Constraints on the X-ray and Very High Energy γ-ray Flux from Supernova Remnant W44

 

 

Multiwavelength SED of W44. See Figure 3 below for more details.

Reference: A. Archer et al. (The VERITAS Collaboration), accepted for publication in Ap. J (2025)

Full text version

ArXiv: ArXiV: 2503.09778

Contacts: Matthew Millard

Observations of GeV gamma-ray emission from the well-studied mixed-morphology supernova remnant (SNR) W44 by Fermi-LAT and AGILE imply that it is a site of significant cosmic ray acceleration. The spectral energy distribution (SED) derived from the GeV data suggest that the gamma-ray emission likely originates from the decay of neutral pions generated by cosmic-ray interactions. It is essential to measure the SED of W44 in the X-ray and very high energy (VHE) gamma-ray bands to verify the hadronic origin of the emission and to gauge the potential contributions from leptonic emission. We report an upper-limit of the nonthermal X-ray flux from W44 of 5 x 10-13 erg cm-2s-1 in the 0.5 - 8.0 keV band based on ~300 ks of XMM-Newton observations. The X-ray upper limit is consistent with previously estimated hadronic models, but in tension with the leptonic models.  We estimate the VHE flux upper limit of ~1.2 x 10-12 erg s-1 cm-2 in the 0.5 - 5.0 TeV range from W44 using data from the Very Energetic Radiation Imaging Telescope Array System (VERITAS). Our non-detection of W44 at VHE wavelengths is in agreement with observations from other imaging atmospheric Cherenkov telescopes (IACTs) and is perhaps consistent with the evolutionary stage of the SNR.
 

FITS files: N/A

 

Figures from paper (click to get full size image):

 

 
Figure 1:  XMM MOS2 counts image of supernova remnant W44 in the 0.5 - 8 keV band smoothed with a gaussian kernel of radius = 3.3′′ and σ = 1.7′′. We extracted the target spectrum from the magenta region and the background spectrum from the white region.
 
Figure 2: XMM-Newton MOS2 spectrum extracted from the radio boundary of SNR W44 (magenta region in Figure 1) with our best-fit model overlaid (solid black curve). The solid red curve shows the best-fit model with the nonthermal flux upper limit included.
 
Figure 3: Multiwavelength SED of W44. Radio data are from Castelletti et al. (2007), Planck data from Planck Collaboration et al. (2016), Fermi-LAT data from Peron et al. (2020), and HESS upper limits from H. E. S. S. Collaboration et al. (2018). The overlaid best-fit hadronic (solid black lines) and leptonic model (dashed magenta lines) are adapted from Figures 4 and 5 of Cardillo et al. (2014) and correspond to models ‘H3’ and ‘L2’, respectively, in their Table 3. The hadronic model includes a bremsstrahlung component, which dominates from ∼ 103 - 107 eV. The grey dotted line denotes the expected emission model from EGRET observations adapted from Figure 8 of Buckley et al. (1998). The XMM-Newton and VERITAS upper limits are from this work.