eROSITA's second release logs 1.9 million X-ray point sources from three stacked sky scans

eROSITA, the primary instrument on the Russian-German Spektr-RG observatory, has been sweeping the entire sky every six months since launch in 2019, and each pass adds depth and sensitivity to the last ^[1][2][7]. DR2 stacks the first 556 days of that observing, so objects too faint to clear the detection threshold on a single pass rise above it in the sum ^[8]. Divide the observing time by three scans and each one runs about 185 days ^[1]. The result lists almost twice as many sources as the first release ^[6].

X-ray positions alone do not say what an object is, which is why the catalog ships with multi-wavelength information linking each detection to its most likely optical and infrared counterpart ^[9]. "X-ray detection is only the first step. By linking X-ray sources to their counterparts at other wavelengths, we can work out what these objects are, where they sit on the cosmic distance ladder, and build clean, well-defined samples on an unprecedented scale," said Mara Salvato, the eROSITA spokesperson and chair of the follow-up working group ^[13][16].

The extragalactic fraction, applied to the point-source count, works out to roughly 1.67 million detections beyond the Milky Way, most of them supermassive black holes accreting at the centers of other galaxies ^[2][10].

The release arrived alongside the Sloan Digital Sky Survey's twentieth data release, whose optical spectroscopy includes work from eROSITA-DE's surveys, the product of nearly a decade of collaboration between the two efforts ^[11]. That pairing is what makes distances available in bulk, and it produced the mission's most striking early claim. "The most luminous black holes at high redshift are like needles in a haystack. Thanks to DR2, we found more needles than expected, suggesting that rapidly growing black holes were more abundant in the early universe than previously thought," said William Roster, lead author of the corresponding study ^[14][17]. Finding more of something in a deeper survey is not yet a measurement of how common it was; the announcement gives the excess as a count, without the sky area, completeness or selection function that would convert counts into space densities ^[4].

The extended sources are reported as a single number covering galaxy clusters and supernova remnants together ^[5]. Salvato's point about identification applies to them as much as to the point sources: something has to decide which is a cluster and which is a remnant before distances or masses follow ^[13]. "This is a dataset of unprecedented scale and completeness, now in the hands of the global astronomical community," said eROSITA Principal Investigator Andrea Merloni ^[15][18].