The high energy Universe at ultra-high resolution: the power and promise of X-ray interferometry

We propose the development of X-ray interferometry (XRI), to reveal the Universe at high energies with ultra-high spatial resolution. With baselines which can be accommodated on a single spacecraft, XRI can reach 100 μ as resolution at 10 Å (1.2 keV) and 20 μ as at 2 Å (6 keV), enabling imaging and imaging-spectroscopy of (for example) X-ray coronae of nearby accreting supermassive black holes (SMBH) and the SMBH ‘shadow’; SMBH accretion flows and outflows; X-ray binary winds and orbits; stellar coronae within $\sim $ ∼ 100 pc and many exoplanets which transit across them. For sufficiently luminous sources XRI will resolve sub-pc scales across the entire observable Universe, revealing accreting binary SMBHs and enabling trigonometric measurements of the Hubble constant with X-ray light echoes from quasars or explosive transients. A multi-spacecraft ‘constellation’ interferometer would resolve well below 1 μ as, enabling SMBH event horizons to be resolved in many active galaxies and the detailed study of the effects of strong field gravity on the dynamics and emission from accreting gas close to the black hole.

A search for supernova light echoes in NGC 6946 with SITELLE

ABSTRACT We present the analysis of 4 h of spectroscopic observations of NGC 6946 with the SITELLE Imaging Fourier Transform Spectrometer on the Canada–France–Hawaii Telescope, acquired to search for supernova light echoes from its 10 modern supernovae. We develop a novel spectroscopic search method: identifying negatively sloped continua in the narrow-band SN3 filter as candidate highly broadened P-Cygni profiles in the H α line, which would be characteristic of the spectra of supernova ejecta. We test our methodology by looking for light echoes from any of the 10 supernovae observed in NGC 6946 in the past 100 yr. We find no evidence of light echoes above the survey surface brightness limit of 1 × 10−15 erg s−1 cm−2arcsec−2.

Tracking the state transitions in changing-look active galactic nuclei through their polarized-light echoes

Context. Variations in the mass accretion rate appear to be responsible for the rapid transitions in spectral type that are observed in increasingly more active galactic nuclei (AGNs). These objects are now labeled “changing-look” AGNs and are key objects for understanding the physics of accretion onto supermassive black holes. Aims. We aim to complement the analysis and interpretation of changing-look AGNs by modeling the polarization variations that can be observed, in particular, polarized-light echoes. Methods. We built a complex and representative model of an AGN and its host galaxy and ran radiative transfer simulations to obtain realistic time-dependent polarization signatures of changing-look objects. Based on actual data, we allowed the system to become several times fainter or brighter within a few years, assuming a rapid change in accretion rate. Results. We obtain time-dependent polarization signatures of distant high-luminosity (quasars) and nearby low-luminosity (Seyferts) changing-look AGNs for a representative set of inclinations. We predict the evolution of the continuum polarization for future polarimetric campaigns with the goal to better understand the physics at work in these objects. We also investigate highly inclined AGNs that experience strong accretion rate variations without appearing to change state. We apply our modeling to Mrk 1018, the best-documented case of a changing-look AGN, and predict a variation in its polarization after the recent dimming of its continuum. Conclusions. We demonstrate that polarization monitoring campaigns that cover the transitions that are observed in changing-look AGNs might bring crucial information on the geometry and composition of all the reprocessing regions within the nucleus. In particular, specific features in the time variation of the polarization position angle can provide a new and efficient method for determining AGN inclinations.

Optical Coherence Tomography in Multiple Sclerosis

Optical coherence tomography (OCT) grew out of a convergence of rapid advancements in femtoseconds optics research and fiber optic commercial technology. The basic concept of OCT is to “see” into tissues using light echoes, analogous to the sound echoes of ultrasonography. Multiple A-scans are assembled into a B-scan two-dimensional image of the tissue of interest. Retina is an ideal tissue for evaluation by OCT, since the eye is designed to minimize light scattering through the anterior chamber and vitreous. OCT has had a significant impact on the field of multiple sclerosis, where it has allowed direct imaging of the myelin-free segments of axons and cell bodies of retinal ganglion cells. Together with precise functional measurements of the afferent visual system, the addition of robust structural measurements of retinal injury has allowed for an unprecedented ability to correlate clinical effects with the degree of neuronal loss. In addition, OCT has proven helpful to distinguish different forms of demyelinating disease, such as multiple sclerosis (MS) and neuromyelitis optica, and has provided ideal outcome measures in remyelination and neuroprotection trials.

Light echoes from the plateau in Eta Carinae’s Great Eruption reveal a two-stage shock-powered event

ABSTRACT We present multi-epoch photometry and spectroscopy of a light echo from η Carinae’s 19th century Great Eruption. This echo's light curve shows a steady decline over a decade, sampling the 1850s plateau of the eruption. Spectra show the bulk outflow speed increasing from ∼150 km s−1 at early times, up to ∼600 km s−1 in the plateau. Later phases also develop remarkably broad emission wings indicating mass accelerated to more than 10 000 km s−1. Together with other clues, this provides direct evidence for an explosive ejection. This is accompanied by a transition from a narrow absorption line spectrum to emission lines, often with broad or asymmetric P Cygni profiles. These changes imply that the pre-1845 luminosity spikes are distinct from the 1850s plateau. The key reason for this change may be that shock interaction with circumstellar material (CSM) dominates the plateau. The spectral evolution of η Car closely resembles that of the decade-long eruption of UGC 2773-OT, which had clear signatures of shock interaction. We propose a two-stage scenario for η Car’s eruption: (1) a slow outflow in the decades before the eruption, probably driven by binary interaction that produced a dense equatorial outflow, followed by (2) explosive energy injection that drove CSM interaction, powering the plateau and sweeping slower CSM into a fast shell that became the Homunculus. We discuss how this sequence could arise from a stellar merger in a triple system, leaving behind the eccentric binary seen today. This gives a self-consistent scenario that may explain interacting transients across a wide range of initial mass.