The Astropy Project supports and fosters the development of open-source and openly developed Python packages that provide commonly needed functionality to the astronomical community. A key element of the Astropy Project is the core package astropy, which serves as the foundation for more specialized projects and packages. In this article, we provide an overview of the organization of the Astropy project and summarize key features in the core package, as of the recent major release, version 2.0. We then describe the project infrastructure designed to facilitate and support development for a broader ecosystem of interoperable packages. We conclude with a future outlook of planned new features and directions for the broader Astropy Project.

Finding electromagnetic (EM) counterparts of future gravitational wave (GW) sources would bring rich scientific benefits. A promising possibility, in the case of the coalescence of a supermassive black hole binary (SMBHB), is that the prompt emission from merger‐induced disturbances in a supersonic circumbinary disc may be detectable. We follow the post‐merger evolution of a thin, zero‐viscosity circumbinary gas disc with two‐dimensional simulations, using the hydrodynamic code flash. We analyse perturbations arising from the 530 km s−1 recoil of a 106 M⊙ binary, oriented in the plane of the disc, assuming either a non‐radiative gamma‐law or a pseudo‐isothermal equation of state for the gas. We find that a single‐armed spiral shock wave forms and propagates outwards, sweeping up ∼40 per cent of the mass of the disc. The morphology and evolution of the perturbations agrees well with those of caustics predicted to occur in a collisionless disc. Assuming that the disc radiates nearly instantaneously to maintain a constant temperature, we estimate the amount of dissipation and corresponding post‐merger light curve. The luminosity rises steadily on the time‐scale of months, and reaches few ×1043 erg s−1, corresponding to ≈10 per cent of the Eddington luminosity of the central SMBHB. We also analyse the case in which gravitational wave emission results in a 5 per cent mass loss in the merger remnant. The mass loss reduces the shock overdensities and the overall luminosity of the disc by ≈15–20 per cent, without any other major effects on the spiral shock pattern.

Using spectroscopically selected galaxies from the Sloan Digital Sky Survey we present a detection of reddening effects from the circumgalactic medium of galaxies which we attribute to an extended distribution of dust. We detect the mean change in the colors of "standard crayons" correlated with the presence of foreground galaxies at z 0.05 as a function of angular separation. Following Peek & Graves, we create standard crayons using passively evolving galaxies corrected for Milky Way reddening and color-redshift trends, leading to a sample with as little as 2% scatter in color. We devise methods to ameliorate possible systematic effects related to the estimation of colors, and we find an excess reddening induced by foreground galaxies at a level ranging from 10 to 0.5 mmag on scales ranging from 30 kpc to 1 Mpc. We attribute this effect to a large-scale distribution of dust around galaxies similar to the findings of Ménard et al. We find that circumgalactic reddening is a weak function of stellar mass over the range M 6 10 9  -M 6 10 10  and note that this behavior appears to be consistent with recent results on the distribution of metals in the gas phase. We also find that circumgalactic reddening has no detectable dependence on the specific star formation rate of the host galaxy.

Small angle scattering by dust grains causes a significant contribution to the total interstellar extinction for any X-ray instrument with sub-arcminute resolution (Chandra, Swift, XMM-Newton). However, the dust scattering component is not included in the current absorption models: phabs, tbabs, and TBnew. We simulate a large number of Chandra spectra to explore the bias in the spectral fit and N H measurements obtained without including extinction from dust scattering. We find that without incorporating dust scattering, the measured N H will be too large by a baseline level of 25%. This effect is modulated by the imaging resolution of the telescope, because some amount of unresolved scattered light will be captured within the aperture used to extract point source information. In high resolution spectroscopy, dust scattering significantly enhances the total extinction optical depth and the shape of the photoelectric absorption edges. We focus in particular on the Fe-L edge at 0.7 keV, showing that the total extinction template fits well to the high resolution spectrum of three X-ray binaries from the Chandra archive: GX 9+9, XTE J1817-330, and Cyg X-1. In cases where dust is intrinsic to the source, a covering factor based on the angular extent of the dusty material must be applied to the extinction curve, regardless of angular imaging resolution. This approach will be particularly relevant for dust in quasar absorption line systems and might constrain clump sizes in active galactic nuclei.

Context. Of the presently known ≈ 3900 exoplanets, sparse spectral observations are available for ≈ 100. Ultra-hot Jupiters have recently attracted interest from observers and theoreticians alike, as they provide observationally accessible test cases. Aims. We aim to study cloud formation on the ultra-hot Jupiter HAT-P-7b, the resulting composition of the local gas phase, and how their global changes affect wavelength-dependent observations utilised to derive fundamental properties of the planet. Methods. We apply a hierarchical modelling approach as a virtual laboratory to study cloud formation and gas-phase chemistry. We utilise 97 vertical 1D profiles of a 3D GCM for HAT-P-7b to evaluate our kinetic cloud formation model consistently with the local equilibrium gas-phase composition. We use maps and slice views to provide a global understanding of the cloud and gas chemistry. Results. The day/night temperature difference on HAT-P-7b (∆T ≈ 2500 K) causes clouds to form on the nightside (dominated by H 2 /He) while the dayside (dominated by H/He) retains cloud-free equatorial regions. The cloud particles vary in composition and size throughout the vertical extension of the cloud, but also globally. TiO 2 [s]/Al 2 O 3 [s]/CaTiO 3 [s]-particles of cm-sized radii occur in the higher dayside-latitudes, resulting in a dayside dominated by gas-phase opacity. The opacity on the nightside, however, is dominated by 0.01 . . . 0.1 µm particles made of a material mix dominated by silicates. The gas pressure at which the atmosphere becomes optically thick is ∼ 10 −4 bar in cloudy regions, and ∼ 0.1 bar in cloud-free regions. Conclusions. HAT-P-7b features strong morning/evening terminator asymmetries, providing an example of patchy clouds and azimuthally-inhomogeneous chemistry. Variable terminator properties may be accessible by ingress/egress transmission photometry (e.g., CHEOPS and PLATO) or spectroscopy. The large temperature differences of ≈2500 K result in an increasing geometrical extension from the night-to the dayside. The chemcial equilibrium H 2 O abundance at the terminator changes by < 1 dex with altitude and 0.3 dex (a factor of 2) across the terminator for a given pressure, indicating that H 2 O abundances derived from transmission spectra can be representative of the well-mixed metallicity at P 10 bar. We suggest the atmospheric C/O as an important tool to trace the presence and location of clouds in exoplanet atmospheres. The atmospheric C/O can be sub-and supersolar due to cloud formation. Phase curve variability of HAT-P-7b is unlikely to be caused by dayside clouds.

We survey the Si K edge structure in various absorbed Galactic low-mass X-ray binaries (LMXBs) to study states of silicon in the inter-and circum-stellar medium. The bulk of these LMXBs lie toward the Galactic bulge region and all have column densities above 10 22 cm −2 . The observations were performed with the Chandra High Energy Transmission Grating Spectrometer. The Si K edge in all sources appears at an energy value of 1844±0.001 eV. The edge exhibits significant substructure which can be described by a near edge absorption feature at 1849±0.002 eV and a far edge absorption feature at 1865±0.002 eV. Both of these absorption features appear variable with equivalent widths up to several mÅ. We can describe the edge structure with several components: multiple edge functions, near edge absorption excesses from silicates in dust form, signatures from X-ray scattering optical depths, and a variable warm absorber from ionized atomic silicon. The measured optical depths of the edges indicate much higher values than expected from atomic silicon cross sections and ISM abundances, and appear consistent with predictions from silicate X-ray absorption and scattering. A comparison with models also indicates a preference for larger dust grain sizes. In many cases we identify Si XIII resonance absorption and determine ionization parameters between log ξ = 1.8 and 2.8 and turbulent velocities between 300 and 1000 km s −1 . This places the warm absorber in close vicinity of the X-ray binaries. In some data we observe a weak edge at 1.840 keV, potentially from a lesser contribution of neutral atomic silicon.

Interstellar extinction includes both absorption and scattering of photons from interstellar gas and dust grains, and it has the effect of altering a sourceʼs spectrum and its total observed intensity. However, while multiple absorption models exist, there are no useful scattering models in standard X-ray spectrum fitting tools, such as XSPEC. Nonetheless, X-ray halos, created by scattering from dust grains, are detected around even moderately absorbed sources, and the impact on an observed source spectrum can be significant, if modest, compared to direct absorption. By convolving the scattering cross section with dust models, we have created a spectral model as a function of energy, type of dust, and extraction region that can be used with models of direct absorption. This will ensure that the extinction model is consistent and enable direct connections to be made between a sourceʼs X-ray spectral fits and its UV/optical extinction.

We analyze the two brightest Chandra X-ray flares detected from Sagittarius A*, with peak luminosities more than 600× and 245× greater than the quiescent X-ray emission. The brightest flare has a distinctive double-peaked morphology -it lasts 5.7 ksec (∼ 2 hours), with a rapid rise time of 1500 sec and a decay time of 2500 sec. The second flare lasts 3.4 ksec, with rise and decay times of 1700 sec and 1400 sec. These luminous flares are significantly harder than quiescence: the first has a power law spectral index Γ = 2.06 ± 0.14 and the second has Γ = 2.03 ± 0.27, compared to Γ = 3.0 ± 0.2 for the quiescent accretion flow. These spectral indices (as well as the flare hardness ratios) are consistent with previously-detected Sgr A* flares, suggesting that bright and faint flares arise from similar physical processes. Leveraging the brightest flare's long duration and high signal-to-noise, we search for intraflare variability and detect excess X-ray power at a frequency of ν ≈ 3 mHz, but show that it is an instrumental artifact and not of astrophysical origin. We find no other evidence (at the 95% confidence level) for periodic or quasi-periodic variability