High-resolution X-ray spectroscopy of astrophysical plasmas with X-ray microcalorimeters

High spectral resolution with a resolving power, $$E/\Delta E \gtrsim 1000$$ E / Δ E ≳ 1000 at 6 keV, is now available in X-ray astronomy. X-ray observations are particularly effective for plasma studies since major atomic transitions appear as spectral features in the X-ray band. High-resolution spectroscopy enables us to probe a wide variety of astrophysical plasmas, which are not obtainable from ground experiments, regarding their temperature, density, magnetic field, gravity, and velocity. In this review, we describe what are the X-ray emitting plasmas in the Universe, along with basic plasma diagnostics, and depict historical development of the techniques used for the X-ray spectroscopy. We outline the X-ray microcalorimeter instrument, soft X-ray spectrometer (SXS), onboard the ASTRO-H satellite. Despite the short lifetime of the satellite in orbit for about a month, observations with the SXS have shown the remarkable power of high-resolution spectroscopy in X-ray astronomy. Observed spectrum of the hot plasma in the core region of the Perseus cluster showed He-like Fe K-line to be clearly resolved into resonance, forbidden and intercombination lines for the first time. The line width indicates that the turbulent pressure amounts to only 4% of the thermal pressure of the plasma. We also describe new findings and constraints obtained from the superb spectrum of the Perseus cluster, which all indicate a great potential of X-ray spectroscopy. The recovery of the spectroscopy science of ASTRO-H is aimed at with XRISM, a Japanese mission planned for launch in early 2020s. In further future, Athena will expand the rich science with its high sensitivity and spectral resolution in early 2030s.

High-resolution VLA low radio frequency observations of the Perseus cluster: radio lobes, mini-halo, and bent-jet radio galaxies

ABSTRACT We present the first high-resolution 230–470 MHz map of the Perseus cluster obtained with the Karl G. Jansky Very Large Array. The high dynamic range and resolution achieved have allowed the identification of previously unknown structures in this nearby galaxy cluster. New hints of sub-structures appear in the inner radio lobes of the brightest cluster galaxy NGC 1275. The spurs of radio emission extending into the outer X-ray cavities, inflated by past nuclear outbursts, are seen for the first time at these frequencies, consistent with spectral aging. Beyond NGC 1275, we also analyse complex radio sources harboured in the cluster. Two new distinct, narrowly collimated jets are visible in IC 310, consistent with a highly projected narrow-angle tail radio galaxy infalling into the cluster. We show how this is in agreement with its blazar-like behaviour, implying that blazars and bent-jet radio galaxies are not mutually exclusive. We report the presence of filamentary structures across the entire tail of NGC 1265, including two new pairs of long filaments in the faintest bent extension of the tail. Such filaments have been seen in other cluster radio sources such as relics and radio lobes, indicating that there may be a fundamental connection between all these radio structures. We resolve the very narrow and straight tail of CR 15 without indication of double jets, so that the interpretation of such head–tail sources is yet unclear. Finally, we note that only the brightest western parts of the mini-halo remain, near NGC 1272 and its bent double jets.

X-ray and Gamma-ray Variability of NGC 1275

Gamma-ray emission from the bright radio source 3C 84, associated with the Perseus cluster, is ascribed to the radio galaxy NGC 1275 residing at the centre of the cluster. Study of the correlated X-ray/gamma-ray emission from this active galaxy, and investigation of the possible disk-jet connection, are hampered because the X-ray emission, particularly in the soft X-ray band (2–10 keV), is overwhelmed by the cluster emission. Here we present a method to spectrally decouple the cluster and active galactic nucleus (AGN) emission in imaging X-ray detectors. We use three sets of simultaneous Niel Gehrels Swift XRT and NuStar data. These observations were made during the period 2015 November to 2017 February, when a huge increase in the gamma-ray emission was observed. We find that the gamma-ray emission shows a very high degree of variability (40%–50%) on time scales of 1–10 days, whereas the hard X-ray emission, associated with the AGN, shows a low variability (∼15%–30%), on various time scales in the range of 0.01–60 days.

Collision of merger and accretion shocks: formation of Mpc-scale contact discontinuity in the Perseus cluster

ABSTRACT Two Mpc-size contact discontinuities have recently been identified in the XMM–Newton and Suzaku X-ray observations in the outskirts of the Perseus cluster (Walker et al.). These structures have been tentatively interpreted as ‘sloshing cold fronts’, which are customarily associated with differential motions of the cluster gas, perturbed by a merger. In this study, we consider an alternative scenario, namely, that the most prominent discontinuity, near the cluster virial radius, is the result of the collision between the accretion shock and a ‘runaway’ merger shock. We also discuss the possible origin of the second discontinuity at ${\sim}1.2{\rm \, Mpc}$.

The galaxy population within the virial radius of the Perseus cluster

Context. The Perseus cluster is one of the most massive nearby galaxy clusters and is fascinating in various respects. Though the galaxies in the central cluster region have been intensively investigated, an analysis of the galaxy population in a larger field is still outstanding. Aims. This paper investigates the galaxies that are brighter than B ≈ 20 within a field corresponding to the Abell radius of the Perseus cluster. Our first aim is to compile a new catalogue in a wide field around the centre of the Perseus cluster. The second aim of this study is to employ this catalogue for a systematic study of the cluster galaxy population with an emphasis on morphology and activity. Methods. We selected the galaxies in a 10 square degrees field of the Perseus cluster on Schmidt CCD images in B and Hα in combination with SDSS images. Morphological information was obtained both from the “eyeball” inspection and the surface brightness profile analysis. We obtained low-resolution spectra for 82 galaxies and exploited the spectra archive of SDSS and redshift data from the literature. Results. We present a catalogue of 1294 galaxies with morphological information for 90% of the galaxies and spectroscopic redshifts for 24% of them. We selected a heterogeneous sample of 313 spectroscopically confirmed cluster members and two different magnitude-limited samples with incomplete redshift data. These galaxy samples were used to derive such properties as the projected radial velocity dispersion profile, projected radial density profile, galaxy luminosity function, supermassive black hole mass function, total stellar mass, virial mass, and virial radius, to search for indications of substructure, to select active galaxies, and to study the relation between morphology, activity, density, and position. In addition, we present brief individual descriptions of 18 cluster galaxies with conspicuous morphological peculiarities.

Propagation of weak shocks in cool-core galaxy clusters in two-temperature magnetohydrodynamics with anisotropic thermal conduction

ABSTRACT We investigate how different magnetohydrodynamic models of propagation of a weak (Mach number ∼1.2) shock in the core of a galaxy cluster affect its observational appearance, using the Perseus cluster as our fiducial model. In particular, we study how thermal conduction, both isotropic and anisotropic, and ion–electron temperature equilibration modify a weak shock. Strong thermal conduction is expected to produce an electron temperature precursor. Less prominent pressure and density precursors are formed as well. A longer equilibration time largely reduces the density precursor but does not change the electron temperature precursor much. When thermal conduction becomes anisotropic, the intracluster magnetic field imprints its characteristic spatial scale on the distortions of the shock induced by heat fluxes.