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

Alicja Wierzcholska, Hubert Siejkowski

TL;DR

Long-term X-ray monitoring of 1ES 0229+200 with Swift and NuSTAR reveals a rare concave spectrum with an upturn near 25 keV in a low X-ray state, challenging the conventional two-bump SED picture. The authors perform multi-model spectral fits (PL, LP, and BKNPL) to 3–40 keV data and analyze X-ray–optical correlations, concluding that the upturn could signal a transition between synchrotron and inverse Compton components or a leptohadronic third bump, with implications for neutrino production. This work highlights the potential for a three-bump SED in HBLs and strengthens the case for hadronic contributions in 1ES 0229+200, while emphasizing the need for further high-energy observations to constrain models.

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 1E 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.

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

TL;DR

Long-term X-ray monitoring of 1ES 0229+200 with Swift and NuSTAR reveals a rare concave spectrum with an upturn near 25 keV in a low X-ray state, challenging the conventional two-bump SED picture. The authors perform multi-model spectral fits (PL, LP, and BKNPL) to 3–40 keV data and analyze X-ray–optical correlations, concluding that the upturn could signal a transition between synchrotron and inverse Compton components or a leptohadronic third bump, with implications for neutrino production. This work highlights the potential for a three-bump SED in HBLs and strengthens the case for hadronic contributions in 1ES 0229+200, while emphasizing the need for further high-energy observations to constrain models.

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 1E 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) -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.
Paper Structure (9 sections, 3 equations, 3 figures, 4 tables)

This paper contains 9 sections, 3 equations, 3 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Data analysis
  3. NuSTAR observations
  4. Swift-XRT and Swift-UVOT observations
  5. Results
  6. Characterisation of the variability
  7. Correlations
  8. Spectral properties
  9. Discussion

Figures (3)

  • Figure 1: Multi-wavelength light curve of 1ES 0229+200 presenting long-term (2008-2024) observations of the blazar. The panels present optical Swift-UVOT observation in U, B, V filters; X-ray (0.3-10 keV) flux; and corresponding power-law photon index. Two epochs of NuSTAR observations taken in 2013 and 2021 are marked with vertical lines and noted as nu13 and nu21, respectively.
  • Figure 2: Correlation plots for 1ES 0229+200. The following figures present flux-flux comparison of Swift/UVOT V and B filter; flux-flux comparison of Swift/UVOT V flux and Swift-XRT flux measured in the energy range of 0.3-10 keV; colour-magnitude diagram for V magnitude and $B-V$ colour; flux-index comparison for Swift-XRT observations.
  • Figure 3: Spectral energy distributions presenting NuSTAR and Swift-XRT observations for six epochs (3 in 2013 and 2021). Swift-XRT observations are corrected for the influence of the hydrogen Galactic column density with N$_H$ provided by Willingale. The last two digits of the label denote the ObsIDs of the NuSTAR observations. The Swift-XRT and NuSTAR observations are distinguished by filled and hollow markers, respectively. The full ObsIDs are listed in Table \ref{['table_nustardata']}. In the case of 2013 NuSTAR data, spectral points are adopted from Wierzcholska_host.