Planetary Nebulae in the eROSITA eRASS1 catalog

TL;DR

This study leverages the eROSITA eRASS1 all-sky X-ray catalog to systematically search for X-ray-emitting planetary nebulae (PNe) by cross-matching with the HASH Galactic PN database. It identifies seven bona-fide eRASS1 PNe—five previously known X-ray PNe and two new detections (IC 1297, NGC 2867)—and rejects three spurious associations, establishing an approximate detection rate of 0.5% within the surveyed sample. The newly detected PNe, along with the known ones, display a spectrum of X-ray properties from soft, diffuse hot-bubble emission to hard, CSPN-dominated emission, with several WR-type central stars likely driving higher X-ray luminosities. The results underscore the limited reach of eRASS1 for faint PNe and motivate deeper follow-up with future missions such as the Einstein Probe to build a larger, more representative census of X-ray-emitting PNe and to probe the link between X-ray properties, binarity, and central-star type.

Abstract

Some planetary nebulae (PNe) host X-ray-emitting hot bubbles shaped by stellar wind interactions and/or harbor X-ray-emitting central stars due to accretion, shocks within their fast stellar winds, or even chromospheric emission from binary companions. In both cases, the properties of the X-ray emission critically probe late-stages of stellar evolution for such low- and intermediate-mass stars. While extant Chandra and XMM-Newton observations have detected X-ray emission in PNe, the numbers known remain very small ($\sim40$) compared to the overall Galactic PNe population ($\sim4000$). We have initiated a project aimed at increasing the sample of known PNe with X-ray emission using both current and new space-based X-ray telescopes such as the Einstein probe. To further investigate their X-ray properties to elucidate what drives current X-ray PN detections, we have cross-searched the SRG {\it eROSITA-DE} eRASS1 source catalogue and Hong Kong (HASH) PNe Database. Five known X-ray PNe have been detected (Abell\,30, NGC\,2392, NGC\,3242, NGC\,5315, and LoTr\,5), two new X-ray PNe are revealed (IC\,1297 and NGC\,2867), one (K\,1-27) is removed from previous X-ray compilations, and another 11 previously detected X-ray emitting PNe are not recovered. A comparison of the X-ray flux of detected and undetected X-ray PNe reveals that eROSITA eRASS1 is sensitive to PNe with X-ray fluxes larger than $\approx2\times10^{-14}$ erg~cm$^{-2}$~s$^{-1}$. The frequency of occurrence is $\simeq$0.5\% among the 1430 HASH True PNe in the eRASS1 footprint.

Figures (7)

• Figure 1: eROSITA (left), B-band (middle) and colour-composite optical (right) images of the five already known X-ray PNe. The X-ray and photographic B-band images (left and middle panels) have a similar $7\times7$ arcmin field of view(LoTr 5 has $14\times14$ arcmin) and are overlaid by X-ray contours. NGC 2392 and LOTR 5's optical images are from J-PLUS. North is up, east to the left. The final colour image is also NE to top left but is zoomed in to provide the PNe morphological detail, except for LoTr 5, which is already well resolved. The first and 4th colour images are from the HST (image credit NASA/HST), while the 4 other blue dominated images are from the very deep amateur narrow-band [OIII] dominated imagery of Peter Goodhew: https://www.imagingdeepspace.com/
• Figure 2: Same as Fig. \ref{['fig:pn_img_ok']} for the two new X-ray PNe uncovered in eRASS1, with NGC 2867 top and IC 1297 bottom. The colour image for NGC 2867 is from the HST (image credit NASA/HST) while that for IC 1297 is from Legacy Surveys/D. Lang (Perimeter Institute) Filters: g, r, z.
• Figure 3: eROSITA (left) and B-band (middle) images of the three PNe with spurious eRASS1 counterparts overlaid by X-ray contours. The X-ray and B-band images have the same $7\times7$ arcmin field of view(PHR J1107$-$5642 has $14\times14$ arcmin). North is up, east to the left. The final image in each case is an RGB colour combination from the SHS and SuperCOSMOS sky survey photographic data, with H$\alpha$ the red channel, SR the green channel, and B$_j$ the blue channel, all adjusted to best reveal the PN. The red arrows on the eROSITA and B-band images for Wray 16-385 show the location of the PN offset from the X-ray source
• Figure 4: eROSITA background-subtracted spectra of the X-ray counterparts of PNe in eRASS1 with limited count number. The peak at low photon energy distributions for the 5 confirmed X-ray emitting PNe compared to the 3 rejected candidates K 1-27, Wray 16-385 and PHR J1107-5642 is clear.
• Figure 5: (top panel) eROSITA background-subtracted X-ray spectrum of NGC 5315 (dots) overlaid with the best-fit model (histogram). (bottom) Residuals of the best-fit model.