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Session 48 - Spiral & Field Galaxies.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center

[48.12] X-Ray Emission from M32: X-Ray Binaries or a \muAGN?

P. B. Eskridge, R. E. White, D. S. Davis (U. Alabama)

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X-Ray emission from M32 was detected by Einstein (Fabbiano \etal 1992 ApJSup 80 531). The L_X (\approx 5 \times 10^37 erg s^-1) is roughly as expected from Low Mass X-Ray Binaries (LMXRBs) for an old population of the L_opt of M32. M32 was observed with the ROSAT PSPC for \approx130 ksec. Many pointings are quite short, and one long pointing is very near the edge of the field. There are two useful long pointings totaling \approx60 ksec. We have retrieved these data from the ROSAT Public Archive, and analysed them with XSPEC. The x-ray spectra of LMXRBs are typically well-fit by bremmstrahlung emission with kT\la5 keV. This is a reasonable model: The best-fit bremmstrahlung solution has kT = 2.5^+1.7_-0.9 keV, with \chi^2=35 for 50 d.o.f. We also fit the data with a power-law model. While this gives a marginally worse fit than bremmstrahlung emission, it is still statistically quite good: The best-fit power-law has \alpha = 1.8^+0.7_-0.2, with \chi^2=39 for 50 d.o.f. An \alpha \approx 1.7 power-law is typical of x-ray emission from AGN (e.g. Mushotzky \etal 1993 ARAA 31 717). While there are no other standard observational signatures of an AGN in M32, high resolution optical imaging (Lauer \etal 1992 AJ 104 552) supports the existence of a central black hole of M_\bullet \approx 3 \times 10^6 M_ødot. The flux from ROSAT is 2 \times 10^-12 erg cm^-2 s^-1, implying a flux in the Einstein band that is \approx3 times larger than that observed by Einstein in 1980, arguing that there is substantial source variability. However, both AGN and LMXRBs are known to be x-ray variable. The emission is coincident with the optical center of M32, and does not appear extended in the (short) nearly on-axis pointings. This would appear to argue for a central AGN, however M32 is very centrally concentrated in the optical, and a population of LMXRBs would be expected to follow the optical emission. Finally, although the observed L_X is in accords with expectations for a population of LMXRBs, the total number of such sources required is only 1--5. This is a small enough number that it is quite possible that there are no LMXRBs in M32. Thus either LMXRBs or emission from a central black hole can explain the current observational data for M32.

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