Decadal brightening in the northeastern non-thermal filament of Cassiopeia A

TL;DR

The paper investigates decadal variability of non-thermal X-ray emission in Cassiopeia A's northeastern filament to probe forward-shock particle acceleration and magnetic-field evolution. It analyzes ~two decades of Chandra ACIS data, performing region-resolved spectroscopy in the 4.2–6 keV band and tracking morphological changes from 2000 to 2019. The main findings show that Region 1 brightened by about $\,\sim80\%$ from 2000–2012 before saturating, while Region 2 remained largely unchanged, yielding an overall filament increase of $>50\%$ largely driven by Region 1. These results reveal strong spatial inhomogeneity in acceleration and magnetic-field amplification within a single SNR filament and motivate continued high-resolution, multi-wavelength and polarimetric studies to map CR acceleration processes in young remnants.

Abstract

We present the decadal brightening of non-thermal emission flux in the northeastern filament of the young supernova remnant Cassiopeia A (Cas A), which highlights dynamic processes in the forward shock. This filament, characterized by the highest particle acceleration rate among Cas A's outer shells, offers an exceptional opportunity to investigate underlying astrophysical mechanisms. Since 2000, the non-thermal flux has increased by several tens of percent before plateauing, while the spectral shape has remained largely unchanged. Over the past two decades, the filament's morphology has evolved significantly, splitting into two distinct sections. Detailed analysis reveals contrasting behaviors: one section shows a flux increase followed by saturation, while the other maintains a steady flux. These differences likely arise from nonlinear effects, including magnetohydrodynamic interactions influenced by magnetic field orientation, interactions with surrounding material, and complex fluid dynamics associated with young supernova remnants. The localized evolution of this filament, captured with high spatial resolution, provides critical insights into the temporal dynamics of non-thermal particles and the generation of cosmic rays from asymmetric supernova explosions.

Figures (7)

• Figure 1: High-resolution images of Cas A in the 0.5--7.0 keV energy band captured by Chandra, illustrating the intricate structures within the supernova remnant. The images highlight the two filamentary structures in the northeast region, outlined with solid lines (Region 1and Region2), with the background region indicated by dashed lines. Alt text: Overall view of CasA.
• Figure 2: Sequential observations of the northeast filaments in Cas A within the 0.5--7.0 keV energy band: this series of images displays the evolving structure of the northeast filaments over a span of fifteen years, captured in 2000, 2009, and 2019, respectively. Each image, progressing from left to right, highlights the dynamic changes occurring within this region of CasA, providing insights into the remnant's ongoing development and the physical processes driving these transformations. Alt text: Expanded view of the analysis area for the years 2000, 2009, and 2019.
• Figure 4: Flux variations obtained by fitting 4.2--6 keV range within the north eastern filament of Cas A over nearly two decades. The top, middle, and bottom panel displays the combined flux changes observed in Region 1 and 2, Region 1, and Region 2, respectively. Alt text: Three scatter plots graphs. In all graphs, the x axis shows observation year from 2000 to 2020. In the top panel, the y axis shows flux in Region1 and 2 from 0.9e-12 to 1.5e-12 ergs per square centimeter per second. In the middle panel, the y axis shows flux in Region1 from 4.5e-13 to 10e-13 ergs per square centimeter per second.In the middle panel, the y axis shows flux in Region2 from 4.0e-13 to 5.2e-13 ergs per square centimeter per second.
• Figure 5: Acceleration and cooling timescales as a function of electron energy, with the magnetic field set at 120 $\mu$G. The solid gray, black, blue, and orange lines are ${ \hbox{$\m@th\tau$} }_{\mathrm{acc}, \perp}$, ${ \hbox{$\m@th\tau$} }_{\mathrm{acc}, \parallel}$, $\tau_{\mathrm{sync}}$ , and $\tau_{\mathrm{acc}}$, respectively. The purple region indicates the maximum electron energy around 30 TeV, as referenced in Eq. \ref{['Emax']}. Alt text: Graph with four lines representing timescales and a region showing the maximum electron energy. The x axis shows the energy from 1.0 to 120 tera electron volt. The y axis shows the time scale from 0.03 to 4000 years.
• Figure 6: (Left) The count rate spectra of Cas A of Chandra, obtained from OBSID 114 (2000), OBSID 12020 (2009), and OBSID 19606 (2019) for Region 1. (Right) Corresponding spectra for Region 2 across the same observational IDs. Alt text: The two panels have scatter plots of fluxes for 2000, 2009, and 2019, respectively. In both graphs, the x axis shows energys from 4.2 to 6.0 kilo electron volt and the y axis shows the number of counts per second per kilo electron volt from 0.002 to 0.03.