The status of the newly born field of stellar X-ray astronomy is considered. The need for further observational capability, the study of stellar surface activity and the development of design criteria for a follow-up mission, with primary emphasis on stellar observations and with general capabilities within the scope of the Explorer program are examined. The criteria which lead to the desired instrumental capabilities are reviewed; and an overview of the proposed instrument and some of its capabilities, as indicated by simulations is presented. Two versions of a trial payload the stellar coronal. Explorer (STCOEX) which meet the observational desirable are developed

Vaiana, G. S.

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The Stellar Coronal X-ray Explorer: STCOEX 
G. S. Vaiana 
Harvard-Smithsonian Center for Astrophysics 
A. Introduction. 
With the end of the observational phase of Einstein Obseri dory mission, it 
behooves us to consider the status of the newlyborn field of stellar x-ray astronomy. 
Although Einstein clearly allowed a quantum leap over previous observations of x-ray emis- 
sion from stars (Vaiana e2 al. lQSl), its relatively short lifespan allowed only a fraction of 
the variety of projects, developed in response to this instrument% capabilities, to be corn 
pleted. In response to the consequent perceived need for further observational capabil- 
ity, we have conducted a overview of the available Einstein data for the purpose of 
deciding which projects will boZh in all probability not be completed by future analysis of 
the Einstein data and, in addltion, seem to us to be of particular importance for the study 
of stellar surface "activitya; a natural outgrowth of this analysis was the development of 
design criteria for a followup mission to Einstein, one whose primary emphasis would be on 
stellar observations, and whose general capabilities lay within the scope of the NASA 
Explorer program. In the fdlowing, I will attempt to brielly review the criteria which lead 
us to the desired instrumental capabihYes 6 8); and pnwide an overview of the proposed 
instrument and some of its capabilities, as indicated by our simulatlons fi C). 
B. Scientific Goals and Design Criteria. 
One of the remarkable aspects of the stellar observations made possible by the Ein- 
stein Observetory was their serendipity: the instrumentation and the observing plans were 
not optimized for stellar observation, but rather for primarily extragalactic studies. In 
spite of this lack of optimization, well in excess of a third of all guest observer programs 
focussed on stars; and the stellar survey constituted one of the major research areas of 
the Consortium groups. What are some of the principal scientffic problems, and to what 
extent have they been resolved? In ths following, I will brielly recap some of the major 
issues, and attempt to define the work wtich remains to be done. 
1. Dependence of C o r d  Plasma Paremeters on the Characteristics 01' the Wnderly- 
ing' Star. The Einstein observations have enabled us to sketch in broad outline the x-ray 
emission pioperties of stars throughout the H R  diagram; however, in many portions of the 
K R  diagram, the data is relatively sparse (because of severe restrictions on the allowable 
&sewing times!. In particular, it has not been possible to assemble uniform, unbiased 
(vir. volumslimited) samples for every spectral types and luminosity classes. In conse 
quence, phenomenological studies of the variation of x-ray luminosity with stellar parame 
ters 8uch as rotation and mass loss rate may remain biased by selection criterla for 
several stellar categories because (ai  least in the beginning of the mission) emphasis was 
largely placed on targets most likely to be x-ray sources. More severe difftculties beset 
the plasma dlagnostlcs (such as temperature analysis). the strong constraints on observ- 
ing time allowed only e very limited sample of stars to be examined by the higher 
resolutlon spectraneters on Einstein, such as the OGS and the SSS. Thus, questions 
regarding possible changes In spectral characteristics along the 'dividing line' separating 
low and hlgh mas8 logs latetype evolved stars (cf. Ayres el a/. 1981); soft x-ray absorp 
tton and possible muttiple temperature component structure in 08 stars; and flare spectros- 
copy, may all be only partially resolved. Furthermore, the quite limited sensitivity of rela- 
tively high resolutlon 8pectto;rcopy (x/u > 100) on Einstein prevented any study of line 
broadening 01 doppler shift effects; the latter 1s particularly of Interest In the case of 08 
https://ntrs.nasa.gov/search.jsp?R=19820018197 2020-03-21T07:35:42+00:00Z
37 2 
stars, since some models predict rapid outflow of the hot plasma (which may be obsew 
able wi.h A/bA - 300-1000). The observation requirements thus call for pointings with 
point source sensitivities of several times 1 6" erg/sec/cm*, in conjunction with spec- 
troscopic struments ca able of resolution In excess of 100, and with line sensitivities of 
2. Temporal Variability. Again because of observing time constraints, virtually all tern 
poral variability data for Einstein stellar targets are restricted to the few thousand 
seconds of contiguous observing time; in only a few select cases (for example, for some 
flare stars and OB associations) are data spanning substantially longer intervals available - and virtually without exception, the longer data base showed evidence for substantial 
variability. It is evident that 'coronal' emission from stars is highly variable on a wide 
range of time scales, from seconds or less (flares), to days and weeks (viz. rotational 
modulation), to months and possibly longer time scales (cycles?). Such Variability in stellar 
'activity' is seen by ground-based observers (e.g., in Ca Ii), but only in a very restricted 
portion of the KR diagram: in contrast, x-rays allow us to look for activity modulation (via 
rotation and/or cycles) in stars Independent of spectra! type or luminosity class, including 
stars in the spectral range 0 - A. T se observations therefore call for the ability to 
carry out e*-tended obs rvations (> 10 sec) which can be repeated on time scales rang 
ing from 10 sec to 10 sec. 
3. Galactic Effects of Stellaf Activity. In addition tc being a major research area in its 
own right. stellar x-ray astronomy forms a new bridge between the stellar astronomers and 
the galactic and extragalactic astronomers: the radiative and particulate emission of 
active stars may well contribute signiflcantly to the radiation fleid and matter content of 
the ga!axy. For example, Rosner et a/. (1 981 ) have shown that latetype dwarf stars carr 
tribute a not insignificant amount of radiation to the galactic component of the diffuse soft 
x-ray background between - 0.28-1.0 keV. Because of Einstein's spectral coverage, the 
stellar contribution at lower energies is not well established (as It depends sensitively on 
the assumed 'coronal' temperature). Furthermore, the limited effective spectral resolution 
did not permit relative elemental abundance analysis, such analysis is of great interest for 
studies of the elemental composition of stellar mass input to the ISM (as it is known from 
solar observations that the photospheric and con>nal/wind abundances are not identical). 
The necessary observational capability to conduct such studies corresponds to that of 8 
9.1 above. 
4. Desired instrument Characteristics. In uder to  place the instrument discussion on 
concrete grounds, we shall adopt the following desirata. 
(i) Energy Range: - 10-200 A; this range encompasses virtually all of the 
significant spectral lines of interest to coronal observations. 
(ii) Optics: Imaging, with spatial resolution better than - 1'; such resolution is 
required both for accurate target identification and for optimal signaktenoise. 
(iii) LhWctors: Photon-counting (i.e., IPC or HRi-like); this requirement follows from 
the necessity of observing flares on sub-second time scale; (so +hat, for exam 
pie, read-out ndse of integrating detectors becomes a significant liability). 
Note, however, tha: CCD detectors nay be preferable for extr6me:yhigh sen+ 
tivit) ..- Jervations for which big;, remporal resolutlon is not reqr*'red. 
(iv) Spectroscopy: High thrxqhput, with resolution in excesd of 100 (300 for OB 
star wind Jiagwgtics), calls for transmission 01 reflection grat,.igs. 
(v) Sensic,uity: Assuming that of order 1r30 pointings (with spectroscopy) are to  
be carried out on a time sca '!ears, with sufffcient sensitivity tu probe 
stars dowr to a level of . O ' o ~ ~ / s a  : at distances of order M) pc (these 
numbers will yield a sufficientkt large samplq to carry out statisr.cal analysis of 
tuminac'ty functi ns throughout the K R  diagr 1, one requires a wint source 
aansitivity @ 10 sec approximately 3-4 times that of €instsit,. 
order loeia erg/sec/cm s . 
?? 
'd 8 
8 
C. Instrumevt Chat8cteristlx~. 
I 
373 ' 
Following the above considerations, we have recently developed two verslons of a 
trial payload (the STellar COronal Explorer) which meets in essentially all respects the 
observational deslrata. Perhaps the most crucial aspect of the design is that the Instru- 
ment is dediccrted to stellar observations. This point is not to be slighted: the character 
of stellar observlng programs (i.e., large numbers of very extended pointlngs, with sub- 
stantlal degree of repetition) is such that it severely conflicts with the requirements of, 
for example, extragalactic observations; in this circumstance, comblned missions may well 
be false aconomy as the principal scientific goals (which presumably underly such ven- 
tures) may not be reallzed. This emphasis on dedication, however, does not exclude the 
possibility of including nmstellar programs whose observing strategies do not overtly 
conflict with stellar observations (such as 0bserva:ions of galactic highluminosity x-ray 
sources). 
Figure 1 shows a schematic of the 'simpler' instrument; the optics consist of a nested 
pair of classical paraboloid-hyperboloid mirror assemblies, together with a movable objec 
tive transmission grating and an HRI and imaging proportional counter (PSPC) in the focal 
plane (mounted on a stage so that the entire dispersed spectrum may be explored); the 
more *complex* design simply calls for two additional nested mirror assemblies, which 
results in a rough doubling of the instrument effective area. The physical characteristics 
of the instruments and the relevant seiisitivities in both the imaging and spectroscopic 
modes are given in Tables 1 and 2; and graphs of the total effective area of the instrv 
ment In the high resolution mode and of the resolving power are shown in Figures 2 and 3. 
It Is to be noted that these results are not speculations, but rather reflect the current 
statsof-thsart in detector and transmission grating technology: no development is 
required to obtain such capabilities. 
In order to  appreciate the power of such an instrument, we show first the range of 
spectral lines (and soutce temperatures) accessible to observatlon (Table 3): the 
STCOEX can observe coronal line emission from plasmc in the temperature range 6.6 < log 
T < 7.5; the lines chosen take advantage of the spectral sensitivity of the instrument, 
and so are not just marginally detectable. A more revealing analysis is to fully simulate a 
typical obserration. Consgijqan 9ervat ion of an RS CVn star at -60 pc, whose volume 
emission measure is -10 cm ; the source spectrum is shown in Figure 4 (using 
recent Raymond 1980 calculations with sdar abundances). We have simulated II STCOEX 
observation by folding this source spectrum through the Instrument response (unless other- 
wise stated, for the 2-mirror system), including the mirror reflectivities, the grating 
transmission, the detector sensitivities, and the instrument resolution (including mirror pfr 
and abberations fzii a coma-corrected grating). If detector noise is nd taken into account 
4 (but photon statistics are), one obtains the spectrum shown in Figure 5 in 10 sec; adding 
detector noise (we assume a background of 1 ct/sec over the entire HRI) th n leads to 
the spectrum shown in Figure d. Comparing Figs. 4 and 6, we see that a 10 sec expo 
sure is sufficient to allow one to resolve most of the strong lines in the source spectrum; 
however, the limited count statistics would not permit sensible line ratlos to be calculated. 
If one goes to longer exposure times, &his restriction Is completely eliminated: Figure 7 
shows what can be accomplished in 10 sec, using the 4-mirror system; again. both pho 
ton statistics and detector noise have been taken into account in the simulation. Spec- 
troscopy of such resolution and sensitivity, in combination with the capability for dedicated 
observations, will clearly allow the observational goals outlined above to be met. 
Support of the above studies by NASA grant NSG 7176 is acknowledged. 
8 
Bibliography. 
Ayres, T. R., Linsky, J. L., Vaiana, G. S., Golub, L., and Rosner, R. 1981, Ap. J. (in press). 
Pallavicini, R., Goiub, L., Rosner, R.. Vaiana, G. S., Ayres, T. R., and Linsky, J. L. 1981, Ab. 
J., 248, 278. 
374 
Rosner, R. et el. 1981, Ap. J. (Leners). 249, L5. 
Vaiana, G. S. et al. 1981, Ap. J., 246, 163. 
t.;,3f?iAL PkGE IS 
Of POOR QUALITY 
Figure 1: Schematic of the STCQEX, showing the principal instrumentation. Only the 2- 
mirror system is shown; an rdternatlve design calls for 2 additional concentric mirror 
assemblies. 
375 
Figure 2: Total effective area of the 2-mlrror STCOW design, including mirror 
reflcctivitfes, transmission grating performance, and detector sensitivity. We show 
results for the HRI with either a Beryllium (solid) or organic (dashed) filter, and for 
the imaging proportional counter (PSPC; solid/dashed). Note that the grating spec- 
tral .'esolution with the high-sensitivity PSPC would be comparable to or somewhat 
better than that of the Einstein SSS. 
200, , . , . , . , , , .. l . , ' , . 1 . , . ,  1 
J 
---. 
O J , .  ( . ,  . ( , ,  , IO 20 30 40 50 60 70 , - ,  80 90 I . ( . ( .  100 lI0 I20 
WAVELENGTH 
Figure 3: Detailed spectral resolution of the trarumlssion grating for two different grating 
periods end the two possible focal plane detectors. 
376 
1 4 1 ,  . 6 . U  . 1 6 -  .I*. - #.I# 
_ . . .  . 
Figure 4: Source spectrum for an 'active' latetypo ?tar, with T = logs8 K; the model is 
that of Raymond (1980). 
Figure 6: Convolution of the source spectrum of Fig. 4 with the totel instrumental 
re-,pgnZe of the 2-mirror deslgn, assuming a source volume emission measure of 
10 ~ m ' ~  at  a distance of 60 pc; we a sumed the Berylllum filter, use of the 
2000 lpmm grating, an exposure time of 10% sec, end no detector noise (photon 
statistics were taken into account). 
377 
. . . . . .  
Figure 6: Same as Fig. 5, but now taking the detector noise into account; note that most 
of the strong llnes in the parent source spectrum are detected, but that line inten- 
sity ratios cannot be reliably calculated. 
8 l C O - u  .. nn I... ...... -1 nMm.0. .. w *.+- CICI, ..................... 
1 . ~ 1 1 ~ 1  ........................... -t .... ._(.%-OS .1.l.rh!r ¶ .................. 
f 
8 
..... . . . . .  .- . . . . .  
. . . . .  
, .. 
6 Figure 7: Same as FIG. 8, h J t  *'-r tha 4-mirror design a t  an exposure of 10 sec. In this 
case, the ;pectroscoplc goals discussed in the text are readily within reach; note 
that the 2-mirror design would lead to somewhat degraded signal-tenoise. 


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