The black hole MAXIJ1820+070 was discovered during its 2018 outburst and was extensively monitored across the electromagnetic spectrum. Following the detection of relativistic radio jets, we obtained four Chandra X-ray observations taken between 2018 November and 2019 June, along with radio observations conducted with the Very Large Array and MeerKAT arrays. We report the discovery of X-ray sources associated with the radio jets moving at relativistic velocities with a possible deceleration at late times. The broadband spectra of the jets are consistent with synchrotron radiation from particles accelerated up to very high energies (>10 TeV) by shocks produced by the jets interacting with the interstellar medium. The minimal internal energy estimated from the X-ray observations for the jets is ∼10 41 erg, significantly larger than the energy calculated from the radio flare alone, suggesting most of the energy is possibly not radiated at small scales but released through late-time interactions. Unified Astronomy Thesaurus concepts: X-ray binary stars (1811); Relativistic jets (1390); Accretion (14); Radio jets (1347); Galactic radio sources (571); Stellar mass black holes (1611)

We discuss the prospects for detecting faint intermediate-mass black holes, such as those predicted to exist in the cores of globular clusters and dwarf spheroidal galaxies. We briefly summarize the difficulties of stellar dynamical searches, then show that recently discovered relations between black hole mass, X-ray luminosity and radio luminosity imply that in most cases, these black holes should be more easily detected in the radio than in the X-rays. Finally, we show upper limits from some radio observations of globular clusters, and discuss the possibility that the radio source in the core of the Ursa Minor dwarf spheroidal galaxy might be a ∼ 10, 000 − 100, 000M⊙ black hole.

Abstract. We have undertaken a survey for blazars by correlating the ROSAT WGACAT database with publicly available radio catalogs, restricting our candidate list to serendipitous flat-spectrum sources (αr ≤ 0.7, fν ∝ ν −α ). We discuss here our survey methods, identification procedure and first results. Our survey is found to be ∼ 95% efficient at finding blazars, a figure which is comparable to or greater than that achieved by other radio and X-ray survey techniques. DXRBS provides a much more uniform coverage of the parameter space occupied by blazars than any previous survey. Particularly important is the identification of a large population of flat-spectrum radio quasars with ratios of X-ray to radio luminosity > ∼ 10 −6 (αrx < ∼ 0.78) and of many low-luminosity flat-spectrum radio quasars. Moreover, DXRBS fills in the region of parameter space between X-ray selected and radioselected samples of BL Lacs.