USING ROSAT TO STUDY NORMAL GALAXIES
Eric M. Schlegel
Smithsonian Astrophysical Observatory, email@example.com
I argue that ROSAT can make a substantial contribution to the study of normal galaxies. It should be used to carry out a survey of as many galaxies within 10-20 Mpc as possible. To date, relatively few galaxy observations have been made.
ROSAT has not been heavily used to study normal galaxies, yet the field of normal galaxies may represent one of the largest domains of astrophysics to which ROSAT can still make a substantial contribution. In this article, I briefly review all of the ROSAT observations obtained to date. I then offer some context for the distribution of those observations followed by two examples of possible surprises that await. astrophysical argument.
|Category||No. of Obs.||Fraction||Category||No. of Obs.||Fraction|
CVs = Cataclysmic Variables; XRBs = X-ray Binaries; SNR = Supernova Remnants
While the above statement may seem outrageous, in the text below I hope to persuade you that it is not. The table (next page) lists the number of observations that have been made in each of the subject categories by ROSAT. The observations include both PSPC and HRI data based upon the MPE master list of the observing log dated 11 March 1996 (available in the anonymous FTP account at GSFC, directory rosat/timelines/). The numbers were obtained by a simple script that counted each entry in the listed categories but did not count multiple observations of an identical target and instrument. In other words, long campaigns using a specific instrument to monitor a particular AGN were counted as one observation. The numbers have an approximately 1% error.
The above numbers must be placed in some context, otherwise we will not be able to judge them. Let's take a few categories for additional consideration. I'll confine myself to the categories for which I have some personal experience. I will not make any comments on the uses of ROSAT for studies of stars, clusters, or AGN because I have little experience in those fields.
The XRBs are probably a relatively small fraction of the data base because ROSAT is not the best instrument for studying XRBs. Most XRBs are located in the Galactic plane at moderate to large distances, so the absorbing column is very high. In addition, the model spectra of many XRBs contain dominant hard components that are not well-matched to the spectral response of either the PSPC or the HRI.
Most of the information we will learn from ROSAT about CVs will be obtained from data already published or in the archive. Let me explain carefully what I mean. Imagine that the current year is, say, 2010, and we are looking back on the ``golden years'' of X-ray astronomy that started in 1990 with the launches of ROSAT and BBXRT. If we ask ourselves how much of the total knowledge of CVs that we learned from ROSAT had been obtained by the start of 1996, I'd argue that the fraction most of the community would agree on is a relatively high number, such as 80%. Does this mean that here, at the start of AO-7, we should not observe any additional CVs? Absolutely not. I can, however, speak from personal experience when I say that most of the CVs feasible for study have been observed with ROSAT. Furthermore, since the HRI is the only instrument available on-board ROSAT, studies of CVs will only yield light curves. I think that there will be little additional knowledge that we learn about CVs from HRI light curves. Again, does this mean that no more CVs should be observed? No, because any specific CV may benefit from an HRI light curve. For example, there have been several CVs recently discovered using EUVE. If a fundamental difference exists between EUV-discovered CVs and X-ray-discovered CVs, the CV community would be very interested in uncovering that difference. Only observations would reveal the extent of such a difference. Therefore, we definitely should observe additional CVs. The marginal contribution of the additional CVs will almost certainly not be large, however.
We can look at a recent addition of the remnant catalog of Green (1991). It contains 190 entries; if we assume that each remnant is observed with the HRI and the PSPC, then there should be 380 observations in the ROSAT database. The number counted above is 367. These two numbers are sufficiently close that I'd argue that most of the SNRs have been observed. Should this be interpreted to mean that no more remnant proposals will be accepted by the peer review panels? Absolutely not. There is a consortium that has made, and continues to make, a strong case for a complete map of the Cygnus Loop, one of the closest remnants available to us for study. A complete map will be very valuable for comparison with images in other wavebands and with models. My point is, however, that the vast majority of the known remnants have been observed, so we at least know how those remnants appear in the X-ray band.
Normal galaxies, in contrast to the XRBs, are a good match to ROSAT's capabilities. They have a relatively soft spectrum unless the galaxy is dominated by the emission of many XRBs. Based upon Einstein data, the X-ray emission of most galaxies was believed to be dominated by XRBs (e.g., Fabbiano 1989). I believe that ROSAT will show that SNRs make a substantial contribution.
The arguments applied to CVs and SNRs are definitely not true for normal galaxies. To estimate the fraction of galaxies observed in the X-ray band, I adopt Tully's Nearby Galaxies Catalog (1988), as it is more likely volume-limited, as opposed to the Shapley-Ames catalog which is a magnitude-limited catalog. In the NGC, there are 2367 entries. If we restrict ourselves to galaxies with distances less than 20 Mpc, there are 1180 entries. If we assume half are AGN (likely an overestimate), then there are 590 galaxies available for study. If we observe each galaxy with the PSPC and with the HRI, there should be 1180 entries in the observing log. From the table above, there are approximately one-third that many. This means that, at most, we have an observation of only one-third of the galaxies within 20 Mpc. Contrast this with the Galactic SNRs, or the CVs. It is even possible to contrast this with AGN because the ratio of ROSAT-observed AGN to the total number within 20 Mpc is very likely almost unity. If we restrict ourselves to galaxies within 10 Mpc, there are 400. None should be AGN. If we again assume 1 HRI and 1 PSPC observation per galaxy, there should be 800 observations in the database, whereas the table above shows that there are about one-half. This is an improvement, but it is not, however, sufficiently good that we can state we have a representative understanding of the X-ray behavior of all sources within 10 Mpc. To state it another way, we do not have images of the X-ray emission of many, or most, of our neighbors. All of the large luminous galaxies have been observed (e.g., M31, M33, M51, M81, and M83) but there are still large, bright galaxies that have not yet been observed.
Figure 1: X-ray luminosity functions for several nearby galaxies. The extreme behavior of NGC 1313 is evident.
I will now describe two examples of ``surprises'' that awaited observations with ROSAT. First, the barred spiral galaxy NGC 1313, a ``C'' target in AO-1, has contributed several surprises. In the first PSPC exposure, three sources (X-1, X-2, and X-3) were present while the Einstein observation showed only two. The new source (X-3) is the first supernova identified at X-ray wavelengths (SN1978K) (Ryder et al. 1993; Schlegel, Petre, & Colbert 1996). SN1978K is undergoing circumstellar interaction and it has an X-ray luminosity of >10 ergs s in the ROSAT band. It has the appearance of a young (X-ray) and an old (optical) remnant. No source like SN1978K exists within 8-10 Mpc of our Galaxy. The other two sources visible in the PSPC exposure also have luminosities >10 ergs s , but both remain unidentified. Stocke et al. (1994, 1995) attempted to identify an optical counterpart to X-2, but could not find one brighter than mag 20. On that basis, they concluded that source X-2 was a cooling neutron star. Petre et al. (1994) argued that the ASCA spectrum of the source is not consistent with a cooling neutron star, but could be consistent with black hole. The third source (X-1) remains unidentified. HRI observations have also been obtained; these observations show that source X-1 is actually composed of several sources (probably 3), at least one of which is extremely time-variable (Petre et al. 1996). The figure presents the luminosity distributions of several nearby galaxies, including NGC 1313.
As a second example, I cite the face-on spiral galaxy NGC 6946 (Schlegel 1994a). There is a supernova remnant in that galaxy that is also very luminous (>10 ergs s ) in the ROSAT band (Schlegel 1994b; Blair & Fesen 1994). The closest similar source is the remnant in NGC 4449 (e.g., Blair et al. 1983). Nothing similar is present in any of the nearest galaxies (our Galaxy, M31, M33, or the LMC). A recent HST image (the PC portion of the WFPC2) shows what might be a multiple-SN event: there are several arcs of emission (Blair, Fesen, & Schlegel 1996). NGC 6946 also reveals a hot component to its interstellar medium. This component appears to follow the spiral arms and appears to be inconsistent with the emission from unresolved point sources. An HRI observation appears to confirm this result (Schlegel et al. 1996). A very hot ISM component is detected in the LMC (Wang et al. 1991), but not in the Galaxy, M31, or M33. The investigation of this component, including its correlation with other galaxy parameters, will shed light on the energy input from supernovae and high-mass clusters of stars. That information will be useful in understanding the dynamics of galactic halos.
The ROSAT HRI offers us the only wide-field ( ) imager that has a good sensitivity, a low internal background, good positional resolution, and, importantly, it is available. The prime observing phase of ROSAT is over; it is time to use ROSAT for large surveys of the sort that AXAF will be unable to carry out.
Observations of normal galaxies will sample the behavior of sources under conditions similar to those in our Galaxy as well as conditions that are very different. This will allow us to investigate source behavior across a greater range of behavior than has been possible to date. Once we have used ROSAT to observe the nearby galaxies, we will have a definitive catalog for investigations using AXAF, Astro-E, or XMM. Should ROSAT be devoted exclusively to galaxies? No, because time-variable studies of AGN or stars will also contribute considerably to our astrophysical understanding. The observation of normal galaxies, however, should be considered very important. We should at least know the appearance of all of our neighbors.
Fabbiano, G. 1989, ARAA, 27, 87
Blair, W., Fesen, R., & Schlegel, Eric M. 1996, in preparation
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