hydrodynamic simulations to verify it. We find that the observed X-ray emission and temperature distributions can be best reproduced with a merger mass ratio of 5 after the first pericentric passage. The NAT radio galaxy is thus more likely to be a single galaxy falling into the cluster center at a relative velocity of 2800 km s−1, a speed constrained by its radio morphology. The infalling subcluster is expected to have a relatively low gas content, because only a gas-poor subcluster can cause central-only disturbances as observed in such an off-axis merger.

Accretion flow properties of XTE J1118+480 during its 2005 outburst

We study spectral and temporal properties of Galactic short orbital period transient black hole XTE J1118+480 during its 2005 outburst using archival data of RXTE PCA and HEXTE instruments in the combined energy range of 3 – 100 keV. Spectral analysis with the physical two component advective flow (TCAF) model allows us to understand the accretion flow properties of the source. We found that this outburst of XTE J1118+480 is an unconventional outburst as the source was only in the hard state (HS). Our spectral analysis suggests that during the entire outburst, the source was highly dominated by the low angular momentum sub-Keplerian halo rate. Since the source was active in radio throughout the outburst, we make an effort to estimate X-ray contribution of jets to total observed X-ray emissions from the spectral analysis with the TCAF model. The total X-ray intensity shows a similar nature of evolution as that of radio and jet X-ray fluxes. This allowed us to define this ‘outburst’ also as a jet dominated ‘outburst’. Total X-ray flux is also found to subside when jet activity disappears. Our detailed spectral analysis also indicated that although the source was only in the HS during the outburst, in the late declining phase the spectrum became slightly softer due to the slow rise in the Keplerian disk rate.

Pulsed-power-driven cylindrical aluminium liner implosions with a high aspect ratio at an 8 MA facility

First experimental investigations of foil aluminium liner implosions with a high aspect ratio of ∼ 103 at an 8 MA pulsed-power facility in China, motivated by possible application for magneto-inertial fusion, will be reported. Key diagnostics including UV cameras, micro-magnetic probes and x-ray pinhole imaging systems were fielded. Quasi-monochromatic UV images and optical streak data showed localized plasma formation at breakdown positions, suggesting that vacuum gaps in contact play a crucial role in long-time and non-uniform initiation. In liner ablation, quasi-periodic striation patterns were directly observed in UV self-emission images and were found to be azimuthally correlated with Rayleigh–Taylor instabilities. An m ≠ 0 azimuthal helical mode was obtained for the unmagnetized liner, which could be attributed to the large surface roughness that may act as an inherently and randomly seeded instability. Precursor plasma formation was confirmed by the observed x-ray emission on-axis at −70 ns by the end-on x-ray framing camera. Further quantitative current measurements taken by micro magnetic probes suggested that the current division due to precursor plasma was approximately 20% of the load current at −66 ns. Side-on imaging diagnostics indicated an evident liner implosion, however with a relatively low convergence ratio of ∼ 3.2 at stagnation. For the observed emission rings with much faster velocity in end-on x-ray images, possible mechanisms were discussed.

Stellar coronal X-ray emission and surface magnetic flux

Context. Observations show that the coronal X-ray emission of the Sun and other stars depends on the surface magnetic field. Aims. Using power-law scaling relations between different physical parameters, we aim to build an analytical model to connect the observed X-ray emission to the surface magnetic flux. Methods. The basis for our model are the scaling laws of Rosner, Tucker & Vaiana (RTV) that connect the temperature and pressure of a coronal loop to its length and energy input. To estimate the energy flux into the upper atmosphere, we used scalings derived for different heating mechanisms, such as field-line braiding or Alfvén wave heating. We supplemented this with observed relations between active region size and magnetic flux and derived scalings of how X-ray emissivity depends on temperature. Results. Based on our analytical model, we find a power-law dependence of the X-ray emission on the magnetic flux, LX ∝ Φm, with a power-law index m being in the range from about one to two. This finding is consistent with a wide range of observations, from individual features on the Sun, such as bright points or active regions, to stars of different types and varying levels of activity. The power-law index m depends on the choice of the heating mechanism, and our results slightly favor the braiding and nanoflare scenarios over Alfvén wave heating. In addition, the choice of instrument will have an impact on the power-law index m because of the sensitivity of the observed wavelength region to the temperature of the coronal plasma. Conclusions. Overall, our simple analytical model based on the RTV scaling laws gives a good representation of the observed X-ray emission. Therefore we might be able to understand stellar coronal activity though a collection of basic building blocks, like loops, which we can study in spatially resolved detail on the Sun.