Observations from the Einstein observatory reveal a previously unreported source, 1e1145.1-6141, within 15 arcmin of 2s1145-619 and of comparable intensity during July 1979. Periodicity analysis of the data shows a 290 + or - 2s period for the 2s source and a 298 + or - 4s period for the lE source, confirming the previous Ariel V report of two periods in this range from this region of the sky

Bignami, G. F.

Hartman, R. C.

Lamb, R. C.

Markert, T. H.

Thompson, D. J.

English

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(N ASA -T!4-800 38^ IWO X-BAY VULSARS: 2S
	 N60-22147
1145-619 AND 1B 1145.1- 6141 (NASA)	 16 p
HC A02/MP A01	 CSCL 03A
unclas
G3/89 18134
RVSA
Technical Memorandum 80638
Two X-Ray Pulsars:
2S 1145-619 and 1E 1145.1-6141
R. C. Lamb, T. H. Markert, R. C. Hartman,
D. J. Thompson and G. F. Signarn!
JANUARY 1980
National Aeronautics and
Space Administration
Goddard Space Flight Center
Greenbelt, Maryland 20771
NASA
d	 Technical Memorandum 8063
Two X-Ray Pulsars:
2S 1145-619 and 1E 1145.1-6141
R. C. lamb, T. H. Markert, R. C. Hartman,
D. J. Thompson and G F. Bignami
V
JANUARY 1980
_	 National Aeronautics and
Space Administration
Goddard Space Flight Center
Greenbelt, Mar&nd 20771
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TWO X-RAY PULSARS: 2S1145-619 AND 1E1145.1-6141
R. C. Lamb*
Department of Physics
W:	 Iowa State University
T. H. Markert
Department of Physics and Center for Space Research
Massachuse,,zts Institute of Technology
R. C. Hartman* and D. J. Thompson*
NASA Goddard Space Flight Center
G. F. Bignami*
LFCTR, Milan, Italy
ABSTRACT
Observations from the Einstein Observatory reveal a previously
unreported source, 1E1145.1-6141, within 15 arcmin of 2S1145-619 and
of comparable intensity during July 1979. Periodicity analysis of the
data shows a 290f2s period for the 2S source and a 298+4s period for the
lE source, confirming the previous Ariel V report of two periods in this
range from this region of the sky.
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*HEAD-2 Guest Investigators
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2I. INTRODUCTION
The Einstein Observatory (Giacconi et al. 1979) has been used to
search for X-ray eoission from the vicinity of several of the unidentified
COS-B Y-ray sources (Mayer-Hassel.wander et al. 1979). In July 1979, data
were obtained with the imaging proportional counter from the vicinity of
the 'y-ray source CG295+0. The 1 0 square field of view clearly showed two
point-like images, separated by 15 arcmin, each of which is located to
an accuracy of about 1 arcmin. One source is 2S1145-619 (Dower et al. 1975,
Bradt, Doxsey, and Jernigan 1979), the other is a previously unreported
source we designate 1E1.145.1,-61+1. Periodicity analysis shows a 290:-2 s period
for the 2S source and a 298±4 s period for the 1E source. This discovery
provides a simple explanation for the previous Ariel V report (White et al.
1978) of two periodicities from this region of the sky since both sources
would be within the Ariel V field of view. Our period values are consistent
with the more precisely determined Ariel V values (1977 epoch) of 291.8 and
297.4 s.
Approximately 15 other X-ray pulsars have been detected out of the more
than 500 known X-ray sources. Such sources are believed to be magnetized
neutron stars orbiting relatively ordinary stars, and the orbital parameters of
six X-ray binary pulsars have been determined sufficiently to establish neutron
star masses as well as other astrophysical. information. Future observations
of these two additional pulsars may also yield similar data.
II. OBSERVATION AND RESULTS
The X-ray picture of the region is shown in .figure 1. The two sources
under discussion are located near the eastern (right) side of the image with
4.
3the 1C source north of (above) the ;S source. In,addition to these point-
like sources there is some diffuse emission consisting of a bright "knot"
and arc-shaped emission surrounding the knot extending southward to the edge
of the field. A detailed examination of the knot and arc will be the focus
of future Einstein observations.
The observation was obtained during 1840 seconds of 13 July 1979. With
the exception of two gaps of 100 and 150 seconds, the data stream was continuous.
Harmonic analyses of the data from each point source showed that each was
periodically variable with periods in the range seen by Ariel V. The best
values of the periods and the significance I.,evels of the detections are
taken from folding analyses in which the arrival times of the photons
were folded into phase bins, the bins were corrected for slight nonuni-
formities in exposure, and Xz values calculated for each trial period over
the range of periods 200 to 400 s. Harmonic analysis showed that 2S1145-619
had significant power at the first overtone frequency as well as the funda-
mental , ;indicating a pulse profile sharper than sinusoidal. Therefore the data
were folded into 10 phase bins for 2S1145-619. For 1L1145.1-6141 there was
no evidence for a non-sinus idil profile. and 4 phase bins were used.
Although the knot source appears to be extended and therefore an improbable
candidate for pulsation, for the sake of completeness i, he arrival times
of the photons from its center were also folded with periods spanning the
a
l
range 200 to 400 s. No significant periodicity for this source was seen.
'
	
	 figure 2 shows the values of Xx per degree of freedom for both point sources
as a function of trial period. For 251145-619 the best period, 290±2 s, has
an associated X  per degree of freedom of 7.4. The probability that random
fluctuations alone are responsible for this peak is less than 10
-10 , there-
4I
fore the detection of periodicity in the intensity 251145-619 is virtually
certain. The best period value for 1E1145.1-6141 is 298±4 s with a confidence
level of < 5 x 10-5 .
 Although this detection is not as strong statistically
as the 2S result, the evidence is higbly suggestive, in view of the corres-
pondence of the period we obtain with the larger Ariel V period, that the
1E source is pulsed and is responsible for the second periodic signal, from
this region of the sky.
Figure 3 shows the pulse profiles of e4ah of the sources. The dominant
feature of the profile for 251145-619 is a relatively narrow peak in intensity
of approximately 4 times the steady value, whereas 1E1145.1-6141 exhibits a
much smoother variation. If we take the steady component of 251145-619 from
the six lowest phase bins we find a pulsed fraction, corrected for background,
of 34±5% and a duty cycle of 40%. The pulsed fraction for 1E1145.1-6141
is 51±111 with a duty cycle of -75%. These large duty cycles are typical of
the values seen in other X-ray pulsars (e.g. Rappaport and Joss 1977).
The energy spectra of these sources have been estimated under the
simplifying assumption that gain variations over the face of the imaging
proportional chamber may be ignored. Such variations are known to be
present and therefore out quoted flux values should be regarded as preliminary
until these gain variations are accounted for by the Einstein analysis program
under development. The energy flux over the band 0.5 to 3.0 keV for 2S1145-619
is approximately 1.3 x 10- 12
 erg/cm 2-s, for 1E1145.1-6141, 0.9 x 10 -11 ergs/cm'-s.
The energy spectrum of the 2S source appears to be cut-off sharply both below
0.5 keV and above 2.5 keV. The spectrum of 1E1145.106141 is•dissimilar in
that it has a ,low energy cut-off near 1 keV and there is no evidence for a,
higher energy cut-off. We have fitted the data to several simple spectral
5models and derived estimates of the hydrogen column density which may be
t	
responsible for the apparent absorption. These estimates are: 5f3 x 1021
` H atoms/cm2 for the 2S source~ and 3±2 x 10 22 11 atoms/cm 2 for the lE source.
If the 2S source is identified with the 9th magnitude B emission line star
HEN 715 (White at al.1979, Dower qt !1-1978) at an estimated distance of
1.5 kpc, and if the line of sight column density is a reliable indicator
of distance, them 1E1145.1- 6141 is at a distance of -9 kpu, implying a
luminosity in the band 0.5 to 3 keV of -10 35 erg/s. Source self-absorption,
which is likely to be present for both sources, may distort the distance and
luminosity estimates considerably.
The best position of 1E1145,1-6141 is a 176°29, d - 61468, with
an estimated error radius of 1 arcmin. We have examined an ESO survey plate
of the region around 1E1145.1-6141. The error circle is located in a rela-
tively crowded region of the galactic plane containing perhaps seven stars
brighter than 17th magnitude with the brightest estimated to be fainter than
13th magnitude. Our best position of 251145-619 is: a = 176°417, 6 = -61°925,
0.8 arcmin from the position of the optical counterpart, HEN 715 and consistent
with it,
III. DISCUSSION
There is no evidence for non-periodic variability of either source over
the half hour interval of our observation. however the combined flux of
P
both sources is nearly 10 2 smaller than the maximum 2-6 keV ,flux quoted
for 4U1145-61 (Forman et a1. 1978). The quoted 4U position and error box
agrees with neither the 2S nor the 1E source position and therefore both
may have contributed significantly to the Uhuru flux values. Therefore one
m,
or the other or both sources are apparently variable by a large amount over
long periods of time. From the variation of the amplitudes of the periodic
•	 signals seen by Ariel V (White et al. 1978) variability of a factor of 7
on a time scale of months may be attributed to 251145-619 and variability
by a factor of 2 on a time scale of days may be attributed to 1.1;1145.1-6141,
The reader may wonder about the apparent improbability of finding two
X-ray pulsars so close together on the celestial sphere. In order to assess
this probability realistically the observed non-uniformity of the distribution
of X-ray pulsars must be considered. For example, of the 15 known galactic
X-ray pulsars four (two of which are discussed in this paper) lie between
galactic longitudes 292° and 296° and within 1° of the plane. This clustering
appears to be an illustration of an enhancement in a distribution of population
I objects near a tangent line to an inner spiral arm. In this ca ge the arm
is the so-called Sagittarius-Carina arm (e.g. Georgelin and Georgelin 1976),
one of the major arms of the galaxy. Because of this non-uniform distribution
the chance coincidence of two X-ray pulsars within 15 aremin of each other
is not negligible.
The association of either X-ray pulsar with the Y-ray source,
CG295+0, is without foundation at this time in view of the lack of
any previous relationship between X-ray pulsars and the unidentified
y-ray sources. However, a search of the COS-B y-ray data for periodic
signals in the range we have observed should be pursued. Future
observations of these X-ray pulsars over extended periods of time
should yield the rate of change of the periods with time, possible
correlations between intensity and spectral, changes, and may yield
orbital determinations from which the neutron star masses can be derived.
7a
i
i
a
We gratefully acknowledge the efforts of many people of the Harvard
Smithsonian Center for Astrophysics who have contributed to the success
of the Einstein observatory and the guest investigator program. We
particularly acknowledge the Delp of F. Seward, W. Forman, R. Harnden,
and C. Janes. We also thank N. White of NASA Goddard Space Flight
Center for his communication with us regarding additional evidence
for the two periods (N. White et al. 1980). This work was supported
in part by the U. S. National Aeronautics and Space Administration
unu`r contract NAS8j-3333+.
f'
i
1
01
REFERENCES
Bradt, H. V., Aoxsey, R. E., and Jernigan, J. G. 1979, X-Ray Astronomy,
edited by Baity, W. A. and Peterson, L. E., Pergamon Press, Oxford.
Dower, R. G., et al. 1978, Nature, 273, 364.
Forman, W., et al. 1978, AP. J. (Sunpl), 38, 357.
Georgelin, Y. M. and Georgelin, Y. P., 1976, Astron. and Astrophy,. A9, 57.
Giacroni, R., et al. 1979, Ate. J., 230, 540.
Mayer-Hasselwander, H. A., et al. 1979, to be published in the 9th
Texas Symposium in Annals of the New York Academy.
Rappaport, S. and Joss, P. C., 1977, Nature, 266, 123.
White, N. E., Parkes, G. E., Sanford, P. W., Mason, K. 0, and Murdin,
P. G., 1978, Nature, 274, 664.
White, N. E., et al. 1980, Ap. J., Following paper.
Figure 1. X-ray picture obtained with cho im. g tig proportiona4 chamber,
'T'he field of vlaw of I * x 1', The two X-ray pulears under dis-
oussion and located near the eastern (right) side of the imago
with the IE source north of (above 'l  the 2S sourco.
Figure 2. Ru-sults of the folding analyses for 2$1145-619 (upper graph)
and IE1145.1-6141 (lower graph). X 2 per degree of freedom is
plotted m: a function of the folding period. The best va"We
of the period for each is indicated in the figure. The confi-
dence level that rcndom processes are responsible for the 2S
result is <IV IO , for the IE result < -5 x 10-5.
Figure 3, Phase plots for 2SI145-619 (upper graph) and IE1145.1-6141
(lower g • aph), The non-source background as a percentage of the
total counts in the phase plot is 4% for 251145-619 and 11% for
1E1145.1- 6141.
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Two X-ray pulsars: 2S 1145-619 and 1E 1145.1-6141

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