It has long been believed that newborn quark stars are inefficient emitters because the quark plasma frequency is so high that it suppresses electromagnetic radiation across the entire spectrum up to energies of several tens of MeV. Here, we show that significant electromagnetic emission can instead originate from the electrosphere of a quark star, where the plasma frequency is much lower, approximately 40 keV. As a result, the cooling of a newborn quark star is expected to proceed through two distinct observational epochs. During the first epoch, blackbody radiation is emitted from the photosphere of an optically thick relativistic wind generated near the electrosphere. For 2SC+ quark star, over the course of a few days following its formation, the photospheric temperature decreases from several MeV to about 22 keV. During the second epoch, the blackbody spectrum evolves into a narrow feature centered around 40 keV, and the emission gradually fades. This spectral evolution provides a characteristic signature of the electrosphere surrounding compact astrophysical objects and may be detectable with current or future x-ray observatories.

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• Quark matter
• Strong electromagnetic field effects
• Electron-positron plasmas