An X-ray burst source was discovered near the X-ray source 3U1915-05. The continuum spectra of both the burst source and the quiescent 3U1915-05 are hard, with kT above 20 keV. The spectrum of 3U1915-05 has a feature at 9.1 keV, which, if attributed to absorption by hydrogen and helium-like iron, suggests the presence of a highly ionized cloud surrounding a central X-ray source

Becker, R. H.

Boldt, E. A.

Holt, S. S.

Pravdo, S. H.

Serlemitsos, P. J.

Smith, B. W.

Swank, J. H.

English

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f	 SPECTRAL CHARACTERISTICS OF
3ulgi -05 1 	 ^.3
BURST SOURCE CANDIDATE
-	 (IliASA-T H -Y-71302) SPECTRAL CHARACTERISTICS 	 N77-24026
OF 3U1915-05, A BURST SOURCE CANDIDATE	 3
(NASA) 13 p HC A02/NF A01	 CSCL 031 Unclas	 -
U3/89 28941
R.!. -BECKER
8. W. SMITH
J. H. SWANK
E: A. BOLDT
S. S. HOLT
S. H. PRAVDO
P. J. SERLEMITSOS
APRIL 1977
--- GODDARD SPACE FLIGHT CENTER
GREENBELT, MARYLAND
X2223 24e,;4^.s.'^
MAY 1977
RECEIVED
NASA STI FACIUTY
INPUT BRANCH
Spectral Characteristics of 301915-05,
A Burst Source Candidate
R. H. Becker* , B. W. Smith% J. H. Swank% E. A. Boldt,
S. S. Holt, S. H. Pravdo	 and P. J. Serlemitsos
Laboratory for High Energy Astrophysics
NASA/Goddard Space Flight Center
Greenbelt, Maryland 20771
ABSTRACT
An X-ray burst source has been discovered near the X-ray source
301915-05. The continuum spectra of both the burst source and the
quiesce , it 301915-05 are hard, with kT above 20 keV. The spectrum
of 3U1915-05 has a feature at 9.1 keV, which, if attributed to absorption
F
by hydrogen and helium-like iron, suggests the presence of a highly
ionized cloud surrounding a central X-ray source.
NASA RC Resident Research Associate.
Also Department of Physics and Astronomy, University of Maryland.
- 2 -
I. INTRODUCTION
In the last year, over 15 X-ray burst sources have been discovered
(Lewin, 1976). The bursts are usually characterized by rise times
of < i s and decay times of 5-50 sec. In many instances, the error
box of the burst source contains a "steady" source of X-rays. During
three days in 1975, the GSFC Cosmic X-ray Spectroscopy Experiment
on OSO-8 observed a series of eleven bursts (Swank et al., 1976a).
During this period, the steady source 301915-05 was also in the field
of view. This paper will report on the spectra and light curves
for both the burst source and 3U1915-05 and discuss the possibility
that they are one and the same source.
II. OBSERVATIONS
During the period between April 11-14, 1976, a series of eleven
X-ray bursts were observed with a pointed xenon proportional counter
with a 50 FWHM field of view. A description of the experiment is given
by Pravdo et al. (1916) and will not be repeated here. The arrival
times of the bursts, the interval between bursts, and the center of
the detector field of view during the burst are given in Table 1.
All the bursts had similar temporal characteristics, with rise times of
ti 0.5 sec and decay times of 5 - 10 secs. The similarity among the
events and the fairly regular occurrence of the events suggest that
all the bursts have the same origin. If so, then an error box for
the locatiun of the burst source can be constructed by taking the inter-
section of all eleven fields of view.
a
J
F- 3 -
The observations also indicated that a continuous X-ray source
was within the field of view of the detector. The only known source
within the field of view during this period was 3U1915-05. We find
that the X-ray intensity from 3UI915-05 varies by as much as a factor
of 3 within 5000 - 10,000 sec. The existence of these rapid variations
do not depend on the assumed location of the source because the orientation
of the detector was changing little over such time intervals. The
average intensity of 3U1915-05 during the observation was 10.0 ± .6
x 10-11 ergs/cm2-sec between 2 - 6 keV and 28.5 x 10 -11 ergs/cm 2-sec
between 2-20 keV. UHURU detected a varying intensity for 301915-05
with a range between 8 - 40 x 10 -11 ergs/cm2-sec (Giacconi et al.,
1974). The average spectrum of 3U1915-05 is shown in Figure 1.
The statistics for a single burst are inadequate to.produce
a meaningful spectrum, so that we have combined that data from all
the bursts to generate an average burst spectrum. This result is also
shown in Figure 1 and has been normalized to the peak burst intensity
of 5.2 ± 1.0 x 10-9 ergs/cm2
 sec between 2 - 6 keV, approximately 50
times the steady intensity of 301915-05.
Attempts to fit the spectra of 3U1915-05 with simple power
law and thermal bremsstrahlung models did not result in acceptable
X2 values. However, the inclusion o{ an absorption edge at 9.1 ± 0.4
keV into either model resulted in acceptable fit3. This feature is
f
significant at the 5Q level. The edge was represented by the analytic
form
exp -(y/E) ±3
 for E ? EEDGE
- 4 -
and zero for E < EEDGE* The best fit power-law is a = 1.60 ± 0.15
and NH = 2.5
+1a5 x 1022 atoms/cm2 with X2 = 16.8 for 13 degrees of
freedom. The thermal bremsstrahlung model requires a kT > 35 keV
with the best fit value of 50 keV and NH . 87
+1 5 x 1022 atoms/cm'
giving a X` = 16.0 for 13 degrees of freedom. In both cases, y =
6.8 + 1.4. The errors represent the range of values of each parameter
within the 4-dimensional 90% confidence contour which is found by allowing
X2 to increase by 7.8 over the best fit. If the absorption edge is
due to hydrogen-like iron ions, the implied column density of hydrogen-
like iron is 1.4 + .3 x 1019 ions/cm2 . If the absorption is due exclusively
to helium-like iron the column density would be roughly a factor of
two less. An upper limit for the equivalent width of any iron line
emission at 7 keV is 176 eV at the 90% confidence level.
Spectral fits'to the burst spectrum did not result in acceptable
X2 values. Since burst spectra have been observed to vary over a
burst, it is not surprising that the time averaged spectrum can not be well-
represented by a simple model. The best fit power-law for the average
burst spectrum is a = 1.7 and NH = 8.7 x 1022
 atoms/cm2 . The best fit
thermal bremsstrahlung resulted in kT = 31 keV and NH = 7.5 x 1022 atoms/
cm2 . The addition of an absorption edge at 9.1 keV did not improve
the fit.
III. DISCUSSION
The bursts occur at intervals of 15,000 - 22,000 sec or multiples
thereof, similar to sequences of bursts observed from 3U1820-30,
MXB 1837+05, MX 1906+00, and MXB 1728-34 (ref. in Lewin, 1976). The
- 5 -
bursts are also similar in intensity and duration to those of previously
discovered burst sources, so that the phenomenology reported here seems
typical.
On the other hand, the spectral character of 3U1915-05 is unique
among published X-ray spectra. The source has a hard spectrum modified
by an apparent absorption edge at 9.1 keV but with relatively little low
energy absorption. Absorption edges at 7.1 keV have been observed in several
•	 A
X-ray binary sources (for example, Becker et al., 1977; Swank et al.,
1976b) and have been attributed to photoelectric absorption due to
neutral iron. The presence of these edges are always accompanied
by substantial low energy absorption. The K-absorption edge energy
for iron depends on the ionization state of the iron and for hydrogen-
like and helium-like iron the edge energy is 9.3 and 8.8 keV, respectively.
Therefore the edge seen in the spectrum of 301915-05 can be attributed
to the K-absorption edges of either hydrogen or helium-'like iron.
The high ionization of the absorbing material naturally explains the
lack of substantial low energy absorption, as all the more abundant
lighter elements would be fully ionized. Therefore, the spectrum of
3U1915-05 is consistent with a hard X-ray source ( .135 keV) enveloped
in a highly ionized cloud of material. The depth of the absorption
edge can be explained by a column density of hydrogen-like iron of
1.4 + .3 x 1019 ions/cm2 . If the abundance of iron in the cloud is
near the photosphere-coronal average of 3.9 x 10 -5
 of hydrogen by number-
(Withbroe, 1971), the column density of equivalent hydrogen atoms is
3.5 + .8 x 1023 cm-2
 since some iron may be fully ionized.
- 6 -
If the cloud as a whole is in statistical equilibrium there
will be as many recombinations as photoionizations. Some fraction
(.3 - .7) depending on T will recombine to the ground state resulting
in an emission edge which will tend to fill in the absorption edge.
Of those which recombine to an excited state, approximately 2/3 will
result in a Lyman a photon at 7 keV: Hence, the depth of the edge only
gives a lower limit to the column density of iron and furthermore,
there should be 2/3 as many Lyman a photons as there are photons missing
due to the edge. Therefore, if the cloud is sphericaliy symmetric
and all the Lyman a phutons escape, we should observe a line of ti 300
eV equivalent width, substantially larger than the observed upper limit.
It is easy to construct models to explain the lack of iron emission
line. If a spherical cloud was divided by an optically thick, planar
disk, the expected iron emission would be halved. In addition, the
optical depth at the line center for iron Lyman a is J O times the
optical depth in the continuum. 'this results in long path lengths
through the cloud, increasing the probability that the line radiation
will be absorbed.
The high ionization of the absorbing material could result from
either collisional ionization or photoionization. Hydrogen-like iron
will be the dominant species of iron in a cloud of 10 8 K temperature.
The continuum intensity from a constant density cloud of 10 8 K temperature
at a distance d with a radius r and a hydrogen column density
7 x 1023 cm 2 (at 108 K, iron is ti 50% fully ionized) is given by
5 x 1025 r/d2 ergs/sec-cm2 . It is unlikely that 3U1915-05, located at
.
- 7 -
III = 31.3 bII = -8,3, is more distant than 10 kpc. Therefore, if
we assume that less than .2 of the observed intensity is from the
108 K absorbing material and that the region is spherical, then the
radius must be less than 1.4 x 10 10 cm and the density greater than
4 x 1013/cm3 , In the limiting case where all the observed intensity
comes from the 108 K cloud, then r < 7 x 10 10
 cm and n  > "0 x 1013
cm 3.
These results are sensitive to the degree that iron is fully
ioni2.d. Therefore, higher values of T lead to smaller limits on r.
If T is greater than 5 x 108 , almost all iron will be fully ionized
and no edge would be observed.
Alternatively, the absorbing material could be photoionized
by X-rays from the hard central source. Hatchett et al. (1976) have
shown that the ionization state and temperature of a gas in photoionizational
equilibrium can be determined from the single parameter C - L x/nrx2 if
the spectrum of the incident X-rays is specified. Here L x
 is the luminosity
of the X-ray source, n is the local atomic density, and r x is the
distance to the X-ray source. For a 50 keV incident spectrum, hydrogen
and helium-like iron will be the dominant species for log ^ = 3. Using
NH calculated from the absorption edge as a lower limit to n rx , we
can solve for r x o If 3U1915-05 is nearer than 10 kpc, then L x <
3,4 x 1036 ergs/sec between 2-20 keV and therefore, r  < 1010 cm if
the cloud is photoionizedo In this model, the kT of the electrons
at a radius where iron is 50% fully ionized.is < 2 keV for an incident
spectrum like that observed for 3U1915-05,
Canizares (1976a, 1976b) has suggested that the time evolution
of spectra from the bursters NGC 6624 and , NXB 1728-34 can be understood
in terms of scattering in a cloud of 10 8' K. Our data indicate that
301915-05 is surrounded by a highly ionized cloud. If this cloud is
in collisional equilibrium, it would have to be very similar to the
t
cloud postulated by Canizares. If the cloud is in photoionization
equilibrium, the temperature of the absorbing material is substantially
cooler. However, this would not exclude the presence of higher temperature
t
material closer to the source of ionizing radiation.
p. 127.
f	 f
REFERENCES
Becker, R. H., Boldt, E. A., Holt, S. S., Pravdo, S. H., Rothschild, R. E.,
Serlemitsos, P. J., Smith, B. W., and Swank, J. H. 1977, Astrophves.
J., in press._
Canizares, C. R. 1976a, Ap. J. (Letters) 207, L101.
Canizares, C. R. 1976b, private communication to Hoffman, J. A. et ai.
1976, Ap. J. (Letters) 210, L13.
Giacconi, R., Murray, S., Gursky, H., Kellogg, E., Schreier, E., Motilsky,
T., Koch, and Tananbaum, H. 1974, Ap. J. Suppl. 237, 27.
Hatchett, S., Buff, J., and McCray, R. 1976, preprint.
Lewin, W. H. G. 1976, preprint.
Pravdo, S, H., Becker, R. H., Boldt, E. A., Holt, S. 5., Rothschild, R. E.,
Serlemitsos, P. J., and Swank, J. H. 1976, Astrophys. J. (Let%ers)
206, L41.
Swank, J. H., Becker, R. H., Pravdo, S. H., Saba, J. R., and Serlemitsos,
P. J. 1976a, I.A.U. Circ. No. 2010.
Swank, J. H., Becker, R. H., Boldt, E. A., Holt, S. S., Pravdo, S. H.,
Rothschild, R. E., and Serlemitsos, P. J. 1976b, Astrophys. J.
(Letters) 209, L65.
WithbraE, G. L. 1971 in the Menzel Symposium, NBS Special Publication 353,
ed. K. B. Gebbie (Washington U.S.A. Government Printing Office);
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Spectral characteristics of 3U1915-05, a burst source candidate

Spectral characteristics of 3U1915-05, a burst source candidate

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