The optical eclipsing binary LR Cen has been eliminated as a candidate for Cen X-3 on the basis of a real discrepancy of orbital periods. We believe that the position coincidence of Wray 795 with Cen X-3 is not statistically significant

Brucato, R. J.

Kristian, J.

Westphal, J. A.

Caltech Authors - Main

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THE ASTROPHYSICAL JOURNAL, 17S:L137-L139, 1972 August 1 
© 1972. The American Astronomical Society. All rights reserved. Printed in U.S.A. 
ON THE OPTICAL SEARCH FOR CENTAURUS X-3 
R. J. BRUCATO, J. KRISTIAN, AND J. A. WESTPHAL 
Las Campanas Observatory and Hale Observatories, Carnegie Institution 
of Washington, California Institute of Technology 
Received 1972 June 5 
ABSTRACT 
The optical eclipsing binary LR Cen has been eliminated as a candidate for Cen X-3 on the basis of 
a real discrepancy of orbital periods. We believe that the position coincidence of Wray 795 with Cen 
X-3 is not statistically significant. 
I. LR CENTAUR! 
Among the more spectacular discoveries made by the Uhuru X-ray satellite is the 
nature of Cen X-3 ( =2U 1119-60) which, from the X-ray data alone, has been found 
to be a member of an eclipsing binary, with most of the X-ray emission in 4~8 pulsations 
(Schreier et al. 1972). The nearby optical eclipsing binary LR Cen was suggested as a 
possible candidate for the source by Kristian, Brucato, and Westphal (1972) and by 
Shklovsky, Cherepaschuk, and Efremov (1972) primarily on the basis of its period, 
which is within 0.5 percent of that of Cen X-3. The difference in periods is small if 
accidental but large in comparison with the observational errors and would require a 
decrease of 0~01 in the period of LR Cen since 1927. However, minima of LR Cen were 
observed on the following dates this year: JD 2441364.870 (Hiltner 1972), JD 2441383.69 
(±0.04), JD 2441394.207 (Peterson 1972), and JD 2441427.741(±0.003). These obser-
vations give a period of 2~0957 ± 0':10002, compared with 2~8712 ± 0~00004 for Cen 
X-3, and definitely eliminate LR Cen as a candidate. Furthermore, the most recent 
position of the X-ray source (Murray, private communication) is greatly improved and 
independently rules out LR Cen as a candidate. 
Note that from the variable-star survey in which LR Cen was found (Uitterdijk 1932 
and references therein) the a priori probability of finding an eclipsing binary with 
period and position in as good agreement is ,..._,10-4• This may have something to say 
about the general validity of a posteriori calculations of the probability of a particular 
observed event. 
II. WRAY 795 
Margon and Wray (1972) have suggested the emission-line star Wray 795 as the X-ray 
source on the basis of position agreement and spectroscopic characteristics. We have 
obtained spectrograms of this star on 1972 March 9 (270 A mm-1) and on 1972 April 8, 
16, 19 (60 .A mm-1), using the image-tube spectrograph attached to the Las Campanas 
40-inch (1-m) telescope. The low-dispersion plate shows emission at Ha and HJ3 with 
the remaining members of the Balmer series in absorption. At the higher dispersion, the 
HJ3 emission is seen to be superposed on a broad absorption feature. The higher members 
of the Balmer series are again in absorption, as are a number of He I lines. We see no 
variations in the spectrum on our plates. We estimate the spectral type of Wray 795 
to be ""B5e with v sin i "-'400-450 km s-1• 
From the distribution of nearby Wray emission-line objects, Margon and Wray 
(1972) find a probability of 0.03 that the coincidence of Wray 795 with the Cen X-3 
error box is accidental. On the basis of a larger sample, we believe that the association is 
not statistically significant. Since the Wray catalog is unpublished, we have cross-
L137 
© American Astronomical Society • Provided by the NASA Astrophysics Data System 
1
9
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J
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7
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L138 R. J. BRUCATO, J. KRISTIAN, AND J. A. WESTPHAL Vol. 175 
correlated Wackerling's (1971) compilation for 7h :::; a < 18h (where a majority of the 
objects are from Wray's catalog) with the X-ray source positions with I b I < 20° listed 
in the 2U catalog (Giacconi et al. 1972). For those sources with error boxes less than 
about 1 square degree, we found 16 coincidences, compared with 13.5 expected on the 
basis of chance. For the complete Wackerling catalog, we found 22 coincidences, com-
pared with 22 expected. 
III. GENERAL REMARKS 
The case of Cen X-3 demonstrates with particular clarity the current impasse in the 
optical identification of X-ray sources. Except for a number of young supernova rem-
nants, the only identified galactic sources are Seo X-1 (near certain), Cyg X-2 (very 
likely), and Cyg X-1 (possible), and these still are based primarily on positional agree-
ment. In the absence of very accurate X-ray positions ("-'10") or theoretical predictions 
of the optical appearance of X-ray sources, identifications must be based on similarities 
to known sources and/ or the correlation of events (pulsation, flares, binary motions) in 
both the optical and X-ray regions. But there has been a conspicuous absence of Seo 
X-1-like objects, and the suggested identifications have been based on "peculiarity" 
according to ill-defined a posteriori criteria which change from source to source. It seems 
clear that none of the many suggested identifications can be considered valid until 
direct evidence is found linking them with the X-ray sources. The binary sources such 
as Cen X-3 are particularly interesting in this regard, since they involve a parameter, 
the period, which can be accurately and unambiguously measured. Even the suggested 
identification of +34°3815 with Cyg X-1, for example (Webster and Murdin 1972; 
Bolton 1972), is not secure. There is no doubt, from the agreement of very accurate 
positions, that it is the radio source discovered by Hjellming and Wade (1971) and Braes 
and Miley (1972). But it is becoming increasingly clear that there is a relatively high 
surface density of faint radio sources in the plane, and there is no good evidence linking 
either +34°3815 or the radio source with Cyg X-1. A determination of activity in the 
X-ray source which is correlated with the 5% period of +34°3815 would be irrefutable. 
In the case of Cen X-3, the elimination of LR Cen as a candidate leaves Wray 795 
as the leading candidate by default, but the evidence either for or against it is not strong. 
Its positional agreement may easily be accidental, and it appears to be a normal Be star 
(as opposed to the more unusual object X Per), which as a class do not seem to be as-
sociated with X-ray sources. 
Using the mass function of Schreier et al. (1972) and an assumed interstellar absorp-
tion of no more than 2.5 mag (Neckel 1967), we estimate the maximum apparent mag-
nitude of the Cen X-3 primary as ,...._,11 mag. There are five stars brighter than ,...._,12 mag 
within the 1.Su box. These are shown in figure 1 (plate L2) along with a number of 
other stars for which we have low-dispersion spectra. Except for Wray 795, none show 
emission at Ha. We believe, however, that all of these stars should be considered as 
optical candidates for Cen X-3. 
We would like to express our appreciation to Mr. Howard Lanning for his assistance 
in obtaining some of the observational material presented here, to Dr. J. P. Swings for 
obtaining a number of spectrograms during his observing run at Las Campanas, and to 
American Science and Engineering, Inc., for providing information on this source in 
advance of publication. This work was supported in part by the National Aeronautics 
and Space Administration under grant NGR 09-140-009 to the Carnegie Institution of 
Washington. 
REFERENCES 
Bolton, C. T. 1972, Nature, 235, 271. 
Braes, L. L. E., and Miley, G. K. 1972, Nature Phys. Sci., 235, 147. 
Giacconi, R., Murray, S., Gursky, H., Kellogg, E., Schreier, E., Tananbaum, H. 1972, Ap. J. (in press). 
Hiltner, W. 1972, !AU Circ., No. 2398. 
© American Astronomical Society • Provided by the NASA Astrophysics Data System 
1972ApJ...175L.137B
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FIG. 1.-Cen X-3. The box shown is the 1.5 <T position from Uhuru measurements (Murray, private communica-
tion) . Low-dispersion (""270 A mm- 1) spectra of the numbered stars show no emission at Ha. Medium-dispersion ("-' 
60 A mm- 1) spectra of the lettered stars show no emission at H ,B. See text for discussion of W795. Identifications 
of the bright stars are: A = HD 306373 (B8); B = H D 98816 (A3) ; C = H D 98724 (F5) ; D = H D 98707 (AO); 
E = HD 98693 (AO); 13 = CPD-59°3378; 14 = CPD-59°3380. Spectral types where shown are from the HD 
catalog. 
BRUCATO, KRISTIAN, AND WESTPHAL (see page L138) 
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No. 3, 1972 CENT A UR US X-3 
Hjellming, R. M., and Wade, C. M. 1971, Ap. J. (Letters), 168, L21. 
Kristian, J., Brucato, R. J., and Westphal, J. S. 1972, I AU Circ., No. 2395. 
Margon, B., and Wray, J. D. 1972, Ap. J. (Letters) (in press). 
Neckel, T. 1967, Heidelberg Vero.if. Vol. 19. 
L139 
Peterson, B. A. 1972, Nature, 236, 449. 
Schreier, E., Levinson, R., Gursky, H., Kellogg, E., Tananbaum, H., and Giacconi, R. 1972, Ap. J. 
(Letters), 172, L79. 
Shklovsky, J. S., Cherepashchuk, A. M., and Efremov, Y. N. 1972, Nature, 236, 448. 
Uitterdijk, F. 1932, B.A.N., 6, 295. 
Wackerling, L. R. 1971, Mem. R.A.S., 73, 153. 
Webster, B. L., and Murdin, P. 1972, Nature, 235, 37. 
© American Astronomical Society • Provided by the NASA Astrophysics Data System 


English

On the Optical Search for Centaurus X-3

19
72
Ap
J.
..
17
5L
.1
37
B
THE ASTROPHYSICAL JOURNAL, 17S:L137-L139, 1972 August 1 
© 1972. The American Astronomical Society. All rights reserved. Printed in U.S.A. 
ON THE OPTICAL SEARCH FOR CENTAURUS X-3 
R. J. BRUCATO, J. KRISTIAN, AND J. A. WESTPHAL 
Las Campanas Observatory and Hale Observatories, Carnegie Institution 
of Washington, California Institute of Technology 
Received 1972 June 5 
ABSTRACT 
The optical eclipsing binary LR Cen has been eliminated as a candidate for Cen X-3 on the basis of 
a real discrepancy of orbital periods. We believe that the position coincidence of Wray 795 with Cen 
X-3 is not statistically significant. 
I. LR CENTAUR! 
Among the more spectacular discoveries made by the Uhuru X-ray satellite is the 
nature of Cen X-3 ( =2U 1119-60) which, from the X-ray data alone, has been found 
to be a member of an eclipsing binary, with most of the X-ray emission in 4~8 pulsations 
(Schreier et al. 1972). The nearby optical eclipsing binary LR Cen was suggested as a 
possible candidate for the source by Kristian, Brucato, and Westphal (1972) and by 
Shklovsky, Cherepaschuk, and Efremov (1972) primarily on the basis of its period, 
which is within 0.5 percent of that of Cen X-3. The difference in periods is small if 
accidental but large in comparison with the observational errors and would require a 
decrease of 0~01 in the period of LR Cen since 1927. However, minima of LR Cen were 
observed on the following dates this year: JD 2441364.870 (Hiltner 1972), JD 2441383.69 
(±0.04), JD 2441394.207 (Peterson 1972), and JD 2441427.741(±0.003). These obser-
vations give a period of 2~0957 ± 0':10002, compared with 2~8712 ± 0~00004 for Cen 
X-3, and definitely eliminate LR Cen as a candidate. Furthermore, the most recent 
position of the X-ray source (Murray, private communication) is greatly improved and 
independently rules out LR Cen as a candidate. 
Note that from the variable-star survey in which LR Cen was found (Uitterdijk 1932 
and references therein) the a priori probability of finding an eclipsing binary with 
period and position in as good agreement is ,..._,10-4• This may have something to say 
about the general validity of a posteriori calculations of the probability of a particular 
observed event. 
II. WRAY 795 
Margon and Wray (1972) have suggested the emission-line star Wray 795 as the X-ray 
source on the basis of position agreement and spectroscopic characteristics. We have 
obtained spectrograms of this star on 1972 March 9 (270 A mm-1) and on 1972 April 8, 
16, 19 (60 .A mm-1), using the image-tube spectrograph attached to the Las Campanas 
40-inch (1-m) telescope. The low-dispersion plate shows emission at Ha and HJ3 with 
the remaining members of the Balmer series in absorption. At the higher dispersion, the 
HJ3 emission is seen to be superposed on a broad absorption feature. The higher members 
of the Balmer series are again in absorption, as are a number of He I lines. We see no 
variations in the spectrum on our plates. We estimate the spectral type of Wray 795 
to be ""B5e with v sin i "-'400-450 km s-1• 
From the distribution of nearby Wray emission-line objects, Margon and Wray 
(1972) find a probability of 0.03 that the coincidence of Wray 795 with the Cen X-3 
error box is accidental. On the basis of a larger sample, we believe that the association is 
not statistically significant. Since the Wray catalog is unpublished, we have cross-
L137 
© American Astronomical Society • Provided by the NASA Astrophysics Data System 
19
72
Ap
J.
..
17
5L
.1
37
B
L138 R. J. BRUCATO, J. KRISTIAN, AND J. A. WESTPHAL Vol. 175 
correlated Wackerling's (1971) compilation for 7h :::; a < 18h (where a majority of the 
objects are from Wray's catalog) with the X-ray source positions with I b I < 20° listed 
in the 2U catalog (Giacconi et al. 1972). For those sources with error boxes less than 
about 1 square degree, we found 16 coincidences, compared with 13.5 expected on the 
basis of chance. For the complete Wackerling catalog, we found 22 coincidences, com-
pared with 22 expected. 
III. GENERAL REMARKS 
The case of Cen X-3 demonstrates with particular clarity the current impasse in the 
optical identification of X-ray sources. Except for a number of young supernova rem-
nants, the only identified galactic sources are Seo X-1 (near certain), Cyg X-2 (very 
likely), and Cyg X-1 (possible), and these still are based primarily on positional agree-
ment. In the absence of very accurate X-ray positions ("-'10") or theoretical predictions 
of the optical appearance of X-ray sources, identifications must be based on similarities 
to known sources and/ or the correlation of events (pulsation, flares, binary motions) in 
both the optical and X-ray regions. But there has been a conspicuous absence of Seo 
X-1-like objects, and the suggested identifications have been based on "peculiarity" 
according to ill-defined a posteriori criteria which change from source to source. It seems 
clear that none of the many suggested identifications can be considered valid until 
direct evidence is found linking them with the X-ray sources. The binary sources such 
as Cen X-3 are particularly interesting in this regard, since they involve a parameter, 
the period, which can be accurately and unambiguously measured. Even the suggested 
identification of +34°3815 with Cyg X-1, for example (Webster and Murdin 1972; 
Bolton 1972), is not secure. There is no doubt, from the agreement of very accurate 
positions, that it is the radio source discovered by Hjellming and Wade (1971) and Braes 
and Miley (1972). But it is becoming increasingly clear that there is a relatively high 
surface density of faint radio sources in the plane, and there is no good evidence linking 
either +34°3815 or the radio source with Cyg X-1. A determination of activity in the 
X-ray source which is correlated with the 5% period of +34°3815 would be irrefutable. 
In the case of Cen X-3, the elimination of LR Cen as a candidate leaves Wray 795 
as the leading candidate by default, but the evidence either for or against it is not strong. 
Its positional agreement may easily be accidental, and it appears to be a normal Be star 
(as opposed to the more unusual object X Per), which as a class do not seem to be as-
sociated with X-ray sources. 
Using the mass function of Schreier et al. (1972) and an assumed interstellar absorp-
tion of no more than 2.5 mag (Neckel 1967), we estimate the maximum apparent mag-
nitude of the Cen X-3 primary as ,...._,11 mag. There are five stars brighter than ,...._,12 mag 
within the 1.Su box. These are shown in figure 1 (plate L2) along with a number of 
other stars for which we have low-dispersion spectra. Except for Wray 795, none show 
emission at Ha. We believe, however, that all of these stars should be considered as 
optical candidates for Cen X-3. 
We would like to express our appreciation to Mr. Howard Lanning for his assistance 
in obtaining some of the observational material presented here, to Dr. J. P. Swings for 
obtaining a number of spectrograms during his observing run at Las Campanas, and to 
American Science and Engineering, Inc., for providing information on this source in 
advance of publication. This work was supported in part by the National Aeronautics 
and Space Administration under grant NGR 09-140-009 to the Carnegie Institution of 
Washington. 
REFERENCES 
Bolton, C. T. 1972, Nature, 235, 271. 
Braes, L. L. E., and Miley, G. K. 1972, Nature Phys. Sci., 235, 147. 
Giacconi, R., Murray, S., Gursky, H., Kellogg, E., Schreier, E., Tananbaum, H. 1972, Ap. J. (in press). 
Hiltner, W. 1972, !AU Circ., No. 2398. 
© American Astronomical Society • Provided by the NASA Astrophysics Data System 
1972ApJ...175L.137B
@ 
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. · 11 ... . 1·0 ?:N' . . .. .. . . . . . ' . ·~ . · · . .,  . ····· . 
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'· ·• · ·s· /: · 5·· ·~ · .~ . ."·-··:~· · ...... ·~ .. . ·•.· ... , :· . .... 
. ... . .; . - .... / ; . ' '""• ' . 
• . . ~··:. ! •• ·-.: -s . . . • : . - . . . . . . . ':- .. . 
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.·· ~ / . . . . . . . .... .. . ..• . . . ... \ . ' . .  . . ·. . . ·• . ·"' . ·• . • . .  .. ' . . .. 
. . .. . . . . . ::·. ·• . .. ':· ·.~ ..... · ·.: .. . ·. .  , ... ' . ' •: 
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FIG. 1.-Cen X-3. The box shown is the 1.5 <T position from Uhuru measurements (Murray, private communica-
tion) . Low-dispersion (""270 A mm- 1) spectra of the numbered stars show no emission at Ha. Medium-dispersion ("-' 
60 A mm- 1) spectra of the lettered stars show no emission at H ,B. See text for discussion of W795. Identifications 
of the bright stars are: A = HD 306373 (B8); B = H D 98816 (A3) ; C = H D 98724 (F5) ; D = H D 98707 (AO); 
E = HD 98693 (AO); 13 = CPD-59°3378; 14 = CPD-59°3380. Spectral types where shown are from the HD 
catalog. 
BRUCATO, KRISTIAN, AND WESTPHAL (see page L138) 
ioij 
r 
~ 
~ 
ti::l 
r 
N 
19
72
Ap
J.
..
17
5L
.1
37
B
No. 3, 1972 CENT A UR US X-3 
Hjellming, R. M., and Wade, C. M. 1971, Ap. J. (Letters), 168, L21. 
Kristian, J., Brucato, R. J., and Westphal, J. S. 1972, I AU Circ., No. 2395. 
Margon, B., and Wray, J. D. 1972, Ap. J. (Letters) (in press). 
Neckel, T. 1967, Heidelberg Vero.if. Vol. 19. 
L139 
Peterson, B. A. 1972, Nature, 236, 449. 
Schreier, E., Levinson, R., Gursky, H., Kellogg, E., Tananbaum, H., and Giacconi, R. 1972, Ap. J. 
(Letters), 172, L79. 
Shklovsky, J. S., Cherepashchuk, A. M., and Efremov, Y. N. 1972, Nature, 236, 448. 
Uitterdijk, F. 1932, B.A.N., 6, 295. 
Wackerling, L. R. 1971, Mem. R.A.S., 73, 153. 
Webster, B. L., and Murdin, P. 1972, Nature, 235, 37. 
© American Astronomical Society • Provided by the NASA Astrophysics Data System 


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On the Optical Search for Centaurus X-3

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