A Complete X-ray View of Supernova Remnant W28 with Einstein Probe: Spatial Distribution of Parameters and Origin of the Thermal-Composite Morphology

Abstract

It has been an unsolved question what leads a supernova remnant (SNR) to a thermal composite rather than a typical shell-like morphology, and what causes recombining plasma inside it. With the 13-ks observation of the Following-up X-ray Telescope onboard the Einstein Probe, we give an overall X-ray picture of W28, one of the prototypical thermal composite SNRs. The observation revealed a shell-like structure west of W28 in radio, optical, and X-ray images, which may revise the known extent of the SNR to $72'\times45'$. Spectral analysis explicitly maps that the special relationship where the plasma experiences recombination in the interior of the remnant, spatially coincident with H$α$ emissions, while in the other regions, the plasma is ionization-dominated. We found that W28 is generally isobaric from its center to the newly discovered shell, and it is even isothermal with a temperature of $\sim0.6$-0.7 keV in the center before the cooling of the plasma. Saturated thermal conduction and cloud evaporation may cool down the plasma within $\sim3$ kyr, the estimated recombination timescale. We revised the SNR dynamical age to $\sim8$ kyr, much younger than previous estimates. The complex structure and complex ionization state distribution may suggest that centrally filled and shell-like morphologies coexist in W28. This state may depend on the environment in which the SNR evolves.

Figures (11)

• Figure 1: Energy-coded X-ray image of W28 complex with FXT (a), consisting of bands in 0.4--0.7 keV (red), 0.7--1.1 keV (green), and 1.1--2.3 keV (blue). The single-band images of each energy band are shown in (b)(c)(d), respectively. The images are in units of count s$^{-1}$ cm$^{-2}$ and in a square-root color scale to enhance weak emissions. All the images are vignetting-corrected and adaptively smoothed. Structures mentioned in this article are marked in (c) and (d). Cyan regions correspond to optical and/or radio SNR candidates Stupar11Stupar18.
• Figure 2: (a): The same as Fig. \ref{['fig: xrayrgb']}(a) but based on the XMM-Newton data. The arc-like patterns of the stray light from GX 5$-$1 is visible in the south, marked between white dashed lines. (b): The effective exposure time of XMM-Newton in units of ks scaled as MOS2 with the Medium filter, which share similar effective area with one FXT module.
• Figure 3: Tri-color image of W28 in radio (red, 1.3 GHz continuum), H$\alpha$ (green), and X-ray (blue, 0.4--2.3 keV) bands. A square root color scale is used to highlight faint diffuse emissions in radio and X-ray images. White polygon regions are used for spectral extraction and analysis and cyan dashed regions correspond to the sky background. The yellow contours correspond to the MWISP CO intensity in the $-20$--10 km s$^{-1}$Tu24. Optical nebulae southeast and northeast of W28 are the Lagoon Nebula Messier 8 and the Trifid Nebula Messier 20, respectively.
• Figure 4: Counts map (not vignetting-corrected) in 3--7 keV to show the effect of the stray light. The cyan contour corresponds to 0.5, 2, and 8 counts for W28 in 0.4--2 keV.
• Figure 7: Maps of the fitting parameters with a subsolar abundance Z=0.3 Z$_\odot$. The value of the parameters and their uncertainty correspond to Table \ref{['tab: fit3']}.