# First detection of VHE gamma-ray emission from TXS~1515--273, study of its X-ray variability and spectral energy distribution

**Authors:** A. Hahn, S. Ventura, A. Berti, D. Green,  MAGIC Collaboration, V. A. Acciari, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, M. Artero, K. Asano, D. Baack, A. Babić, A. Baquero, U. Barres de Almeida, J. A. Barrio, I. Batković, J. Becerra González, W. Bednarek, L. Bellizzi  
**arXiv:** [2107.09413](https://arxiv.org/abs/2107.09413)  
**Date:** 2021-07-20  
**Summary page:** https://www.summarizepaper.com/en/paper/2107.09413v1/

## Key points

- Comprehensive multi-wavelength (MWL) campaign on blazar TXS 1515-273 conducted in 2019
- First detection of very-high-energy (VHE) gamma-ray emission from TXS 1515-273 with a statistical significance of 7.6$\sigma$
- Average integral VHE flux determined to be 6 $\pm$ 1% of the Crab nebula flux above 400 GeV
- X-ray coverage provided by Swift-XRT, XMMNewton, and NuSTAR during the MWL campaign
- Hour-scale flares observed in continuous X-ray observations with approximately 9-hour intervals
- Longer rise and decay timescales in soft X-ray band compared to hard X-ray band indicating particle cooling regime within emission region
- Constraints on parameters such as emission region size and magnetic field strength derived from X-ray variability timescales
- Synchrotron peak frequency determination classifying TXS 1515-273 as a flaring high synchrotron-peaked object without extreme levels reached
- Modeling efforts involving simple one-zone model failing to reproduce radio and optical emissions adequately, while two-component leptonic model successfully replicating emissions across radio to VHE bands

## Summary

We present the results of a comprehensive multi-wavelength (MWL) campaign on the blazar TXS 1515-273 conducted in 2019. The campaign spanned from radio to very-high-energy gamma rays (VHE) and provided valuable insights into the unique characteristics and behavior of this source. Prior to this study, TXS 1515-273 had not been extensively observed in MWL. However, our observations with the MAGIC telescopes led to the first detection of VHE gamma-ray emission from TXS 1515-273 with a statistical significance of 7.6$\sigma$. The average integral VHE flux was determined to be 6 $\pm$ 1% of the Crab nebula flux above 400 GeV. The MWL campaign included X-ray coverage provided by Swift-XRT, XMMNewton, and NuSTAR. Continuous X-ray observations revealed clear hour-scale flares separated by approximately 9 hours. Analysis of the XMM-Newton data indicated longer rise and decay timescales in the soft X-ray band compared to the hard X-ray band, suggesting a particle cooling regime within the emission region. By leveraging X-ray variability timescales, we were able to constrain parameters such as emission region size and magnetic field strength. Furthermore, synchrotron peak frequency determination allowed for classification of TXS 1515-273 as a flaring high synchrotron-peaked object that did not reach extreme levels. Modeling efforts based on constraints from X-ray data involved two approaches: a simple one-zone model failed to reproduce radio and optical emissions adequately while a two-component leptonic model incorporating interacting components successfully replicated emissions across radio to VHE bands. In conclusion, our comprehensive MWL study sheds light on the unique characteristics and behavior of TXS 1515-273, providing valuable insights into its spectral energy distribution and underlying physical processes driving its emission properties.