1. Facts about SNe and their remnants Evolution of an SNR sensitively depends on its environment. Observed SNRs are typically produced by SNe in relative dense environments. But most (> 80%) of core-collapsed SNe explode in superbubbles (e.g., van Dyk et al. 1996; Higdon et al. 1998). Most of Type Ia SNe probably also occur in low density regions (Galactic halo and bulge). Most of SNRs are “missing”!

2. Missing Supernova Remnants Q. Daniel Wang (Univ. of Massachusetts Amherst) In collaboration with Shukui Tang, Yang Chen, David Smith, Fangjun Lu, et al. Characteristics of SNRs in low density regions Accumulated X-ray emission from such SNRs

3. Examples of SNRs in low density evironments NameL, bshellPWNX-ray Crab Nebula 184.6, -5.8NOYesNonthermal G54.1+0.354.1, 0.3NOYesNonthermal DA 53093.3, 6.9D=27’ in radio NO? RX J193264, 5.32.4’ in X-ray NO? G28.6–0.128.6, –0.110’ x 8’ in X-ray NoNonthermal?

4. DA 530 1420 MHz (Landecker et al. 1999) PSPC observation: n 0 ~ 0.02-0.05 cm -3 kT ~ 10-15 keV n e t ~ 8 × 10 10 cm -3 s

5. RX 193214.6+300741 Exposure: PSPC 3.3 ks Diameter: ~7 pc (D/10kpc) vertial distance: ~ 1 kpc L(0.1-2.4): ~10 34 ergs/s Mass: ~ 0.7Msun n 0 ~ 0.02 cm -3 Age: ~ 7 x 10 3 yrs (V e /10 3 km/s) Thermal spectrum of a very high T or a Power law No optical and radio counterparts yet. 40 ks Chandra obs. approved

6. G28.6–0.1 Image: Chandra ACIS-I observations Contours: VLA 20cm radio Ueno et al. 2003 Diameter: ~20 (D/8kpc) L(2-10): ~3 x 10 32 ergs/s n e t ~ 10 11 cm -3 s Thermal spectrum of T ~ 5.4 keV or a Power law of a photon index ~ 2

7. T ≤10 7 K SNRs in superbubbles

8. 30 Dor C in the LMC East half West half Smith & Wang 2004 100 pc

9. SNRs in the 3-phase ISM McKee & Ostriker 1977 The interstellar space is dominated by a hot phase maintained by SNe and/or superbubbles.

10. Missing SNRs and Galactic ridge X-ray emission A toy model for the GRXE: SNRs are in a hot medium and emit thermal X-ray emission. A GRXE spectrum samples the entire evolution of an SNR, according to the model of McKee & Ostriker (1977). The intermixing between the X-ray emission and absorption is approximately uniform. T 0 ≤10 6 K

11. Galactic ridge X-ray spectrum in the Chandra deep survey field SNR thermal Nonthermal (Valinia et al. 2000) Extragalactic SN rate ~ 1/(15-30)yr Total N H ~1.5  10 23 cm -2 T 0 ~ 0.01 keV Abundance: 0.5  0.1 Luminosity (0.8-10keV) ~ 9  10 38 erg s -1

12. Conclusions Candidates of SNRs in low density medium have been identified and are yet to be carefully examined. Most of such SNRs are not observed individually. They can be detected collectively and may explain the GRXE. They may have lasting impacts on the Galactic ecosystem.

14. ~ 10 55 erg, or ~ 10 4 Type Ia SNe over the past ~ a few x 10 7 years. Snowden et al. 1997