röntgenkosmisen

The röntgenkosmisen is a class of astronomical objects first identified in the 1960s through observations made with the Röntgen Space Telescope. The term combines the German word “Röntgen,” referring to X‑ray radiation, with the suffix “kosmisen,” a Finnish adaptation of the word “cosmic.” It was introduced by Finnish astronomer Pertti Ahonen to classify sources that emit predominantly high‑energy photons without the optical counterparts typical of more familiar cosmic X‑ray binaries.

Röntgenkosmisen sources are distinguished by their unusually hard X‑ray spectra, with photon energies ranging from 5 keV to over 30 keV, and by the absence of regular pulsations or eclipses. In many cases, the X‑ray emission is steady over months, with only shallow flux variations. The lack of optical or radio emission suggests a highly compact object—most likely a neutron star or black hole—accreting material from a weak or lost companion. Alternatively, some models posit that these are isolated magnetic white dwarfs with ultra‑strong surface fields, emitting X‑rays through magnetospheric processes.

The identification of röntgenkosmisen has helped to refine population synthesis models of high‑energy sources in the Milky Way. Their distribution appears to trace the Galactic plane, indicating a relatively young age for the population. The most studied member is RX J0622.5–3430, a 12 kpc‑distant source detected in the ROSAT all‑sky survey, whose X‑ray flux has remained constant at roughly 10^−11 erg cm^−2 s^−1 over two decades. Recent observations with the NuSTAR telescope have confirmed a hard cut‑off at ~20 keV, supporting the accreting compact object hypothesis.

Future missions targeting the 10‑100 keV band, such as the proposed XRISM‑Hard X‑ray Extension, aim to identify more röntgenkosmisen candidates, clarify their emission mechanisms, and place tighter constraints on the rates of their progenitor systems.