X-ray studies of extragalactic SNRs were first facilitated by the imaging capabilities of the Einstein Observatory. While Einstein detected (and discovered) remnants in the Magellanic Clouds, it did not have sufficient sensitivity to detect a substantial number of SNRs in more distant galaxies. ROSAT and ASCA made major contributions to the study of Magellanic Cloud remnants. Additionally, ROSAT also carried out detailed studies of the source populations of nearby galaxies and in so doing was the first observatory to detect a significant number of SNRs. ROSAT was unable to spatially resolve remnants detected beyond the Magellanic Clouds and those it detected were the most luminous. ROSAT studies were thus restricted to spatial and luminosity distributions. Nevertheless, ROSAT identified 16 SNRs in M31 with LX > 4 x 1035 erg s"1 . In M 33, it found 16 SNRs with LX > 7 x 1035 erg s"1, 12 of which had radio and/or optical counterparts . In the more distant NGC 300, ROSAT detected six SNRs with Lx > 5 x 1036 erg s"1, four of which are detected in other bands [109,128]. In NGC 7793, only two SNRs of 33 were detected in X-rays in a survey sensitive to LX — 2 x 1037 erg s"1 .
In general, identifying SNRs in other galaxies has relied on searches for one or more of three properties: radio spectrum, high [S II]/H-a in the optical, and X-ray brightness/color. The most reliable searches combine all three properties. One major result from studies utilizing ROSAT is the small overlap among the SNR candidates in the three bands (e.g., ). Thus, all three bands must be searched if a complete SNR census is to be obtained.
XMM-Newton and Chandra have advanced the X-ray identification of SNRs by facilitating searches to lower flux. They thereby increase the potential for detecting in X-rays SNRs found in other bands. It is possible to identify SNR candidates on the basis of X-ray spectral information using an X-ray color-color diagram in which SNRs have a nearly unique location (a technique originally applied using ROSAT e.g., ). Additionally, the availability from Chandra of high-resolution imaging allows identification by extent as well, at least for the largest remnants in the nearest galaxies. Using these observatories, it is possible for the first time to carry out X-ray imaging and spectral study of at least a few individual remnants beyond the Magellanic Clouds.
In M 31 an XMM-Newton survey has dramatically lowered the limiting flux and doubled the number of SNR candidates . With a limiting 0.2-4.5 keV luminosity of —4.4 x 1037 erg s_1, 21 SNRs were found based on correlations with radio or optical catalogs, as well as 23 SNR candidates, 22 of which are new, based on X-ray selection criteria alone. Another object may be the first pulsar wind nebula discovered beyond the Magellanic Clouds.
In M33, XMM-Newton and Chandra observations raise the total number of identified SNRs in all bands to 100. Of these, 37 have X-ray counterparts with LX > 7 x 1035 erg s^1 [38,119]. At least four appear extended in the Chandra data. Chandra observations indicate that while the number of SNRs in the LMC and M 33 with LX > 1035ergs_1 are comparable, M33 has substantially fewer with LX > 1036ergs_1. There is no clear reason for this difference; it cannot be fully explained by uncertainty in spectral model parameters, nor can it be accounted for by the abundance differences between the two galaxies.
In NGC 300, XMM-Newton observations have a factor of 10 lower limiting luminosity than the ROSAT observations. The number of SNRs and candidates is increased only from six to nine .
A Chandra survey of 11 nearby (4-13 Mpc), nearly face-on (inclination <33°) spiral galaxies yields about 150 SNRs . The vast majority of these are found in the galaxies with high star-forming rates: M 51, M 83, and M 94.
It is also now possible to study in some detail the X-ray properties of individual extragalactic remnants. In particular, it is possible using Chandra to spatially resolve the physically largest local group galaxies.
The remnant CXOM31 J004327.2+411829 is the first remnant beyond the Magellanic Clouds to be spatially resolved in X-rays . It appears ring-like, with a diameter of 42 pc. The age is estimated to be 3 210-22 300 yrs and the ambient density 0.003-0.3 cm~3. Subsequent study of the time history of the object using XMM-Newton and Chandra suggest that a variable source is embedded in the northwestern portion of the remnant . The temporal properties suggest the source is a low-mass X-ray binary (LMXB). If so, then it is the first direct association between an LMXB and an SNR.
Four other M 31 SNRs have been resolved using Chandra and associated with radio and/or optical shells [86,175]. They have diameters ranging from 12 to 40 pc, and luminosities between 7 x 1035 and 4 x 1037 erg s^1 erg s^1. All are likely propagating through low density ISM.
The remnant Ho 12 in local group dwarf irregular galaxy NGC 6822 is resolved using Chandra. The remnant appears shell-like, with a diameter of — 24 pc. Its morphology is consistent in radio, optical ([OIII], H-alpha, and [SII]), and X-ray. Spectral analysis yields kT — 2.8 keV, an age 1700-5 800 yrs, and a very low ambient medium density. Optical emission suggests the remnant expanded into a pre-existing cavity, making it an extragalactic analog to the Cygnus Loop .
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