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Multi-GeV Fermi-LAT Detection of PSR B1259-63

D. Malyshev, M. Chernyakova, A. Finn Gallagher, A. Kuzin, N. Matchett, A. Santangelo, Iu. Shebalkova, B. van Soelen

TL;DR

This study targets the high-energy emission from the gamma-ray binary PSR B1259-63/LS 2883 in the 10–100 GeV range around periastron, addressing the gap between GeV and TeV observations. Using >17 years of Fermi-LAT data and ON–OFF alongside 3D likelihood analyses, it reports the first firm detection with $Γ = 1.9 ± 0.1$ and $F_{10-130GeV} = (1.0 ± 0.1) × 10^{-10}$ ph cm$^{-2}$ s$^{-1}$ over $-400$ to $+100$ days (TS ≈ 64). The results reveal significant pre-periastron emission and a spectral connection to the TeV regime, supporting an inverse-Compton origin and suggesting a turnover near $100$ GeV. A simple one-zone IC/synchrotron model with $Γ_e = 1.8$, $E_{cut,e} = 10$ TeV, and $B_0 ≈ 1$ G at $d ≈ 2$ AU can reproduce the broadband SED, constraining particle acceleration and radiative timescales and motivating continued observations with CTA.

Abstract

PSR B1259-63/LS 2883 is a classical gamma-ray binary detected from radio to TeV energies near periastron. Using over 17 years of Fermi-LAT observations, we report the first significant detection ($8σ$ in likelihood analysis) of the system in the 10-100 GeV energy range, over orbital phases from -400 to +100 days relative to periastron. The observed spectrum is well described by a power law with photon index $Γ= 1.9 \pm 0.1$ and shows a flux level consistent with that measured at TeV energies by H.E.S.S. The smooth connection between the Fermi-LAT and TeV spectra suggests that the detected multi-GeV emission traces the rising tail of the inverse-Compton component extending into the TeV regime. The presence of detectable emission hundreds of days before periastron indicates high-energy activity over a larger orbital phase range than previously established, enabling new constraints on particle-acceleration and radiative processes in the system.

Multi-GeV Fermi-LAT Detection of PSR B1259-63

TL;DR

This study targets the high-energy emission from the gamma-ray binary PSR B1259-63/LS 2883 in the 10–100 GeV range around periastron, addressing the gap between GeV and TeV observations. Using >17 years of Fermi-LAT data and ON–OFF alongside 3D likelihood analyses, it reports the first firm detection with and ph cm s over to days (TS ≈ 64). The results reveal significant pre-periastron emission and a spectral connection to the TeV regime, supporting an inverse-Compton origin and suggesting a turnover near GeV. A simple one-zone IC/synchrotron model with , TeV, and G at AU can reproduce the broadband SED, constraining particle acceleration and radiative timescales and motivating continued observations with CTA.

Abstract

PSR B1259-63/LS 2883 is a classical gamma-ray binary detected from radio to TeV energies near periastron. Using over 17 years of Fermi-LAT observations, we report the first significant detection ( in likelihood analysis) of the system in the 10-100 GeV energy range, over orbital phases from -400 to +100 days relative to periastron. The observed spectrum is well described by a power law with photon index and shows a flux level consistent with that measured at TeV energies by H.E.S.S. The smooth connection between the Fermi-LAT and TeV spectra suggests that the detected multi-GeV emission traces the rising tail of the inverse-Compton component extending into the TeV regime. The presence of detectable emission hundreds of days before periastron indicates high-energy activity over a larger orbital phase range than previously established, enabling new constraints on particle-acceleration and radiative processes in the system.

Paper Structure

This paper contains 3 sections, 7 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Data Analysis
  3. Results and Discussion

Figures (3)

  • Figure 1: Left panel: Count map of the PSR B1259$-$63 region in galactic coordinates above 10 GeV smoothed with $0.1^\circ$ gaussian kernel. The green ellipses illustrate positions of nearby 4FGL catalogue sources. The thin green dashed circle stands for an extended source HESS J1303-631. The solid green and dashed magenta circles correspond to the ON and OFF regions around the PSR B1259$-$63 position. Right panel: Arrival times of the photons above 10 GeV in $0.1^\circ$-radius circle around the PSR B1259$-$63 position vs. the energy of the photons. Vertical dotted lines present the times of periastron, shaded regions illustrate $(-400; +100)$ days around the periastron.
  • Figure 2: Orbital light curve of PSR B1259$-$63/LS 2883 above 10GeV. Left: Light curve from the ON–OFF analysis. Red points show the ON-source count rate within $0.1^\circ$ of the PSR B1259$-$63 position. Blue points show the background (OFF) count rate in an annulus between $0.1^\circ$ and $0.4^\circ$. The significance above each point corresponds to the Poisson significance of $N_{ON}$ counts above the expected background. Right: Orbital light curve from the likelihood analysis. The dashed blue line shows the combined flux upper limit around apastron (+100 to –400 days), while the magenta point indicates the average flux detected between –400 and +100 days. For points with $TS>1$, the corresponding TS values are shown above the markers.
  • Figure 3: Multiwavelength spectrum of PSR B1259$-$63/LS 2883. Left: Joint Fermi-LAT spectrum (–400 to +100 days around periastron, averaged over 2008–2025) and H.E.S.S. spectrum (–23 to +127 days around the 2021 periastron Hess2024). The red dashed line and shaded region show the best-fit power-law model to Fermi-LAT data above 10 GeV and its $1\sigma$ statistical band. Dotted magenta lines show H.E.S.S. spectra from different intervals in 2021. Right: Broadband spectrum from X-rays to TeV. The cyan shaded region indicates the flux range observed during –400 to +100 days around the 2004 periastron chernyakova06. Black dotted and dash-dotted lines show synchrotron and inverse-Compton components of a simple one-zone model (see text).