EUV astronomy in the 21st century

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11.1 Looking back

The astrophysical research discipline we now know as Extreme Ultraviolet astronomy is approximately 30 years old. An observational technique once dismissed as impossible has become established as a significant branch of space astronomy and a major contributor to our knowledge of the Universe. In several areas, the science obtained from EUV observations is unique. For example, the presence of the He II Lyman series in this spectral range provides a diagnostic tool for the study of the second most abundant element in the Universe in the atmospheres of hot stars and in interstellar space. The determination of the ionisation fraction of helium in the local ISM could not have been carried out in any other spectral range.

EUV astronomy has passed through the development phases that might be deemed typical of a discipline depending on access to space. Beginning with the sounding rocket borne experiments of the early 1970s, the longer duration Apollo-Soyuz Test Project highlighted the potential of the field, with the first reported source detections in 1975. However, it was a further 15 years before the next major advance with the first EUV all-sky survey of the ROSATWFC(in 1990) followed by the wider spectral coverage of the EUVE survey in 1992. The underlying reasons for this hiatus had more to do with national and international politics, together with the economics of funding opportunities and even the launch delays following the Challenger disaster, than technological limitations. However, the ROSAT and EUVE missions were certainly able to take advantage of the development in mirrors, detectors and filters that were made during the late 1970s and early 1980s, providing instrumentation of greater sensitivity than might have been possible had the surveys been carried out earlier. It is notable that the first EUV sky surveys were performed with imaging telescopes, in contrast to the first X-ray surveys. Indeed, the first imaging X-ray survey was that carried out by the ROSAT mission, with the WFC on board.

The level of development in the WFC and EUVE instrumentation arising from all the delays may, in the end, have placed the future of EUV astronomy on a firmer footing than would have been possible earlier. In particular, had the ROSAT WFC been launched in 1986, as originally planned, the telescope would have carried thicker thin film filters than were ultimately flown. As a consequence, the achieved sensitivity would have been significantly lower and the number of sources detected much smaller - probably less than one quarter of the published survey catalogues. At this level, with «25 white dwarfs, and «75 late-type stars, the statistical basis of the studies of these populations would have been seriously undermined. Furthermore, of the less populous source categories such as CVs and AGN, only a very few examples would have been detected. In the event, the «1000 now known EUV sources have provided a rich scientific output, forming a substantial basis for future EUV astronomy.

A foresighted enhancement of the EUVE mission, made possible by the long delays to the project, with the addition of a spectroscopic capability, has played a major role in defining what the future of EUV astronomy should be. This particular instrument has enabled «0.5-1.0 A spectroscopy to be carried out on almost all the brightest EUV sources, allowing the study of the composition of white dwarf photospheres, stellar coronae, density and ionisation of the local interstellar medium and emission mechanisms in CVs and AGN. Without this, the next step in EUV astronomy would have been the first spectroscopic instrument and a consequent delay to the rapid progress that has been made during the past ten years.

11.2 Limitations

Although the ROSAT WFC and EUVE missions can be claimed as outstanding successes, their telescopes were of comparatively limited aperture. Typical effective areas for the survey instruments ranged from «10-20 cm2, while the peak effective area of the EUVE spectrometer was «1 cm2. These figures can be contrasted with the >200 cm2 effective area of the ROSAT XRT with its PSPC detector. Clearly, it would be desirable to construct new EUV instrumentation with much higher effective areas than already flown.

The practicality of enhanced EUV astronomy missions depends on a combination of science, politics, funding and technology. X-ray astronomy has an impressive track record of continuing development of larger and larger telescope collecting areas accommodated on satellites of increasing size and complexity. Continuity of these missions seems to be assured well into the future. It would be nice to enjoy a similar situation in the field of EUV astronomy.

Unfortunately, the nature of the subject of EUV astronomy encompasses a few problems that make it more difficult to justify the magnitude of expenditure required by the large X-ray observatories. The principal limitation is the absorption by the interstellar medium that defines the horizon within which sources can be observed. While, as seen from the all-sky surveys, there are a few 'windows' in the galactic ISM through which extragalactic sources can be observed, these are small in number. Therefore, although in principle EUV observations are important for many astrophysical environments, they are mostly restricted to a local region of our galaxy. The source environment may play an additional role, since absorption by surrounding material may prevent the escape of EUV photons in detectable quantities. Since EUV astronomy is going to be of limited interest to non-stellar astronomers, except for studies of the ISM, it may never be possible to justify an expenditure on future missions similar to the large X-ray or UV observatories.

Nevertheless, it is clear from the content of this book that future EUV observations will be tremendously important for the study of white dwarfs, stellar coronal plasma, CVs and the local ISM. Hence, following the end of the ROSAT WFC and EUVE missions it is important to consider what new instrumentation should be developed and flown. However, such instrumentation will probably need to fit within the cost envelopes of the smaller missions rather than observatory class satellites, implying that the payloads will not be much more massive than those already flown.

11.3 New EUV science

Two general aims can probably be stated for EUV astronomy, although these must be justified in detail for individual science goals. The first is a requirement for greater effective areas to allow the observation of fainter sources. The second is the need for higher resolution spectroscopy than available from EUVE. While greater source sensitivity is a goal that can exist in isolation, increased collecting area is an essential adjunct to improving spectral resolution, to maintain the signal-to-noise.

11.3.1 A deeper sky survey

Approximately 1100 sources of EUV radiation are now catalogued, residing mainly in the region of the galaxy defined by the local bubble (see chapter 7). The bubble defines a volume beyond which the interstellar column density increases so rapidly that very few sources are detectable. Therefore, a new, more sensitive survey would probably be limited to searching for less luminous sources than those currently known out to distances of 100-200 pc. There are certainly many nearby stars that were not detected by the WFC that might be revealed in such a survey (see section 3.8). Only 25% of all stars within 10 pc were detected by the WFC and fewer than 5% of all those in the CNS3 nearby star catalogue (distance limit 25 pc).

One of the main surprises from both main sky surveys was the low number of white dwarf detections («100) compared with the 1000-2000 expected from earlier predictions. This was eventually interpreted as arising from the presence of heavy elements in the photospheres of those stars with effective temperatures above «40 000 K (see section 3.6.2). However, those hotter DA white dwarfs that were detected do not necessarily represent the most extreme cases of heavy element opacity. Therefore, the existing survey data have only probed the tip of what must be a large iceberg in studying the pattern of photospheric opacity across the population of hot white dwarfs.

11.3.2 High resolution EUV spectroscopy

The spectral resolution of EUVE ranged from 0.4 A at the shortest wavelengths to 2.5 A at the longest. While enormously superior to the very limited spectral information available from broadband photometry, this resolution places restrictions on the scientific information that can be extracted. Resolving powers (R = X/AX) in the range 10 000-30 000 have been available routinely in the far-UV with IUE and HST. With these instruments it has been possible, for example, to resolve the individual heavy element absorption lines in stellar atmospheres, resolve multiple components of interstellar absorption lines and study the dynamics of stellar systems through radial velocity variations. As EUV observations provide some unique information about a number of astronomical objects, it will be important to develop a similar spectroscopic capability in this band.

An important diagnostic measurement to be made on any spectrum is the radial velocity of an individual absorption line, with respect to the observer. Differences in line velocities can allow the discrimination of stellar and interstellar components or chart dynamical motions of the components of a stellar system or motions of gas within it. Hence, the important point is how a particular spectral resolution translates into velocity space. A resolving power of 10 000 is equivalent to a velocity of 30kms-1, equal to 0.15 ¿A at a far-UV wavelength of 1500 A. In the EUV, this velocity discrimination corresponds to a wavelength resolution of 0.015 A at 150 A.

To define what performance may be required by a future EUV spectrometer, it is necessary to look at what current astrophysical problems may be addressed particularly by EUV observations and what resolution is needed to solve them. A good guide is to consider the questions left unanswered by the EUVE observations discussed in this text.

11.3.3 Photospheric opacity in white dwarf atmospheres

EUV astronomy has clearly played an important role in understanding the presence of opacity sources in the atmospheres of hot white dwarfs, in the H-rich DA stars in particular (see section 3.6, chapter 8). In conjunction with far-UV observations, EUV spectra have proved to be important diagnostic tools for determining general abundance levels (e.g. Lanz et al. 1996) and, uniquely, for studying the depth dependence of the absorbing material (e.g. Holberg et al. 1999a; Barstow et al. 1999). However, the limited spectral resolution of EUVE was unable to resolve the individual absorption lines in the EUV; those features that could be seen were in fact blends of large numbers of individual lines. This presents a particular problem in understanding the role of helium, whether in the photosphere or ISM, in those hot DAs which have significant quantities of heavy elements in their atmospheres. While, in principle, the He Lyman series lines should be detectable in the EUV spectra, the large number of other absorption features can easily mask their presence. Furthermore, although a component of He II is needed to completely explain the EUV spectral shape of G191-B2B, it is not known whether this is photospheric, circumstellar or interstellar.

11.3.4 Stellar coronae and coronal dynamics

A large number of coronal spectra were obtained by EUVE. The identification of many individual emission lines associated with high atomic ionisation stages provided important diagnoses of the structure and composition of the hot plasma (see chapter 6). Since the coronal plasma features are in the form of emission lines, set against a low level or even negligible continuum flux, the EUVE spectral resolution is less problematic, with all the most prominent emission lines being well separated in most cases. Only a few lines are blended (see table 5.3). However, these data are limited in other ways. The coronal sources are much less luminous than the white dwarfs. Even though the fluxes are confined to a number of narrow emission lines, long exposures have been required to obtain spectra of sufficient signal-to-noise for detailed analysis. However, coronal sources are highly variable, either through the natural cycles similar to the variability of activity on the Sun (including stellar flares), by rotational modulation of emitting regions or by interaction and eclipses in active binary systems. Hence, in most spectroscopic observations only the global time averaged coronae have been examined, compared to the shorter time scale variability.

Many of the most coronally active stars are in binary systems. In principle, the spectra of the individual stars could be separated by their relative Doppler shifts but this was not possible with EUVE. Furthermore, the long exposures often required (compared with the binary orbital periods) will have tended to smear out the line emission components in any case. In single stars, where rotational velocities are high, changing Doppler shifts of the emission from localised active regions would be lost within the EUVE spectral resolving power. Even if stellar motions are too slow for dynamical Doppler effects to be important, there should be information in the Doppler width of the emission lines, if this could be resolved.

11.3.5 Cataclysmic variables and related objects

The issues here are similar in a number of ways to the limitation of coronal source observations. Like these objects, CV-type objects are intrinsically faint in the EUV but are also highly variable objects, exhibiting outbursts, spin/rotation-modulated emission and flickering effects. Apart from the long duration flux changes associated with outbursts, the exposures required for good s/n spectra average out all the other effects. Some CV spectra contain what appear to be broad emission lines. However, it is not clear whether these are true emission features or artifacts produced by large numbers of blended absorption features. Higher resolution observations are essential to solve this problem. Acquiring exposures over shorter time-scales is also essential to limit any smearing affects associated with the dynamics of these systems, which otherwise limit the resolution that can be achieved.

11.3.6 Interstellar medium

EUV studies of the ISM have concentrated on the detection of absorption edges of He I and He II, with analysis of the continuum absorption by H (see chapter 7). The absorption edges are actually composed of a series of converging absorption lines, which give a rounded shape to the edge at the resolution of EUVE. However, in principle, the width of the lines has information about the temperature of the absorbing gas. The analysis of the EUVE data is not very sensitive to assumptions about the value of the Doppler broadening parameter, but high spectral resolution would make measurement of this feasible.

Interpretation of all the ISM results recorded with EUVE has to assume that all the absorbing gas is associated with a single component. However, this is not necessarily true. Far-UV observations often show ISM absorption at several different velocities that might be associated with discrete interstellar clouds. The work of Barstow et al. (1997) indicates that the total column density measured along the lines-of-sight to several white dwarfs is larger than might be expected if the absorbing material was primarily associated with the local interstellar cloud. Therefore, a higher resolution may well reveal multiple absorbing clouds of helium along these lines-of-sight.

It had been anticipated that, apart from the detection of the He resonance absorption edges, other interstellar absorption lines would be detected in the EUVE spectra. However, apart from the discovery of the He I autoionisation transitions (Rumph et al. 1994), no such lines have been detected. Interstellar lines are intrinsically very narrow, and convolution with the EUVE spectral response would weaken them with respect to the photospheric continuum against which they are viewed. Many of the interstellar absorption lines that might be observed in the EUV are associated with hot gas and could provide important information about the ionisation balance of the local cloud and bubble. If viewed against more distance objects, such as AGN, it should be possible to study the hot halo believed to surround our own galaxy. A particularly exciting scientific programme would be to search for absorption associated with the He3 isotope in the local ISM, to study the He3:He4 ratio, which is an important cosmological parameter.

The above discussion places its emphasis on probing the ISM by studying its absorption on the spectra from distant objects. However, it is also possible to detect the emission from hot gas directly. The EUVE spectrometers were optimised to study point sources rather than diffuse emission and were of limited use in studying the hot interstellar gas. The Cosmic Hot Interstellar Plasma Spectrometer (CHIPS, figure 11.1) is a University-Class Explorer (UNEX) mission funded by NASA. It will carry out all-sky spectroscopy of the diffuse background at wavelengths from 90 to 260 A with a peak spectral resolution of A/150. CHIPS data will help scientists determine the electron temperature, ionisation conditions, and cooling mechanisms of the million-degree plasma believed to fill the local interstellar bubble. The majority of the luminosity from diffuse million-degree plasma is expected to emerge in the poorly explored CHIPS band, making CHIPS data of relevance in a wide variety of galactic and extragalactic

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Fig. 11.1. Three-dimensional layout of the CHIPS spectrograph (courtesy M. Hurwitz).

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Fig. 11.1. Three-dimensional layout of the CHIPS spectrograph (courtesy M. Hurwitz).

astrophysical environments. The CHIPS instrument will be carried into space on board a dedicated spacecraft and is expected to be launched in 2002.

11.3.7 Requirements for new EUV instrumentation

Astronomers always require bigger and better telescopes for the next phase of their research. A corollary of this statement is that what astronomers actually get is a compromise between the available resources and the ideal, reduced to the minimum required to make a large enough scientific advance to justify the expenditure. The scientific questions/problems outlined in the preceding sections allow us to define such minimum requirements of the next generation of instrumentation. From the point of view of spectral resolution, a factor of 20 improvement (to R « 4000 on the resolution available with EUVE is needed to resolve the heavy element complexes in white dwarf atmospheres. With a corresponding velocity discrimination of 75 km s-1, this would not be adequate to separate ISM and photospheric absorption features on velocity grounds, where differences are typically a few to a few tens of kms-1. It would probably be possible to determine the nature of any He II absorption lines found, as those in the photosphere will be broadened compared to the ISM. However, to truly resolve ISM and photospheric components will require a further factor of ten improvement in spectral resolution to R « 40 000, similar to that available with the current generation of far-UV instrumentation.

Similar levels of performance to those discussed above will also be needed to obtain dynamical information. For example, a velocity resolution of 75 km s-1 will allow the study of relatively close binary systems such as CVs or pre-CVs, which have radial velocity amplitudes of a few hundred km s-1. However, in those systems without a compact object the radial velocities will usually be much lower, requiring a correspondingly higher resolution.

While the resolution requirements are straightforward, depending directly on the scientific goals, it is more difficult to specify the effective area demands without carrying out detailed simulations for specific categories of source. In simplistic terms, if the spectral resolution is enhanced by a particular factor, the effective area must increase by the same amount to maintain the signal-to-noise in each pixel. However, when the spectral resolution is improved for a system of otherwise unresolved lines, the contrast of the line cores to continuum level is enhanced, so scientific goals may still be achieved without an automatic enlargement of the effective area. An increased ability to carry out time resolved studies, where the exposure times are determined by the time scale of the phenomenon being studied, is much more dependent on improvements in telescope effective area. Unlike the exposures of non-variable sources, any decrease in signal-to-noise commensurate with increased dispersion cannot be counteracted by increasing the exposure time.

11.4 Advanced instrumentation for EUV astronomy

The future of EUV astronomy depends significantly on advances in technology. Development of grating technology is needed to improve the achievable spectral resolution, coupled with improvements in efficiency as a contribution to enhancing the overall effective area. Similarly detector spatial resolution needs to evolve to accommodate the better spectral resolution. However, there does not currently seem to be much prospect of increasing detector quantum efficiencies. A primary limitation on the achievable effective area is the mirror technology. As in X-ray astronomy, the use of grazing incidence telescopes, like those employed on the ROSAT WFC and EUVE is the standard technique. Increasing the effective area significantly implies larger, more massive telescope systems, which must then be carried on board larger, more expensive satellites. As discussed earlier, the X-ray astronomy route of increasingly large observatory class missions is probably not practical for EUV astronomy. Therefore, more innovative approaches to mirror design are required.

It is well understood that grazing incidence systems are relatively inefficient light collectors, a single mirror only intercepting light in a thin annulus subtended by the leading element, when compared with the use of the full aperture by normal incidence telescopes. The ALEXIS mission has pointed the way forward by being the first experiment to use multilayer normal incidence optics for non-solar EUV astronomy. As discussed in section 4.4.1, the penalty for improved effective area, produced by the multilayers, is a decrease in the practical wavelength coverage. However, it is possible to envisage the production of low cost, highly efficient, EUV instruments that have wavelength range tuned to addressing particular astrophysical problems. We discuss here recent developments that have adopted this approach in producing the next generation of instruments.

11.4.1 The Joint Plasmadynamic Experiment

The Joint Plasmadynamic Experiment (J-PEX) was a collaborative project, led by the US Naval Research Laboratory (NRL) with contributions from the US Lawrence Livermore National Laboratory (LLNL), University of Leicester UK and the Mullard Space Science Laboratory (MSSL) of University College London. Its main aim was to answer the question regarding the presence of He II in the spectrum of G191-B2B, by resolving the individual He lines from the other heavier element species. As discussed in section 11.3.7, this requires an instrument with a resolving power ^X/AX = 5000, higher than previous instrumentation has delivered.

Fig. 11.2. Single J-PEX diffraction grating manufactured by Carl Zeiss.

The J-PEX experiment was a sounding rocket borne telescope, one of the first in a new generation of instruments employing optics operating at near-normal incidence. The conventional grazing incidence mirrors found in earlier instruments were replaced by MoSi multilayer coated reflection gratings which acted as both dispersion and light collection elements (Kowalski et al. 1999). Manufactured by Carl Zeiss, using multilayer coatings developed at NRL and LLNL, four gratings (figure 11.2) had a total geometric area of 512 cm2 and had a spherical figure with a 2 m focal length. This arrangement enabled J-PEX to achieve an effective area in excess of 5 cm2 and a resolving power of «4000 at 235 A, more than ten times that of EUVE. The optical layout of the telescope is shown in figure 11.3.

The diffracted radiation was focused onto a microchannel plate detector, mounted along the grating optical axis to minimise optical aberrations. Developed and built at the University of Leicester, the detector design consisted of two 37 mm square Photonis MCPs with 6 ^m pores, mounted in a chevron configuration. The front MCP was coated with a Csl photocathode to enhance detection efficiency. Imaging was by means of a progressive geometry vernier conductive anode readout which, along with the electronics and HV supply, has been developed and supplied by MSSL. The entire detector assembly was mounted in its own vacuum chamber with an integral ion pump, to allow operation of the detector on the ground and protect the photocathode. In the flight configuration, the front MPC was replaced by a circular 33 mm diameter IKI plate, having 10 ^m pores, due to problems with the quantum efficiency of the Photonis MCPs (see Bannister 2001). The spatial resolution achieved by the detector was better than 20 ^m (Bannister et al. 2000b; Lapington et al. 2000).

The J-PEX instrument was a slitless spectrometer. Hence, the absolute pointing requirements were not very critical. However, to achieve the high spectral resolution, the attitude drift needed to be very stable to avoid smearing of the spectral features in the dispersion direction. A new digital attitude control system (ACS) was utilised, coupled with an optical telescope to track residual drifts. The optical telescope consisted of a 12.5 cm diameter, 2 m focal length spherical mirror and a CCD detector. By imaging the target throughout the entire

Fig. 11.3. Schematic view of the J-PEX high resolution spectrometer payload, designed and constructed for launch by a Terrier boosted Black Brant IX sounding rocket.

period of observation, this optical telescope ensures that post-flight aspect reconstruction can be performed, thus recovering resolution. A normal incidence multilayer EUV imaging telescope, coaligned with the spectrometer and focused on the spectrometer MCP detector, provided a backup system for attitude reconstruction by monitoring the position of the EUV target image.

The primary goal of J-PEX was to acquire an EUV spectrum of sufficient resolution to separate lines of He II, if present, from the large numbers of features from heavier elements such as C, N, O, Fe and Ni. The measured resolution of the instrument matched that required for this task. Furthermore, by analysing the width and depth of any He II features, the location of the material (photospheric or circumstellar/interstellar) can be determined.

J-PEX was first flown from White Sands Missile Range (WSMR) on a two-stage Black Brandt IX sounding rocket on February 25 2000, with an expected exposure time «300 s. Unfortunately, the rocket veered off course and had to be destroyed shortly before burn-out. However, although the instrument pointing could not be controlled and no science data were acquired, the telescope operated correctly and was recovered intact. The telescope was flown for a second time on February 21 2001 (NASA 36.195), completing its mission flawlessly.

Analysis of the J-PEX high resolution EUV spectrum of G191-B2B reveals some exciting results when comparing the observed spectrum (error bars in figure 11.4) with predictions based on a homogeneous composition stellar atmosphere and including interstellar H I, He I and He II absorption. The H I and He I column densities were fixed at values obtained from analysis of the broader band, lower resolution EUVE spectrum and the temperature and gravity taken from the latest optical analysis (T = 54 000 K, log g = 7.5; see Barstow et al. 1999 and 1998 respectively). The interstellar He II column density and photospheric He abundance were allowed to vary freely and the best match to the model obtained by a x2 minimisation technique. The best fit model folded through the J-PEX instrument response, assuming a nominal 0.05 A resolution (fwhm), is shown in figure 11.4 (solid line histogram).

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Fig. 11.4. (a) The spectrum of G191-B2B obtained by the J-PEX spectrometer, spanning the wavelength range 222-234 A (error bars). The solid histogram is the best fit theoretical model of the star and ISM, as described in the text. The strongest predicted lines of He, C, N, O, and P are labelled with their ionisation state and wavelength. Lines of Fe and Ni are too numerous to include and account for unlabelled individual features and broader absorption structures. (b) As (a) but spanning the wavelength range 232-244 A.

236 238 240

Wavelength [A]

Fig. 11.4. (a) The spectrum of G191-B2B obtained by the J-PEX spectrometer, spanning the wavelength range 222-234 A (error bars). The solid histogram is the best fit theoretical model of the star and ISM, as described in the text. The strongest predicted lines of He, C, N, O, and P are labelled with their ionisation state and wavelength. Lines of Fe and Ni are too numerous to include and account for unlabelled individual features and broader absorption structures. (b) As (a) but spanning the wavelength range 232-244 A.

The good agreement between model and data is striking, with the most prominent resolved feature a strong OIV absorption line at 233.5 A. Many other features are present, which are mainly blends of large numbers of Fe V and Ni V lines. Between «227 and 232 ¿A is a broad 'bump' which is a characteristic of the overlapping series of interstellar He II absorption lines superposed on a continuum. Taken with the strong depression of the flux below 227 A, this provides conclusive proof that interstellar or circumstellar He II is present along the line-of-sight. With a total column density of 3.8 x 1017 atoms cm-2, the implied He ionisation fraction of «0.7 is substantially higher than typical of the LISM (0.25-0.5). However, a circumstellar C IV component in the far-UV has recently been identified (Bannister et al. 2001a) and suggesting that part of the ionised He II may be circumstellar. The best fit model spectrum implies that there is also photospheric helium present. However, the strongest

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