The quest for binary and dual supermassive black holes (SMBHs) at the dawn of the multi-messenger era is compelling. Detecting dual active galactic nuclei (AGN)-active SMBHs at projected separations larger than several parsecs-and binary AGN-probing the scale where SMBHs are bound in a Keplerian binary-is an observational challenge. The study of AGN pairs (either dual or binary) also represents an overarching theoretical problem in cosmology and astrophysics. The AGN triggering calls for detailed knowledge of the hydrodynamical conditions of gas in the imminent surroundings of the SMBHs and, at the same time, their duality calls for detailed knowledge on how galaxies assemble through major and minor mergers and grow fed by matter along the filaments of the cosmic web. This review describes the techniques used across the electromagnetic spectrum to detect dual and binary AGN candidates and proposes new avenues for their search. The current observational status is compared with the state-of-the-art numerical simulations and models for formation of dual and binary AGN. Binary SMBHs are among the loudest sources of gravitational waves (GWs) in the Universe. The search for a background of GWs at nHz frequencies from inspiralling SMBHs at low redshifts, and the direct detection of signals from their coalescence by the Laser Interferometer Space Antenna in the next decade, make this a theme of major interest for multi-messenger astrophysics. This review discusses the future facilities and observational strategies that are likely to significantly advance this fascinating field.

Aims. We investigate two multi-shell galactic supernova remnants (SNRs), Kes 79, and G352.7−0.1, to understand the causes of this morphology. Methods. The research was carried out based on new and reprocessed archival VLA observations and XMM-Newton archival data. The surrounding gas was investigated based on data extracted from the HI Canadian Galactic Plane Survey, the 13 CO Galactic Ring Survey, and the HI Southern Galactic Plane Survey.Results. The present study infers that the overall morphology of both SNRs is the result of the mass-loss history of their respective progenitor stars. Kes 79 is likely to be the product of the gravitational collapse of a massive O9 star evolving near a molecular cloud and within the precursor's wind-driven bubble, while G352.7−0.1 should be the result of interactions of the SNR with an asymmetric wind from the progenitor together with projection effects. No radio point source or pulsar wind nebula was found to be associated with the X-ray pulsar CXOU J185238.6+004020 in Kes 79. The X-ray study of G352.7−0.1 found that most of the thermal X-ray radiation completely fills the interior of the remnant and originates in heated ejecta. Characteristic parameters, such as radio flux, radio spectral index, age, distance, shock velocity, initial energy, and luminosity, were estimated for both SNRs.

The XMM-Newton Serendipitous Ultraviolet Source Survey (XMM-SUSS) is a catalogue of ultraviolet (UV) sources detected serendipitously by the Optical Monitor (XMM-OM) on board the XMM-Newton observatory. The catalogue contains UV-detected sources collected from 2417 XMM-OM observations in one to six broad-band UV and optical filters, made between 2000 February 24 and 2007 March 29. The primary contents of the catalogue are source positions, magnitudes and fluxes in one to six passbands, and these are accompanied by profile diagnostics and variability statistics. XMM-SUSS is populated by 753 578 UV source detections above a 3σ signal-to-noise ratio threshold limit which relate to 624 049 unique objects. Taking account of substantial overlaps between observations, the net sky area covered is 29-54 deg 2 , depending on UV filter. The magnitude distributions peak at m AB = 20.2, 20.9 and 21.2 in UVW2 (λ eff = 2120 Å), UVM2 (λ eff = 2310 Å) and UVW1 (λ eff = 2910 Å), respectively. More than 10 per cent of the sources have been visited more than once using the same filter during XMM-Newton operation, and >20 per cent of sources are observed more than once per filter during an individual visit. Consequently, the scope for science based on temporal source variability on time-scales of hours to years is broad. By comparison with other astrophysical catalogues we test the accuracy of the source measurements and define the nature of the serendipitous UV XMM-OM source sample. The distributions of source colours in the UV and optical filters are shown together with the expected loci of stars and galaxies, and indicate that sources which are detected in multiple UV bands are predominantly star-forming galaxies and stars of type G or earlier.

Aims. With the purpose of producing the first detailed full view of Puppis A in X-rays, we carried out new XMM-Newton observations covering the missing regions in the southern half of the supernova remnant (SNR) and combined them with existing XMM-Newton and Chandra data. Methods. Two pointings toward the south and southwest of Puppis A were observed with XMM-Newton. We combined these data with archival XMM-Newton and Chandra data and produced images in the 0.3−0.7, 0.7−1.0, and 1.0−8.0 energy bands. Results. We present the first sensitive complete X-ray image of Puppis A. We investigated its morphology in detail, carried out a multiwavelength analysis, and estimated the flux density and luminosity of the whole SNR. The complex structure observed across the remnant confirms that Puppis A evolves in an inhomogeneous, probably knotty interstellar medium. The southwestern corner includes filaments that perfectly correlate with radio features suggested to be associated with shock/cloud interaction. In the northern half of Puppis A the comparison with Spitzer infrared images shows an excellent correspondence between X-rays and 24 and 70 μm emission features, while to the south there are some matched and other unmatched features. X-ray flux densities of 12.6 × 10 −9 , 6.2 × 10 −9 , and 2.8 × 10 −9 erg cm −2 s −1 were derived for the 0.3−0.7, 0.7−1.0, and 1.0−8.0 keV bands, respectively. At the assumed distance of 2.2 kpc, the total X-ray luminosity between 0.3 and 8.0 keV is 1.2 ×10 37 erg s −1 . We also collected and updated the broad-band data of Puppis A between radio and GeV γ-ray range, producing its spectral energy distribution. To provide constraints to the high-energy emission models, we re-analyzed radio data, estimating the energy content in accelerated particles to be U min = 4.8 × 10 49 erg and the magnetic field strength B ∼ 26 μG.