/

/v/7-s/_ /

Coordinated XTE/EUVE

Observations of Algol

NASA P.O. No. S-73068-Z

Final Report

Submitted To:

National Aeronautics and Space Administration

Goddard Space Flight CenterGreenbelt MD 20771

Principal Investigator:

Robert A. Stern

Dept Hl-12 Bldg 252

Solar and Astrophysics Laboratory

Lockheed Martin ATC

3251 Hanover St.

Palo Alto, CA 94304

(415) 424-a272

October 10, 1997

https://ntrs.nasa.gov/search.jsp?R=19980004494 2020-03-11T22:34:16+00:00Z

1. Summary

We performed a coordinated RXTE observation of the prototypical eclipsing binary Algol

in conjunction with EUVE and ASCA observations. Observing the X-ray spectrum of

Algol with ASCA, EUVE, and RXTE provides critical, near-simultaneous constraints on

the distribution of the hottest temperature plasma in the Algol system: this is required

to unambiguously determine the coronal Fe abundance. After some initial shuffling of

observatory schedules with the help of mission planners, the coordinated multiwavelength

observation was successfully performed in February, 1996. Analysis of the data, including

development of new, IDL-based global model fitting routines, was accomplished from

mid-1996 to mid-1997. The results indicate that the initial estimates of [Fe/H] ,-_ 0.3

dex (Stern et al. 1995) are confirmed, and the Algol high temperature plasma thus has

an Fe deficiency relative to the (Anders and Grevesse 1989) solar abundance.

2. Technical Progress

The results of this investigation will be published (in CD ROM format) in the proceedings

of the 10th Workshop on Cool Stars, Stellar Systems, and the Sun, Cambridge, 1997,

ASP Conference Series, eds. J.A. Bookbinder & R.A. Donahue (San Francisco: ASP).

A hardcopy version of this paper is appended to this report.

Multiwavelength EUVE/ASCA/RXTE Observations of

Algol and the [Fe/H] Abundance

Robert A. Stern 1, James R. Lemen t Sandy Antunes 2 Stephen A.Drake 3, Fumiaki Nagase 4 Jfirgen H.M.M. Schmitt s KulinderpalSingh 6, Nicolas E. White 7

Abstract.

EUVE, ASCA, and XTE observed the eclipsing binary Algol (BetaPer) from 1-7 Feb 96. The coordinated observation covered _ 2 binaryorbits of the system, with a net exposure of _ 160 ksec for EUVE, 40 ksec

for ASCA (in 4 pointings}, and 9G ksec for XTE (in 45 pointings). Wediscuss results of modeling the combined EUVE, ASCA, and XTE datausing continuous differential emission measure distributions, and provide

constraints on the Fe abundance in the Algol system.

1. Introduction

The prototype eclipsing binary Algol (/3 Per; P = 2.87 d.), studied intensivelyin nearly all wavelength regions from IR to X-rays, is now beginning to revealmore of its multifaceted nature through both ASCA (Antunes, Nagase, andWhite 1994), and EUVE spectrometer observations (Stern et al. 1995). A keyresult in both of these investigations is that the highly ionized Fe X-ray line

complexes or individual EUV lines are considerably weaker than expected (rel-ative to the continuum or other spectral features) from models using the solarabundances of, e.g. Anders and Grevesse (1989). However, because these ob-servations were performed many months apart, and because of Algol's inherentvariability, a combined analysis of the earlier data is not possible. To remedythis, a coordinated EUVE/ASCA/XTE observation was undertaken to settlethe issue of the Fe abundance in Algol's corona and place limits on the form of

the differential emission measure (DEM) from 106-10 s K, using the presence ofthe strong Fe L complexes and the line-to-continuum ratios of the Fe lines.

I Solar and Astrophysics Lab/Lockheed Martin

2Hughes/STX

3NASA/USRA

4ISAS

5MPE/Garching

6Tara Inst. of Fund. Res.

7NASA/GSFC

2. Observations

EUVE observed Algol for more than 2 binary orbits, with a net exposure time of

140 ksec. ASCA co-observed for a total of_ 40 ksec in four separate pointings,and XTE in 40 pointings for _ 90 ksec. The ASCA data were taken largelyduring quiescent periods, which will be helpful in the combined emission measureand Fe abundance analysis. The XTE data sampled a much larger portion of the

EUVE observation, with concentrated periods of observation designed to overlapwith the ASCA observations. The times of the coordinated EUVE/ASCA/XTEobservations are indicated in Figure 1. Because of the late launch of XTE, theEUVE and ASCA observations were delayed by about 1 week: the high levelof coordination and cooperation among the various observatories and schedulersmade this last minute shift possible.

3. Results

3.1. EUVE Lightcurve

A lightcurve from the EUVE Deep Survey/Lexan filter (A -,_ 70-170/_) is shownin Figure 2. Note the nearly continuous variability of the Algol system in this

wave band, which is similar to the coverage of the EUVE Short Wavelength(SW) spectrometer. The flare occurring at about midday on Feb 3 is relativelysmall by Algol standards, with an exponential decay time £ 4 hr. Folding theDS/Lexan data with the orbital period of Algol, 2.867 d, results in the lightcurveshown in Figure 3. Although parts of the Algol lightcurve are reproducible fromorbit to orbit, successive orbits of the Algol system reveal numerous changes dueeither to small flares or evolution of activity patterns. For example, what appearsto be a secondary eclipse during the first orbit (_b _ 0.5) is masked by what isprobably a decaying flare during the second orbit. This lack of reproducibility

on an orbit-to-orbit basis suggests caution in attempting a detailed model ofEUV eclipses.

3.2. RXTE lightcurve

The RXTE observation was designed to provide _ one (spacecraft) orbit expo-sures, sampled throughout the EUVE observation. The RXTE PCA (Propor-tional Counter Array) data are strongly affected by instrumental background,which is highly variable during a given observation. Hence a correct backgroundcorrection algorithm is crucial. Using the software developed by Keith Jahodaand provided by the RXTE GOF, we have obtained a background-corrected

PCA lightcurve for the Algol observation. This is shown in Figure 4. Note thatthe RXTE and EUVE lightcurves are in rough correspondence, but the peak onthe larger flare is missed by RXTE, and there are still likely some systematiceffects present in the background subtraction.

3.3. EUVE/ASCA/RXTE Spectra and Continuous EM Model Fit

By filtering the EUVE spectrometer data, removing periods of high count rate (_

i c/s), we obtain what may be termed a "quiescent" spectrum in the EUVE SW

EUVE Start

04:00

Coordinoted EUVE/ASCA/XTE Times

08:00 12:00 16:00 20:00 00:[30

UT from 01-Feb-96 00:00:00

ICO:O0 04:00 08:00 12:00 16:00 20:00 00:00

UT from 02-Feb-96 00:00:00

00:00 04:00 08:00 12:00 16:00 20:00 00:00

UT from 05-F'eb-96 00:00:00

00:00 04:00 08:00 12:00 16:00 20:00 00:00

UT from 04-Feb-96 00:00:00

I0(3:00 04:00 08:00 12:00 16:00 20:00 00:0(3

UT from 05-Feb-96 00:00:00

00:00 04:00 08:00 12:00 16:00 20:00 00:(30

UT from 06-F'eb-96 00:00:00

I00:00 04:00 08;00 12:00 16:00 20:00 00:(30

UT from 07-Feb-96 00:00:00

Figure 1. Times of coordinated EUVE, ASCA, and XTE observa-tions. The EUVE observation start and end times are indicated by

vertical dashed lines. The ASCA times are indicated by the four line

shaded regions with the wider spacing, and the XTE observation times

by the more numerous line shaded regions with the smaller spacing.The indicated observation periods include some times of SAA passes,

earth occultations, etc.

and MW bands. Corresponding to this, we have selected the final ASCA obser-

vation, and the overlapping RXTE observations to match this "quiescent" data.

We have developed a multiwavelength version (MULTI_FIT) of the EUVE_FIT

software used to fit a spectrum to an emission measure (EM) distribution de-

scribed by a sum of Chebyshev polynomials (see Stern et aL 1995). The ASCA

and RXTE response matrices and effective areas were all computed using the

data analysis software provided by their respective GOFs. The output FITSfiles were then read into IDL and converted into data structures for use by

the MULTI_FIT software. The MULTI__FIT software was adapted to use the

models produced by the SPEX software of Mewe, Kaastra and Liedhal (1995),

EUVE DS/Lexon Light Curve + ASCA Coord Obs (Shoded)

Figure 2. EUVE Deep Survey/Lexan lightcurve (50-170 h). Thetimes of the coordinated ASCA observations are indicated by the line-

shaded regions.

with lines from each element separately calculated to allow for varying elementalabundances. In addition, the normalizations of the 3 instruments were allowed

to vary independently to allow for intercalibration effects, but never were > 15%from their nominal values. A fixed absorption of N_/_ 4 × 102° was applied tothe ASCA and RXTE data to account for known calibration problems with theASCA SIS.

We experimented with Chebyshev polynomials with nterms -- 6 to 25, andallowing the elements Fe, Mg, S, and Si to vary. In general, the fits, though notformally accepted, provided good representations of the spectra. A sample ofthe best fit results is given in Table 1. As can be seen, changing the numberof terms above ,,, 10-13 did not have a significant affect on the fit, and the

range of [Fe/H] values (relative to Anders and Grevesse 1989) was ,,_ 0.26-0.35.Attempting to place a formal confidence limit on this range is difficult, becauseof the dominance of systematic effects in the instrument calibrations and atomiccodes.

For the purposes of illustration, we show the fit results for nterms -- 11.The spectral fit in the EUVE SW and MW bands, the ASCA SIS and the RXTEPCA is shown in Figures 5-8 and the resulting EM distribution in Figure 9.

O3

O3

Z

O(D

2.0

1.5

1.0

0.5

Folded DS/Lexan LightcurveT _ T _ T _ i

0 ¢ = 0.00-0.99 !

A _ = 1.00-1.99 ?[] _ = 2.00-2.99

+÷ + ,I*+ ÷

+

44 4 +

, *I,+

44444

0.2 0.4 0.6 0.8 1.0

Orbitol Phase

Figure 3. Phased lightcurve for DS/Lexan data. Different plottingsymbols, indicated in the legend, refer to successive orbits.

4. Summary

The coordinated EUVE, ASCA, and XTE observations of Algol obtained inFebruary 1996 offer a unique opportunity to determine the EM distribution and

element abundances of the Algol system. Coordinated multiwavelength dataindicate consistency with earlier ASCA and EUVE results, i.e., an [Fe/H]30%. The simultaneous fitting of these results leaves no doubt that, unless theatomic physics data is wrong to the same degree in both the EUV and X-rayregions for the Fe L lines, the derived underabundances are real. The question ofthe photospheric Fe abundance of Algol is, however, still an unresolved problem.

Acknowledgments. This work was supported by NASA Contracts NAS5-

96052 (EUVE), PO# S-57785-Z (ASCA), PO# S-73068-Z (XTE) and the Lock-heed Martin Independent Research Program.

5

09

}--Z

00

6O

40

2O

o

Ol -Feb

I IRXTE PCA Light Curve

tt

I'

I I

t tltt i

I , I _ I , I , I , I

02-Feb 03-Feb 04-Feb 05-Feb 06-Feb 07-FebStort Time (01-Feb-95 00:00:00)

Figure 4. RXTE PCA Lightcurve for Algol observation.

References

Anders, E., and Grevesse, N., 1989, Geochim. Cosmochim. Acta, 53, 197.

Antunes, A., Nagase, F., and White, N.E., 1994, ApJ, 436, L83.

Mewe, R., Kaastra, J., and Liedhal, D., 1995, SPEX code.

Stern, R.A., Lemen, J.R., Schmitt, J.H.M.M., and Pye, J.P., 1995, ApJ, 444,L45.

6

Table 1. Chebyshev Polynomial Fit Results

Nterms X_ NFREE Fe Mg S Si O6 1.39 741 0.26 0.48 0.08 0.29 N/A

10 1.34 737 0.27 0.44 0.08 0.28 N/A15 1.35 732 0.26 0.42 0.09 0.28 N/A20 1.36 727 0.27 0.44 0.08 0.28 N/A25 1.37 722 0.27 0.44 0.08 0.28 N/A

11 1.52 737 0.26 0.73 N/A 0.41 N/A

11 1.49 737 0.35 0.90 0.20 N/A N/A11 1.39 737 0.31 N/A 0.18 0.40 N/A

11 1.48 737 0.31 1.1 N/A N/A 0.

$W

(fl

E

0(D

3O0

250

2O0

150

1O0

5O

0

i: :- _i

-:_...--',-.--_.._-_-:: ... !_. 1!:! . ,.:: , : , . . . : :.

(Drr" 80 1O0 120 140

Wovelength(/_,)

Figure 5. EUVE SW Spectral fits and residuals.

80-

60

0 i_ : _ :i _'ii _i_:h,i!_

_'_, : ' ! ' i

-20b.._,-4 :_., :_,;r.,_.::.',.._,%_'._

_ :""::"'":';:.I'"" '_!i_;:_"

1.50250 300

2.00 Wave_e ng_.h(,,&.")

55O

Figure 6.

_D

0

EUVE MW Spectral fits and residuals.

1000

BOO

600

400

ASCA

,/'200 . ,,,r_ . ___-_-_.

cE 0 wavelength

Figure 7.ASCA SIS Spectral fits and residualS.

8

8OO

6OO

03

C400

00

2OO

A

bo 0

_o

XTEi

........ [.--.

..L._

...__..J

:!

• .:_..i.--U'i....,i!;i_:,i,y=

...... i i

- :,_i "_

13d 1 2 3 4

Wovelength(_,)

5

Figure 8. RXTE PCA Spectral fits and residuals.

Ou3

tad

3000

2000

1000

NTERMS = 11 NFREE =I I I

[, , , , I , , , , I , , , |, I i , ,

5.5 6.0 6.5 7.0

Log T

755

Figure 9. DEM for Nterms=ll Chebyshev Polynomial fit.

Form Approved

REPORT DOCUMENTATION PAGE OMB NO. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, includingthe time for reviewing instructions,searchingexisting data sources,gathering and maintaining the data needed, and completing and reviewing the collection of information Send comments regarding this burden estimate or any other aspect of thiscollection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 JeffersonDavis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office ot Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.

1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

October 1997 Contractor Report

4, TITLE AND SUBTITLE 5. FUNDING NUMBERS

Coordinated XTE/EUVE Observations of ALGOL

6. AUTHOR(S)

Robert A. Stern

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS (ES)

Lockheed Martin Corporation

3251 Hanover Street

Palo Alto, CA 94304-1191

9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS (ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

S-73068-Z

8. PEFORMING ORGANIZATION

REPORT NUMBER

LMMS/VMD-97-622

10. SPONSORING I MONITORINGAGENCY REPORT NUMBER

CR-203894

11.SUPPLEMENTARY NOTES

Technical Monitor: J. Swank, Code 662

12b. DISTRIBUTION CODE12a. DISTRIBUTION I AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category: 90

Report available from the NASA Center for AeroSpace Information,

800 Elkridge Landing Road, Linthicum Heights, MD 21090; (301) 621-0390.

13. ABSTRACT (Maximum 200 words)

EUVE, ASCA, and XTE observed the eclipsing binary Algol (Beta Per) from 1-7 Feb. 96. The

coordinated observation covered - 2 binary orbits of the system, with a net exposure of - 160 ksec for

EUVE, 40 ksec for ASCA (in 4 pointing), and 90 ksec for XTE (in 45 pointings). We discuss results of

modeling the combined EUVE, ASCA, and XTE data using continuous differential emission measure

distributions, and provide constraints on the Fe abundance in the Algol system.

14. SUBJECT TERMS

EUVE, ASCA, XTE, Algol, Fe/H Abundance

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

NSN 7540-01-280-5500

18. SECURITY CLASSIRCATIONOF THIS PAGE

Unclassified

19. SECURITY CLASSIFICATIONOF ABSTRACT

Unclassified

15. NUMBER OF PAGES

10

16. PRICE CODE

20. LIMITATION OF ABSTRAC_

UL

Standard Form 298 (Rev. 2-89)