X-ray Observations Research Articles

X-ray surface brightness fluctuations in smooth galaxy cluster atmospheres

Abstract We measure surface brightness fluctuations in Chandra X-ray images of the cores of the galaxy clusters Abell 2029, Abell 2151, Abell 2107, RBS0533, and RBS0540. Their relatively structureless X-ray atmospheres exhibit the thermodynamic properties of cool cores including short central cooling times and low entropy. However, unlike typical cool-core clusters, molecular gas, star formation, and bubbles associated with radio jets are faint or absent near their central galaxies. Four clusters show typical gas density fluctuation amplitudes of ∼ 10percnt on the scales probed, apart from RBS0540, which exhibits lower amplitudes, suggesting that its gas is mildly disturbed. Under the assumption that gas density fluctuations are indicative of random gas velocities, we estimate scale-dependent velocity amplitudes of gas motions across all studied clusters, which range from 100 $\rm km~s^{-1}$ to 200 $\rm km~s^{-1}$ in Abell 2029, Abell 2151, and Abell 2107. These velocity estimates are comparable to the atmospheric velocity dispersion in the Perseus cluster measured by the Hitomi X-ray Observatory. The turbulent heating rates implied by our measurements are of the same order as the radiative cooling rates. Our results suggest that atmospheric sloshing and perhaps turbulent motion may aid radio jets in stabilizing atmospheric cooling.

Directional miscentering dependence in weak lensing mass bias

Abstract Galaxy cluster masses estimated from parametric modeling of weak lensing shear observations are known to be biased by inaccuracies in observationally determined centers. It has recently been shown that such systematic effects can be non-isotropic when centers are derived from X-ray or Compton-Y (Sunyaev-Zeldovich effect) observations, which is often the case in practice. This fact challenges current methods of accurately correcting for weak lensing mass biases using simulations paired with isotropic empirical miscentering distributions, in particular as the effect on determined masses is currently a dominant source of systematic uncertainty. We use hydrodanamical cosmological simulations taken from the Magneticum Pathfinder simulations to show that the non-isotropic component of the mass bias can be reduced to within one percent of the mass when considering the center of mass, rather than the bottom of the gravitational potential, as the reference center of a galaxy cluster.

Hydrogen Column Density Variability in a Sample of Local Compton-thin AGN II

Abstract We present the multiepoch analysis of 13 variable, nearby (z ≲ 0.1), and Compton-thin (1022 < N H < 1.5 × 1024 cm−2) active galactic nuclei (AGN) selected from the 105-month BAT catalog. Analyzing all available archival soft and hard X-ray observations, we investigate the line-of-sight hydrogen column density (N H,los) variability on timescales ranging from a few days to approximately 20 yr. Each source is analyzed by simultaneously modeling the data with three physical torus models, providing tight constraints on torus properties, including the covering factor, the cloud dispersion, and the torus average hydrogen column density (N H,av). Our final sample includes 27 variable, Compton-thin AGN after implementing another 14 AGN analyzed in our previous work. We find that all sources require either flux or N H,los variability. Based on the degree of variability, we classify 37% of them as “N H,los Variable,” 44% as “Nonvariable in N H,los,” and 19% as “Undetermined.” Noticeably, there is no discernible difference between geometrical and intrinsic properties among the three variability classes, suggesting no intrinsic differences between the N H,los-variable and nonvariable sources within the considered sample. We measure the median variation in N H,los between any observation pair of the same source to be 25% with respect to the lowest N H,los measure in the pair. Furthermore, 48% of the analyzed sources require the inclusion of a Compton-thick reflector in the spectral fitting. Among these, 30% exhibit recorded 22 GHz water megamaser emission, suggesting a potential shared nature between the two structures.

Possible explanations for the formation of ear-like structures in young Type Ia supernova remnants

Several young type Ia supernova remnants (SNRs) exhibit apparent axisymmetrical deviations from spherical symmetry, manifested as two opposite ear-like protrusions in their projected morphologies. The origin of this specific feature remains debated, with several physical mechanisms proposed as possible explanations. In this work, we propose two scenarios to explain the formation of ear-like structures: (i) the feedback of cosmic-ray (CR) acceleration via an effective adiabatic index, γ_ eff, with spatial variations, and (ii) the large-scale pre-existing density gradient in the local interstellar medium (ISM). Our cylindrical hydrodynamic simulation results reveal that both scenarios can produce prominent protrusions well beyond the main shell, resembling the peculiar features observed in several young type Ia SNRs. Additionally, based on the detailed analysis of the simulation data, we attempt to elucidate the ear-formation process and suggest that the relative positions of contact discontinuity can serve as an observational diagnosis in pursuing the origin of ear-like structures. We also discuss the following points: (i) the theoretical prediction of type Ia SNRs with "ears" and the visibility of an ear-like structure in observations; (ii) the simulation-based inference concerning the emission properties of the ear-like regions and the implications of a qualitative comparison of our results with X-ray observations; and (iii) the potential combination of different scenarios. Despite various existing models, we tend to regard our proposed scenarios as potential alternatives, whereby the ear-like structures originate in pure ejecta-ISM interaction, distinct from the ejecta-circumstellar medium (CSM) or jet-CSM models.

How the cool-core population transitions from galaxy groups to massive clusters. A comparison of the largest Magneticum simulation with eROSITA, XMM-Newton, Chandra and LOFAR observations

Our aim is to understand how the interplay between black hole (BH) feedback and merge processes can effectively turn cool-core galaxy clusters into hot-core clusters in the modern universe. Additionally, we also aim to clarify which parameters of the BH feedback model used in simulations can cause an excess of feedback at the scale of galaxy groups while not efficiently suppressing star formation at the scale of galaxy clusters. To obtain robust statistics of the cool-core population, we compare the modern Universe snapshot (z = 0.25) of the largest Magneticum simulation ( Box2b/hr ) with the eROSITA eFEDS survey and Planck SZ-selected clusters observed with XMM-Newton. Additionally, we compare the BH feedback injected by the simulation in radio mode with Chandra observations of X-ray cavities, and LOFAR observations of radio emission. We confirm a decreasing trend in cool-core fractions towards the most massive galaxy clusters, which is well reproduced by the Magneticum simulations. This evolution is connected with an increased merge activity that injects high-energy particles into the core region, but it also requires thermalisation and conductivity to enhance mixing through the intra-cluster medium core, where both factors are increasingly efficient towards the high mass end. On the other hand, BH feedback remains as the dominant factor at the scale of galaxy groups, while its relative impact decreases towards the most massive clusters. The problems suppressing star formation in simulations are not caused by low BH feedback efficiencies. They root in the definition of the black hole sphere of influence used to distribute the feedback, which decreases as density and accretion rate increase. Actually, a decreasing BH feedback efficiency towards low-mass galaxy groups is required to prevent overheating. These problems can be addressed in simulations by using relations between accretion rate, cavity power, and cavity reach derived from X-ray observations.

A multi-wavelength view of the isolated neutron star eRASSU J065715.3+260428

On the premise of a soft spectral distribution and absence of counterparts, the X-ray source was recently identified as a likely thermally emitting isolated neutron star (XINS) in a search in the All-Sky Survey. We investigated the nature and evolutionary state of the neutron star through a dedicated multi-wavelength follow-up observational campaign with and complemented by the analysis of archival LAT observations. The coherent timing analysis of the X-ray observations unveiled the rotation period of the XINS, P=261.085400(4),ms, and its spin-down rate P s,s^-1 (errors are $1σ$ confidence levels). The nearly sinusoidal pulse profile has a pulsed fraction of ∼15,% ($0.2-2$,keV). No optical counterparts are detected down to 27.3,mag ($5σ$, R band) in the FORS2 imaging, implying a large X-ray-to-optical flux ratio above 5200. The X-ray spectrum of the source is best described by a composite phenomenological model consisting of two thermal components, either a double blackbody continuum with temperatures 90,eV and 220,eV or a hydrogen neutron star atmosphere of temperature log(T/ K )∼ 5.8 combined with a hot blackbody of 250,eV, in both cases modified by an absorption feature at low energies, ∼0.3,keV with an equivalent width of ∼100,eV. The presence of faint non-thermal hard X-ray tails is ruled out above $(2.1±1.8)$,% of the source unabsorbed flux. Radio searches at $1-1.5$,GHz with yielded negative results, with a deep upper limit on the pulsed flux of 1.4,μJy ($10σ$). Similarly, no significant spatial or pulsed signals were detected in sixteen years of LAT observations. The most likely interpretation is that the source is a middle-aged spin-powered pulsar, which can also be identified as The absence of non-thermal X-ray, radio, or gamma-ray emission within current limits suggests either an unfavourable viewing geometry or unusual magnetospheric properties. Additional observations are needed to check for faint hard X-ray tails, investigate the presence of diffuse emission from a pulsar-wind nebula, and obtain a more accurately sampled timing solution.

Peculiar radio-bright behaviour of the Galactic black hole transient 4U 1543−47 in the 2021–2023 outburst

ABSTRACT Correlated behaviours between the radio emission and the X-ray emission in Galactic black hole X-ray binaries (BH XRBs) in the X-ray hard state are crucial to the understanding of disc-jet coupling of accreting black holes. The BH transient 4U 1543–47 went into outburst in 2021 following $\sim$19 yr of quiescence. We followed it up with $\sim$ weekly cadence with MeerKAT for about one year and a half until it faded into quiescence. Multi-epoch quasi-simultaneous MeerKAT and X-ray observations allowed us to trace the compact jet emission and its X-ray emission. In its hard spectral state across three orders of magnitude of X-ray luminosities above $\sim$10$^{34}$ ergs s$^{-1}$, we found the correlation between radio and X-ray emission had a power-law index of 0.82$\pm$0.09, steeper than the canonical value of $\sim$0.6 for BH XRBs. In addition, the radio versus X-ray correlation show a large range of the power-law normalization, with the maximum significantly larger than that obtained for most BH XRBs, indicating it can be particularly radio-bright and variable in the X-ray binary sample. The radio emission is unlikely diluted by discrete jet components. The observed peculiar radio-bright and variable behaviours provide the evidence for the relativistic effects of a variable Lorentz factor in the range between 1 and $\sim$2 of the compact jet.

Serendipitous detection of an intense X-ray flare in the weak-line T Tauri star KM Ori with SRG/eROSITA

Abstract Weak-line T Tauri stars (WTTS) exhibit X-ray flares, likely resulting from magnetic reconnection that heats the stellar plasma to very high temperatures. These flares are difficult to identify through targeted observations. Here, we report the serendipitous detection of the brightest X-ray flaring state of the WTTS KM Ori in the eROSITA DR1 survey. Observations from SRG/eROSITA, Chandra X-ray Observatory, and XMM-Newton are analysed to assess the X-ray properties of KM Ori, thereby establishing its flaring state at the eROSITA epoch. The long-term (1999–2020) X-ray light curve generated for the Chandra observations confirmed that eROSITA captured the source at its highest X-ray flaring state recorded to date. Multi-instrument observations support the X-ray flaring state of the source, with time-averaged X-ray luminosity ( $L_\mathrm{0.2-5\ keV}$ ) reaching $\sim 1.9\times10^{32}\mathrm{{erg\ s^{-1}}}$ at the eROSITA epoch, marking it the brightest and possibly the longest flare observed so far. Such intense X-ray flares have been detected only in a few WTTS. The X-ray spectral analysis unveils the presence of multiple thermal plasma components at all epochs. The notably high luminosity ( $L_\mathrm{0.5-8\ keV}\sim10^{32}\ \mathrm{erg\ s}^{-1}$ ), energy ( $E_\mathrm{ 0.5-8\ keV}\sim10^{37}$ erg), and the elevated emission measures of the thermal components in the eROSITA epoch indicate a superflare/megaflare state of KM Ori. Additionally, the H $\alpha$ line equivalent width of $\sim$ $-5$ Å from our optical spectral analysis, combined with the lack of infrared excess in the spectral energy distribution, were used to re-confirm the WTTS (thin disc/disc-less) classification of the source. The long-duration flare of KM Ori observed by eROSITA indicates the possibility of a slow-rise top-flat flare. The detection demonstrates the potential of eROSITA to uncover such rare, transient events, thereby providing new insights into the X-ray activity of WTTS.

Bayesian insights into the Tycho supernova remnant: A detailed mapping of ejecta properties

Context. While Tycho’s supernova remnant (SNR) is one of the most studied type Ia Galactic SNRs, a global view of the physical properties of its ejecta is lacking to be able to understand its mysteries. In particular, the spatial distribution of the Si-rich ejecta line- of-sight velocity presents a unexplained large-scale asymmetry, with the north dominantly blueshifted and the south redshifted. Aims. To investigate the origin of this line-of-sight velocity asymmetry in the ejecta and its current dynamics, we carried out a detailed X-ray spatially resolved spectral analysis of the entire shocked ejecta in Tycho’s SNR to determine the physical properties of its various components. This study is based on the archival deep X-ray observations from the Chandra space telescope. Methods. The spatially resolved spectral analysis in 211 regions over the entire SNR is based on a tessellation method applied to the line-of-sight velocity map. We modelled the ejecta emission with two thermal non-equilibrium ionisation components of different compositions for intermediate-mass elements (IME) and iron-rich ejecta. We included Doppler shift and line broadening and added a power law for the synchrotron emission, and additional constraints. A Bayesian tool was used to conduct the fitting, using a nested sampling algorithm. This allowed us to us to obtain a complete view of the statistical landscape. Results. We provide maps of the physical parameters of the various components across the SNR ejecta. The Doppler shift map spectrally confirms the large-scale north-south asymmetry in the line-of-sight velocity that was obtained from a general morphological component analysis. We reveal different spatial distributions of temperature and ionisation time for IMEs and for iron-rich ejecta, but none of these maps shows a structure associated with the large-scale north-south asymmetry in the line-of-sight velocity distribution. In the IME component, we observed an overall anti-correlation between the temperature and ionisation time that could arise from different ionisation histories. The abundance maps show spatial variations, depending on the element, perhaps due to an origin in different layers during the explosion. We compare these abundances with some nucleosynthesis models. In addition, we observe for the first time an emission line at 0.654 keV possibly related to oxygen. Its spatial distribution differs from the other elements, so this line may arise in the ambient medium.

The Mega-MUSCLES Treasury Survey: X-Ray to Infrared Spectral Energy Distributions of a Representative Sample of M Dwarfs

We present 5–1 × 107 Å spectral energy distributions (SEDs) for 12 M dwarf stars covering spectral types M0–M8. Our SEDs are provided for community use as a sequel to the Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems (MUSCLES) survey. The 12 stars include eight known exoplanet hosts and four stars chosen to fill out key parameter space in spectral type and rotation period. The SEDs are constructed from Hubble Space Telescope ultraviolet spectroscopy and XMM Newton, Chandra, and/or Swift X-ray observations, and completed with various model data, including Lyα reconstructions, PHOENIX optical models, APEC coronal models, and differential emission measure models in the currently-unobservable extreme ultraviolet. We provide a complete overview of the Mega-MUSCLES program, including a description of the observations, models, and SED construction. The SEDs are available as MAST high-level science products and we describe the various data products here. We also present ensemble measurements from our sample that are of particular relevance to exoplanet science, including the high-energy fluxes in the habitable zone and the far-ultraviolet to near-ultraviolet ratio. Combined with MUSCLES, Mega-MUSCLES provides SEDs covering a wide range of M dwarf spectral types and ages such that suitable proxies for any M dwarf planet host of interest may be found in our sample. However, we find that ultraviolet and X-ray fluxes can vary even between stars with similar parameters, such that observations of each exoplanet host star will remain the gold standard for interpreting exoplanet atmosphere observations.

Spectroscopic confirmation of the galaxy clusters CARLA J0950+2743 at z = 2.363 and CARLA-Ser J0950+2743 at z = 2.243

Galaxy clusters are the largest gravitationally bound structures in the Universe and therefore are a powerful tool for studying mass assembly at different epochs. At z > 2, they provide the unique opportunity to place solid constraints not only on the growth of the dark matter halo, but also on the mechanisms of galaxy quenching and morphological transformation when the Universe was younger than 3.3 Gyr. However, the currently available sample of confirmed z > 2 clusters remains very limited. We present the spectroscopic confirmation of the galaxy cluster CARLA J0950+2743 at z = 2.363 ± 0.005 and a new serendipitously discovered cluster, CARLA-Ser J0950+2743 at z = 2.243 ± 0.008, in the same region. We confirm eight star-forming galaxies in the first and five in the second cluster by detecting [OII], [OIII], and Hα emission lines. The analysis of an archival X-ray Chandra dataset that covers the cluster position revealed a counterpart with a total luminosity of L0.5−5keV = 2.9 ± 0.6 × 1045 erg s−1. Because the depth of the X-ray observations is limited, we cannot distinguish the 1D profile of the source from a point spread function model, but our statistical analysis of the 2D profile favors an extended component that might be associated with a thermal contribution from the intracluster medium. If the extended X-ray emission is due to the hot intracluster medium, the total combined dark matter mass for the two clusters would be M200 ≈ 3.0−0.23(stat)+0.20 −0.85(sys)+1.13 × 1014 M⊙, assuming a ∼30% contribution from the active galactic nucleus. Our two clusters are therefore interesting targets for studies of the structure growth in the cosmological context. However, future investigation will require deeper high-resolution X-ray and spectroscopic observations to rule out the hypotheses that the emission is entirely due to the active galactic nucleus or that it originates from other contaminating radio galaxies and structures.

Hydrostatic mass bias for galaxy groups and clusters in the FLAMINGO simulations

ABSTRACT The masses of galaxy clusters are commonly measured from X-ray observations under the assumption of hydrostatic equilibrium (HSE). This technique is known to underestimate the true mass systematically. The fiducial FLAMINGO (Full-hydro Large-scale structure simulations with All-sky Mapping for the Interpretation of Next Generation Observations) cosmological hydrodynamical simulation predicts the median hydrostatic mass bias to increase from $b_\text{HSE} \equiv (M_\text{HSE,500c}-M_\text{500c})/M_\text{500c} \approx -0.1$ to −0.2 when the true mass increases from group to cluster mass scales. However, the bias is nearly independent of the hydrostatic mass. The scatter at fixed true mass is minimum for $M_\text{500c}\sim 10^{14}~\text{M}_\odot$, where $\sigma (b_\text{HSE})\approx 0.1$, but increases rapidly towards lower and higher masses. At a fixed true mass, the hydrostatic masses increase (decrease) with redshift on group (cluster) scales, and the scatter increases. The bias is insensitive to the choice of analytic functions assumed to represent the density and temperature profiles, but it is sensitive to the goodness of fit, with poorer fits corresponding to a stronger median bias and a larger scatter. The bias is also sensitive to the strength of stellar and active galactic nucleus feedback. Models predicting lower gas fractions yield more (less) biased masses for groups (clusters). The scatter in the bias at fixed true mass is due to differences in the pressure gradients rather than in the temperature at $R_\text{500c}$. The total kinetic energies within $r_\text{500c}$ in low- and high-mass clusters are sub- and supervirial, respectively, though all become subvirial when external pressure is accounted for. Analyses of the terms in the virial and Euler equations suggest that non-thermal motions, including rotation, account for most of the hydrostatic mass bias. However, we find that the mass bias estimated from X-ray luminosity weighted profiles strongly overestimates the deviations from HSE.

Accretion onto WD 2226-210, the central star of the Helix Nebula

ABSTRACT The central star of the Helix Nebula, WD 2226$-$210 presents enigmatic hard X-ray emission and mid-IR excess. The latter has been attributed to a dusty disc or a cloud-like structure around WD 2226$-$210 formed from material of Kuiper Belt-like or comet-like objects in highly eccentric orbits. We present here a detailed analysis of multi-epoch Chandra and XMM–Newton X-ray observations of WD 2226$-$210, comparing these to previous Einstein and ROSAT data. The luminosity of the hard X-ray component of WD 2226$-$210 has remained basically constant in the decade from 1992 to 2002, with very subtle evidence for variability in time-scales of hours. Under the assumption that the X-ray emission from WD 2226$-$210 is due to accretion of material, an accretion rate of $\dot{M}\approx 10^{-10}$ M$_\odot$ yr$^{-1}$ is estimated. The origin of the material accreted by WD 2226$-$210 is uncertain, and can be attributed to the disc-like structure around it or to a substellar donor companion. The accretion rate proposed for the continuous replenishment by bombardment of the mid-IR-emitting structure around WD 2226$-$210 cannot match that required by the X-ray emission.

X-ray cavities in TNG-Cluster: AGN phenomena in the full cosmological context

ABSTRACT Active galactic nuclei (AGNs) feedback from supermassive black holes (SMBHs) at the centres of galaxy clusters plays a key role in regulating star formation and shaping the intracluster medium, often manifesting through prominent X-ray cavities embedded in the cluster’s hot atmosphere. Here we show that X-ray cavities arise naturally due to AGN feedback in TNG-Cluster. This is a new suite of magnetohydrodynamic cosmological simulations of galaxy formation and evolution, and hence of galaxy clusters, whereby cold dark matter, baryon dynamics, galactic astrophysics, and magnetic fields are evolved together consistently. We construct mock Chandra X-ray observations of the central regions of the 352 simulated clusters at z = 0 and find that $\sim$39 per cent contain X-ray cavities. Identified X-ray cavities vary in configuration with some still attached to their SMBH, while others have buoyantly risen. Their size ranges from a few to several tens of kpc. TNG-Cluster X-ray cavities are underdense compared to the surrounding halo and filled with hot gas ($\sim 10^8$ K); 25 per cent of them are surrounded by an X-ray bright and compressed rim associated with a weak shock (Mach number $\sim$1.5). Clusters exhibiting X-ray cavities are preferentially strong or weak cool-cores, are dynamically relaxed, and host SMBHs accreting at low Eddington rates. We show that TNG-Cluster X-ray cavities originate from episodic, wind-like energy injections from central AGN. Our results illustrate the existence and diversity of X-ray cavities simulated in state-of-the-art models within realistic cosmological environments and show that these can form without necessarily invoking bipolar, collimated, or relativistic jets.

Probing the heating of the neutral atomic interstellar medium in the Dwarf Galaxy Survey through infrared cooling lines

Star formation in galaxies is regulated by dynamical and thermal processes. In the Milky Way and star-forming galaxies with similar metallicity, the photoelectric effect on small dust grains usually dominates the heating of the neutral atomic gas, which constitutes the main star-forming gas reservoir. In more metal-poor galaxies, the lower dust-to-gas mass ratio together with the higher occurrence and luminosity of X-ray sources suggest that other heating mechanisms may be at play. We aim to determine the contribution of the photoelectric effect, photoionization by UV and X-ray photons, and ionization by cosmic rays to the total heating of the neutral gas in a sample of 37 low-metallicity galaxies. In particular, we wish to assess whether X-ray sources can be a significant source of heating. We also attempt to recover the intrinsic X-ray fluxes and compare them with observations when available. We used the statistical code MULTIGRIS together with a photoionization grid of Cloudy models propagating radiation from stellar clusters and potential X-ray sources to the ionized and neutral gas. This grid includes physical parameters such as metallicity, gas density, ionization parameter, and radiative source properties. We describe a galaxy as a combination of many 1D components linked by a few physical hyperparameters. We used infrared cooling lines as constraints to evaluate the most likely combinations and parameters. We constrained the heating fractions for the main mechanisms for the first time in a low-metallicity galaxy sample. We show that for the higher metallicity galaxies, the photoelectric effect dominates the neutral gas heating. At metallicities below $1/8$ the Milky Way value, cosmic rays and photoionization can become predominant. We computed an observational proxy for the photoelectric effect heating efficiency on polycyclic aromatic hydrocarbons (PAHs) using the total cooling traced by We show that this proxy can match theoretical expectations when accounting for the fraction of the heating due to the photoelectric effect according to our models. Finally, we show that it is possible to predict the X-ray fluxes reasonably well in the $0.3-8$\,keV band from the gas cooling lines for most of the galaxies observed in this band. With the current grid and assumptions, determining the exact heating fraction due to cosmic rays remains difficult, but we speculate that heating from X-ray sources is more important. As expected from the low abundance of dust and PAHs in metal-poor galaxies, heating mechanisms other than the photoelectric effect heating must be accounted for. Bright X-ray sources may deposit their energy on large scales in such transparent, dust-poor interstellar medium, and thus they represent promising avenues to understand the physical properties of the main star-forming gas reservoir in galaxies. The modeling strategy adopted here makes it possible to recover the global intrinsic radiation field properties when X-ray observations are unavailable, such as in early universe galaxies.

Unfolding X-Ray Spectral Data: Conditions and Applications

We present conditions for which X-ray spectra can be “unfolded” to present an accurate representation of the true source spectra. The method we use to unfold the data is implemented in the Interactive Spectral Interpretation Software and distinguishes itself as being model-independent. We find that this method of unfolding makes accurate representations of the true source spectra: (1) The detector is high-resolution, and (2) The spectrum is not steeply sloped. These criteria are not simple conditions that give concrete determinations; each detector and spectrum must be judged individually. We find that both grating and imaging detectors can be unfolded with minimal distortions as compared to both continuum and local spectral features, the latter CCD detectors being much more energy dependent. We also provide example use cases for unfolding in the context of current generation X-ray observatories and important caveats.

Simultaneous radio and X-ray observations of the transitional millisecond pulsar candidate 3FGL J1544.6−1125

ABSTRACT Transitional millisecond pulsars (tMSPs) are neutron star systems that alternate between a rotation-powered radio millisecond pulsar state and an accretion disc-dominated low-mass X-ray binary (LMXB)-like state on multi-year time-scales. During the LMXB-like state, the X-ray emission from tMSPs switches between ‘low’ and ‘high’ X-ray brightness modes on a time-scale of seconds to minutes (or longer), while the radio emission shows variability on time-scales of roughly minutes. Coordinated Very Large Array (VLA) and Chandra observations of the nearby tMSP PSR J1023+0038 uncovered a clear anticorrelation between radio and X-ray luminosities such that the radio emission consistently peaks during the X-ray low modes. In addition, there are sometimes also radio/X-ray flares that show no obvious correlation. In this paper, we present simultaneous radio and X-ray observations of a promising tMSP candidate system, 3FGL J1544.6$-$1125, which shows optical, $\gamma$-ray, and X-ray phenomena similar to PSR J1023+0038, but which is challenging to study because of its greater distance. Using simultaneous VLA and Chandra observations, we find that the radio and X-ray emission are consistent with being anticorrelated in a manner similar to PSR J1023+0038. We discuss how our results help in understanding the origin of bright radio emission from tMSPs. The greater sensitivity of upcoming telescopes such as the Square Kilometre Array will be crucial for studying the correlated radio/X-ray phenomena of tMSP systems.

X-Ray, UV, and Optical Observations of Proxima Centauri’s Stellar Cycle

Proxima Cen (GJ 551; dM5.5e) is one of only about a dozen fully convective stars known to have a stellar cycle, and the only one to have long-term X-ray monitoring. A previous analysis found that X-ray and mid-UV observations, particularly two epochs of data from Swift, were consistent with a well-sampled ∼7 yr optical cycle seen in All Sky Automated Survey project (ASAS) data, but not convincing by themselves. The present work incorporates several years of new ASAS-SN optical data and an additional 5 yr of Swift XRT and UVOT observations, with Swift observations now spanning 2009–2021 and optical coverage from late 2000. X-ray observations by XMM-Newton and Chandra are also included. Analysis of the combined data, which includes modeling and adjustments for stellar contamination in the optical and UV, now reveals clear cyclic behavior in all three wavebands with a period of 8.0 yr. We also show that UV and X-ray intensities are anticorrelated with optical brightness variations caused by the cycle and by rotational modulation, discuss possible indications of two coronal mass ejections, and provide updated results for the previous finding of a simple correlation between X-ray cycle amplitude and Rossby number over a wide range of stellar types and ages.

Variable structures in the stellar wind of the HMXB Vela X-1

Context. Strong stellar winds are an important feature in wind-accreting high-mass X-ray binary (HMXB) systems. Exploring their structure provides valuable insights into stellar evolution and their influence on surrounding environments. However, the long-term evolution and temporal variability of these wind structures are not fully understood. Aims. This work probes the archetypal wind-accreting HMXB Vela X-1 using the Monitor of All-sky X-ray Image (MAXI) instrument to study the orbit-to-orbit absorption variability in the 2 − 10 keV energy band across more than 14 years of observations. Additionally, the relationship between hardness ratio trends in different binary orbits and the spin state of the neutron star is investigated. Methods. We calculated X-ray hardness ratios to track absorption variability, comparing flux changes across various energy bands, as the effect of absorption on the flux is energy-dependent. We assessed variability by comparing the hardness ratio trends in our sample of binary orbits to the long-term averaged hardness ratio evolution derived from all available MAXI data. Results. Consistent with prior research, the long-term averaged hardness ratio evolution shows a stable pattern. However, the examination of individual binary orbits reveals a different hardness ratio evolution between consecutive orbits with no evident periodicity within the observed time span. We find that fewer than half of the inspected binary orbits align with the long-term averaged hardness evolution. Moreover, neutron star spin-up episodes exhibit more harder-than-average hardness trends compared to spin-down episodes, although their distributions overlap considerably. Conclusions. The long-term averaged hardness ratio dispersion and evolution are consistent with absorption column densities reported in literature from short observations, indicating that a heterogeneous wind structure – from accretion wakes to individual wind clumps – likely drives these variations. The variability observed from orbit to orbit suggests that pointed X-ray observations provide limited insights into the overall behaviour of the wind structure. Furthermore, the link between the spin state of the neutron star and the variability in orbit-to-orbit hardness trends highlights the impact of accretion processes on absorption. This connection suggests varying accretion states influenced by fluctuations in stellar wind density.

Simulating the Peculiar Periphery of the Cygnus Loop

The middle-aged Galactic supernova remnant (SNR)- the Cygnus Loop (CL)- displays a peculiar morphology in X-rays, featuring a blowout in the southern region. The underlying process accounting for the formation of the peculiar periphery remains a mystery. To this end, we conduct hydrodynamical simulations to investigate the SNR evolution coupled with a tailored stellar-wind model: a bipolar stellar wind emanating from a runaway red supergiant progenitor, excavating a wind-blown cavity elongated along the −z-direction. Our simulation results reveal that the forward shock of the consequent SNR sweeps up the modified ambient media, shaping the overall morphology with a blowout comparable to that of CL. Besides, a series of simulation runs are performed to assess the impacts of different model parameters and the projection effect (observational angle θ obs) on the final SNR profile. Three physical quantities are extracted from simulation results to characterize the simulated SNR and make a direct comparison with the X-ray observations of CL. We find that the final SNR morphology is sensitive to both stellar-wind properties and θ obs. A Cygnus-Loop-like SNR could be reproduced under appropriate parameter combinations at θ obs = 0°. While for θ obs ≲ 30°, the projected morphology akin to CL could be also generated under specific conditions.