Disentangling two spectral components in the X-ray emission of the blazar 1ES 0229+200

¹ Institute of Nuclear Physics, Polish Academy of Sciences, ul. Radzikowskiego 152, 31-342 Kraków, Poland
² Landessternwarte, Universität Heidelberg, Königstuhl 12, D 69117 Heidelberg, Germany
³ Academic Computer Centre CYFRONET of the AGH University of Krakow, Nawojki 11, 30-950 Kraków P.O. Box 6, Poland

^⋆ Corresponding author: alicja.wierzcholska@ifj.edu.pl

Received: 28 April 2025
Accepted: 10 September 2025

Abstract

X-ray observations are essential to achieve a deeper understanding of the broadband emission mechanism in blazars. Here, we present a long-term spectral and temporal analysis of X-ray and optical observations of 1ES 0229+200 collected with the Neil Gehrels Swift Observatory from 2008 to 2024, complemented by hard X-ray observations from the Nuclear Spectroscopic Telescope Array (NuSTAR). The blazar 1ES 0229+200 is a high-frequency, peaked BL Lac object, known for its exceptionally hard very high-energy (VHE) γ-ray spectrum extending up to 10 TeV. In August 2021, NuSTAR observed the source in a low X-ray state, revealing a concave spectral shape with a distinct upturn around 25 keV. This feature contrasts with previous observations performed with NuSTAR and Swift-BAT, which showed no such spectral upturn. Previous observations of 1ES 0229+200 and broadband SED (spectral energy distribution) modelling suggest that its X-ray emission extends beyond 100 keV without a significant cutoff. The newly detected spectral upturn may indicate a transition between the synchrotron and inverse Compton components or could be linked to photohadronic processes involving high-energy neutrinos. We discuss the implications of this finding in the context of blazar spectral energy distributions, particularly the potential existence of a third SED bump in the kiloelectronvolt to megaelectronvolt range. The observed spectral features support the hypothesis that 1ES 0229+200 could be a source of high-energy neutrino emission.