IDENTIFICATION OF THE TeV GAMMA-RAY SOURCE ARGO J2031+4157 WITH THE CYGNUS COCOON

Abstract

The extended TeV gamma-ray source ARGO J2031+4157 (or MGRO J2031+41) is positionally consistent with the Cygnus Cocoon discovered by Fermi-LAT at GeV energies in the Cygnus superbubble. Reanalyzing the ARGO-YBJ data collected from 2007 November to 2013 January, the angular extension and energy spectrum of ARGO J2031+4157 are evaluated. After subtracting the contribution of the overlapping TeV sources, the ARGO-YBJ excess map is fitted with a two-dimensional Gaussian function in a square region of 10 • × 10 • , finding a source extension σ ext = 1. • 8 ± 0. • 5. The observed differential energy spectrum is dN/dE = (2.5 ± 0.4) × 10 −11 (E/1 TeV) −2.6±0.3 photons cm −2 s −1 TeV −1 , in the energy range 0.2-10 TeV. The angular extension is consistent with that of the Cygnus Cocoon as measured by Fermi-LAT and the spectrum also shows a good connection with the one measured in the 1-100 GeV energy range. These features suggest to identify ARGO J2031+4157 as the counterpart of the Cygnus Cocoon at TeV energies. The Cygnus Cocoon, located in the star-forming region of Cygnus X, is interpreted as a cocoon of freshly accelerated cosmic rays related to the Cygnus superbubble. The spectral similarity with supernova remnants (SNRs) indicates that the particle acceleration inside a superbubble is similar to that in an SNR. The spectral measurements from 1 GeV to 10 TeV allows for the first time to determine the possible spectrum slope of the underlying particle distribution. A hadronic model is adopted to explain the spectral energy distribution.

The ARGO-YBJ legacy to next generation wide field-of-view experiments

“…is shown in the second column. The TeV flux averaged over the Cygnus region 65 • < l < 85 • cover the energy range 400 GeV -2 TeV and follow a power law with spectral index -2.65±0.44, a value very close to that found for TeV emission from the Cygnus Cocoon and for the p+He component measured by ARGO-YBJ, indicating the presence of young CRs accelerated by a nearby source [30].…”

“…Milagro measured the diffuse gamma ray emission from the region 65 discrepancy can be explained taking into account that the contribution of all the discrete gamma ray sources was not completely removed from the Milagro data. According to the ARGO-YBJ data, the 1 TeV flux associated to the Cygnus Cocoon is of the same order of the diffuse flux [30].…”

“…5, upper plot). Moreover, considering that the Milagro result does not take into account the contributions from the Cygnus Cocoon (not yet discovered at the time of the measurement) [30] and from overlapping point and extended sources, the discrepancy with the Fermi/LAT predictions is almost cancelled out. Therefore, the full set of measurements with ground-based detectors is in agreement with direct observations by Fermi/LAT, and the evidence of any "TeV excess", requiring additional sources or particle production processes other than those producing Galactic cosmic rays, is ruled out.…”

“…However, as will be discussed in the following, the Milagro result does not take into account the contributions from the Cygnus Cocoon and from the overlapping point or extended sources TeV J2032+4130, VER J2019+407 and VER J2016+372 [30].…”

TeV γ-ray astronomy with ground-based air-shower arrays

“…A more recent analysis of the ARGO-YBJ full data set (November, 2007 to January, 2013) [17] suggests that the location of the extended TeV -ray source ARGO J2031+4157 (or MGRO J2031+41) is consistent with the Cygnus Cocoon discovered by Fermi-LAT at GeV energies in the Cygnus superbubble [18]. The significance map around the Cygnus Cocoon as observed in the ARGO-YBJ full data set is shown in the left panel of Fig.…”