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Stereo observations of CTA 1 with SST-1M

Bastien Lacave

TL;DR

This paper investigates discrepancies between VERITAS and LHAASO VHE measurements of the CTA1 SNR by conducting dedicated SST-1M stereo observations. Using ~55 hours of data (≈30 h after quality cuts) and a pipeline incorporating Hillas parametrization, RF-based gamma/hadron separation, and multiple background estimators, the authors map the region around CTA1. They find a 3.5σ excess offset by ~0.25° north of the pulsar and show that the 8 TeV flux from this northern region aligns with LHAASO's PLC spectrum with a cutoff at $E_c\approx110\ \mathrm{TeV}$, while exceeding VERITAS extrapolations. The results support an energy-dependent morphology in CTA1 and motivate deeper SST-1M observations (Summer 2025) to achieve >5σ detection, constrain the emission region, and refine the PWN particle transport and magnetic-field models.

Abstract

CTA~1 is a composite supernova remnant featuring a shell structure and an inner Pulsar Wind Nebula. The shell is visible in the radio band, while Fermi has detected the radio-quiet pulsar PSR J0007+7303 at its core. Gamma-ray detectors such as LHAASO and VERITAS have detected TeV emission in the vicinity of the pulsar. However, the derived SEDs from LHAASO WCDA and VERITAS show significant discrepancies, which could be due to a complicated energy-dependent morphology not accounted for in the spectral analysis, and different angular resolution of the two experiments. CTA~1 has been a target for dedicated observations by the SST-1M telescopes, a pair of small-sized Imaging Atmospheric Cherenkov Telescopes (IACTs) capable of operating in both mono and stereo modes. Located at the Ondřejov Observatory in Czech Republic, these telescopes are sensitive to the high energy range of the gamma-ray spectrum, spanning from 1 to 300 TeV. To investigate the very high-energy emission of CTA~1, the SST-1Ms have accumulated approximately 30 hours of selected observations, aiming to further constrain the characteristics of the source's high energy emission, and to shed some light into the discrepancy between different experiments.

Stereo observations of CTA 1 with SST-1M

TL;DR

This paper investigates discrepancies between VERITAS and LHAASO VHE measurements of the CTA1 SNR by conducting dedicated SST-1M stereo observations. Using ~55 hours of data (≈30 h after quality cuts) and a pipeline incorporating Hillas parametrization, RF-based gamma/hadron separation, and multiple background estimators, the authors map the region around CTA1. They find a 3.5σ excess offset by ~0.25° north of the pulsar and show that the 8 TeV flux from this northern region aligns with LHAASO's PLC spectrum with a cutoff at , while exceeding VERITAS extrapolations. The results support an energy-dependent morphology in CTA1 and motivate deeper SST-1M observations (Summer 2025) to achieve >5σ detection, constrain the emission region, and refine the PWN particle transport and magnetic-field models.

Abstract

CTA~1 is a composite supernova remnant featuring a shell structure and an inner Pulsar Wind Nebula. The shell is visible in the radio band, while Fermi has detected the radio-quiet pulsar PSR J0007+7303 at its core. Gamma-ray detectors such as LHAASO and VERITAS have detected TeV emission in the vicinity of the pulsar. However, the derived SEDs from LHAASO WCDA and VERITAS show significant discrepancies, which could be due to a complicated energy-dependent morphology not accounted for in the spectral analysis, and different angular resolution of the two experiments. CTA~1 has been a target for dedicated observations by the SST-1M telescopes, a pair of small-sized Imaging Atmospheric Cherenkov Telescopes (IACTs) capable of operating in both mono and stereo modes. Located at the Ondřejov Observatory in Czech Republic, these telescopes are sensitive to the high energy range of the gamma-ray spectrum, spanning from 1 to 300 TeV. To investigate the very high-energy emission of CTA~1, the SST-1Ms have accumulated approximately 30 hours of selected observations, aiming to further constrain the characteristics of the source's high energy emission, and to shed some light into the discrepancy between different experiments.

Paper Structure

This paper contains 5 sections, 4 figures.

Table of Contents

  1. Introduction
  2. Observations and data selection
  3. Results
  4. Conclusion
  5. Acknowledgements

Figures (4)

  • Figure 1: Excess and significance as a function of observation time for the selected stereo dataset, in a signal region of 0.3° radius.
  • Figure 2: Sky maps of the CTA 1 region using the ring background estimation method. The black circle represents the region of the analysis. The black star is the position of PSR J0007+7303 2012ApJ...744..146A. (Left) : Significance map. The white dashed circle is the LHAASO signal region and the beige one the VERITAS signal regions. (Right) : Excess map of the same region.
  • Figure 3: Significance map of the CTA 1 region using the FoV background estimation method. The contours represent the 2D gaussian fit performed to lecate the excess. The black star is the position of PSR J0007+7303.2012ApJ...744..146A. The white dashed circle is the LHAASO signal region and the beige one the VERITAS signal regions.
  • Figure 4: The differential spectrum of CTA 1. The red data point and upper limits are from this work (SST-1M stereo). The grey band represents the power-law model from VERITAS (2013), while the black line and green band show the power-law with exponential cutoff model from LHAASO (2024) and its extrapolation.