The Ariel-V satellite monitored the X-ray light curve of A1524-62 almost continuously from 40 days prior to maximum light until its disappearance below the effective experimental sensitivity. The source exhibited maximum light on approximately 4 December 1974, at a level of 0.9 the apparent magnitude of the Crab Nebula in the energy band 3-6 keV. Although similar to previously reported transient sources with a decay time constant of approximately 2 months, the source exhibited an extended, variable pre-flare on-state of about 1 month at a level of greater than approximately 0.1 maximum light. The four bright (greater than 0.2 of the Crab Nebula) transient sources observed during the first half-year of Ariel-V operation are indicative of a galactic disk distribution, and a luminosity at maximum in excess of 10 to the 37th power ergs/sec

Boldt, E. A.

Eadie, G.

Holt, S. S.

Kaluzienski, L. J.

Pounds, K. A.

Ricketts, M. J.

Serlemitsos, P. J.

Watson, M.

English

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PREPRINt.
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THE LIGHT CURVE OF A TRANSIENT
X-RAY SOURCE
L. J. KALUZIENSKI
S. S. H O LT
E. A. BOLDT
P. J. SERLEMITSOS
G. EADIE
K. A. POUNDS
M. J. RICKETTS
M. WATSON
JULY 19715
(NASA-IM-X-70036)	 THE LIGHT CURVE CF A	
N75-28978
TRANSIENT X-F'4Y SCUE C E (NASA)	 14 p HC $3-25CSCL 03B
Unt,las
G3/90 29855
GODDARD SPACE- FLIGHT CENTER
GREENBELT, MARYLAND
A
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THE LIGHT CURVE OF A TRANSIENT X-RAY SOURCE
L. J. Kaluzienski
University of Maryland, College Park, Maryland
S. S. Holt, E. A. Boldt and P. J. Serlemitsos
Laboratory for High Energy Astrophysics
NASA/Goddard Space Flight Center, Greenbelt, Maryland
G. Eadie, K. A. Pounds, M. J. Ricketts and M. Watson
University of Leicester, Leicester, United Kingdom
ABSTRACT
The Ariel-V satellite has monitored the X-ray
light curve of A1524-62 almost continuously from 40
days prior to maximum light until its disappearance
below the effective experimental sensitivity. The
source exhibited maximum light on — 4 December 1974,
at a level of 0.9 the apparent magnitude of the Crab
Nebula in the energy band 3-6 keV. Although similar
to previously reported transient sources with a decay
time constant of — 2 months, the source exhibited an
extended, variable pre-flare on-state of — 1 month
at a level of 3t 0.1 maximum light. The four bright
(> 0.2 cf the Crab Nebula) transient sources observed
during the first half-year of Arie1 V operation are
indicative of a galactic disk distribution, a lumi-
nosity at maximum in excess of 10 37 ergs-sec -1 , a
frequency of occurrence which may be as high as 100
year- 1 , and a median decay time which is < 1 month.
INTRODUCTION
The Ariel-V satellite was launched into a low, near-circular equa-
corial orbit from the San Marco platform in the Indian Ocean on 15 October
1974. It contains a complement of six separate X-ray astronomy experi-
ments, including an UHURU-type Sky Survey Instrument (SSI) which views
in the satellite equatorial plane, and an All-Sky Monitor (ASM) which
observes more than 37 steradians of the celestial sphere each orbit.
f'
The object of the latter experiment is the continuous measurement of
strong sources for anomalous intensity variations, and the detection of
new transient sources.
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Shortly after launch, a strong new X-ray source appeared in the
constellation Triangulum Australis near the highly variable source
Cir X-1. It was first reported to be at maximum on 22 November 1974
from the SSI by Pounds (1974), when it was out of the field of view of
the ASM. The presently reported observations of the ASM and SSI indicate
that the 22 November 1974 maximum was only a precursor to a prolonged
transient source which was above the ASM level of detectability (— 100
UHURU counts-sec-1 ) for several months. While its decay phase is quite
similar to that of the previously reported transient sources Cen X-2,
Cen X-4, and 3U1543-47, it apparently has an extended, variable low
phase prior to maximum light which was not observed in these earlier
sources.
Because all but one of the pre-Ariel-V transients had decay times
> 1 month, such long-lived sources have been considered typical of the
phenomenon. New results from Ariel-V, however, indicate that experi-j
	
mental bias may have favored the discovery of such prolonged sources
relative to a more numerous population of sho r ter-lived transients.
II. EXPERIMENTAL RESULTS
The ASM consists of a pair of 1 cm2 pinhole cameras which utilize
one-dimensional position-sensitive proportional counters to image the
X-ray sky in spacecraft latitude. The — 10 rpm spacecraft rotation
carries the camera fan beam response around in longitude, so that the
entire sky can be covered (with the exception of dead spots near the
spacecraft poles and a dead band at the equator). Data storage restric-
tions limit the ASM to 512 discrete data words per orbit, so that the
A7
I.
7
7
3
sky can be divided into approximately 100 x 100 elements if no dif-
ferential energy resolution or temporal resolution finer than the
— 100 minute orbit time is attempted. The energy acceptance window is
well-defined, however, with an efficiency of — 60% in the band 3-6 keV
which is relatively independent of source spectral form. The duty cycle
for the observation of any source in the sky with the pinhole camera is
- 1%. A detailed description of the ASM may be found in Hult (1975).
The SSI consists of a set of proportional counters viewing out from
the side of the spinning spacecraft and divided into two pairs, each
pair being associated with one arm of the X-shaped field of view. Each
arm of the collimator has FWHM angles of 44 arc mins and 10.6 degrees,
with a separation between the arms of 50 degrees. The effective area
(behind the collimator) of each detector pair is 290 cm 2 , with one pair
covering the energy range 1.2 - 5.8 keV and the other pair the range
2.4 - 19.8 keV. Limitations in the capacity of the on-board data store
require a trade-off between the arc of sky covered and the spectral
detail obtained from any one orbit observation. Most of the present
results were obtained in the '1800 sector mode', in which a selected
1800 x 200 band of sky in the spin plane of the satellite (the galactic
plane in this case) is monitored by both pairs every second orbit, the
complementary 1800 sector being observed in the intervening orbits.
A more detailed description of the SSI may be found in Pounds et al.
(1975). It is important to note that the complementary fields of view
of the SSI and ASM are mutually exclusive, so that they do not simul-
taneously view the same parts of the sky.
4
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Figure 1 shows the overall 3-6 keV X-ray light curve constructed
for A1524-62 via the ASM (points) and SSI (solid curve), the decline
phase of which is reminiscent of the earlier transient sources Can X-4
(Evans et al., 1970) and 2U1543-47 (Matilsky et al., 1972). This
phenomenology is characterized by an apparent plateau region lasting
about 3 weeks where the source is relatively constant at — 80% of the
peak intensity, followed by a gradual decay back to the unobservable
pre-flare state with an e-folding time of 2 months. A significant
feature not observed in the earlier transients is the prolonged and
variable low-level activity, including at least one pronounced pre-
cursor peak, prior to the onset of the main flare. After its discovery
by the SSI, a search of earlier ASM data revealed a possible presence
of A1524-62 as early as day 301, if Cir X-1 (with which it may be con-
fused in the ASM) was truly < 50 UHURU counts-sec-1 at this time.
Observations of the SSI over the period day 314-337 find Cir X-1 at a
level of < 10 UHURU counts-sec -1 and, in fact, Cir X-1 has not been
positively detected by any - experiment since the launch of Ariel-V.
In Figure 2 data from the SSI are plotted differentially in time,
with the upper trace the ratio of the low to the high energy counting
rates, and the lower trace their sum. Clearly, the ratio implies a
,
spectral softening of the source up until the maximum of 22 November 1974,
after which the ratio measurements are consistent with spectral stability
following this precursor peak. Four channel pulse-height spectra were
obtained during days 328 through 330, which were not fittable with an
optically thin isothermal source. They were, however, each consistent
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with the same power-law approximation to the source spectrum dN = (5.2
+ 0.8) E -(2.5 ± 0.1) cm-2 sec -1 keV-1 , with no measurable absorption by
cold interstellar material at a 1Q upper limit of 1.4 keV.
III. DISCUSSION
The existence of an extended early on-state in the evolution of
transient X-ray sources would appear to rule out models which are
characterized by a lack of substantial X-ray emission prior to a critical
instability which flashes the source. Such models include the thermo-
nuclear shell flash mechanism proposed by Van Horn and Hansen (1974), as
well as X-ray analogies to the optical nova model of hose (1968).
Amnuel at al. (1974) have suggested that transient sources are
generically similar to the bright, variable X-ray sources in the galaxy,
in the sense that the site of X-ray emission is the ronnact member of
a binary system. In contrast to the bright sources, " vever, the
transients may have primaries which are dwarfs of spectral class later
than F rather than giants or supergiants. These sources may be expected
to appear as weak, relatively constant X-ray emitters prior to outburst,
but could still supply a flare lasting several months at > 10 37 ergs-sec-1
with a mass transfer rate of — 10 15 gm-sec-1 (, 10-11 M 0 year-l) over
1 104 years. We note that the difference between a "flare" in a known
source as a new "transient" source may be a strong function of detector
threshold. Unlike the factor of — 3 increase in Cyg X-1 (Cursky et al.,
1975; Holt at al., 1975a) or even the order-of-magni-,-de increase in
Aquila -1 (Buff, J., 1975; Holt at al., 1975b) recently detected,
A1524-62 had to increase by more than two orders of magnitude to be
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reconcilable with its absence from the UHURU catalog.
The spectral data are not inconsistent with a binary system inter-
pretation of the source geometry, as well-established binaries such as
Cyg X-1 and Her X-1 are characterized by spectra which are well-fit by
power law approximations over this energy range. This does not imply
that the production of X-rays is ne!essarily non-thermal; in fact, such
spectra may be synthesized by a thermal gradient in the source in
combination with photon transport effects (cf. Davidson, 1974). We
suggest that the spectral softening observed prior to the precursor
peak may be a manifestation of the lessening of the optical thickness
of circumstellar matter, as expected from models (cf. Pringle, 1973)
in which transient sources result from a decrease in the mass accretion
rate to the compact member, with the source emergence a consequence of
the depletion of the optically thick blanket of accreting matter which
previously smothered the X-ray output.
Additional information concerning the galactic distribution of the
transient sources is emerging from Ariel-V observations. A total of
four major transient sources (> 20% of the apparent magnitude of the
Crab Nebula) have now been observed from the spacecraft, all within 5°
of the galactic plane. Furthermore, no new sources exceeding this flux
and lasting longer than a few days have been detected by the ASM outside
the plane. Silk (1973) elegantly demonstrated the galactic origin of
transient sources, but found the pre-Ariel-V transients to be recon-
cilable with a halo distribution. The new sources clearly favor a
Population I distribution at distances ;21 1 kpc. With apparent magnitudes
i7
in X-rays comparable to the Crab Nebula, therefore, their maximum
luminosities may be as high as the Hddington-limited p 1038 ergs-sec-1
of the strongest galactic sources,
If the transient sources all have comparable intrinsic luminosities
and occur uniformly in the galactic plane, the four sources with lumi-
nosity at maximum > 10 -8 ergs-cm 2-sec- 1 over the first half-year of
Ariel-V operation imply an average peak luminosity ;2^ 10 37 ergs-sec-1
if there are e^, 100 of these sources occurring in the galaxy each year
(after Silk, 1973). It is formally possible to con-s train the scale
height of the transient source disk by demanding that the upper limit to
a galactic ridge be consistent with present measurements (cf. Holt et al.,
1974), but this approach is meaningful only if many unresolvable sources
are "on" at the same time. In this regard, ar important difference
between the presently reported source and others recently detected by
Ariel-V is one of decay timescale. A1118-61 was reported with a decay
time of — 1 week (Ives, Sanford and Bell-Burnell, 1975), and A0535+26
had a decay time of — 19 days (Kaluzienski at al., 1975). Earlier, the
other UHURU source positively identified as a transient, 3U1735-28,
was found to have a similarly short decay time (Giacconi et al., 1974).
It would appear, then, that the "typical" decay time of — 2 months for
A1524-62 may actually be quite atypical.
The relative paucity of UHURU transients is a clear indication that
long-lived sources of the type reported here with absolute luminosities
at maximum ;b 1037 ergs-sec -I certainly cannot have a frequency of occur-
rence as large as 10 year-1 in the plane without contributing a signif-
icant ridge component, or conflicting with the UHURU results (as their
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8
detection would require coverage of a large fraction of the plane in a
time 4 1 week). The sensitivity of the ASM is not good enough to detect
them all (especially in source-confused regions), but over the antici-
pated - 2 year lifetime of Ariel-V we may expect the catalog to grow
sufficiently to allow a first determination of the transient source
frequency, timescale and luminosity distributions.
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REFERENCES
Amnuel, P. R., Guseinov, 0. 11. and Rakhamimov, Sh. Ju. 1974, Astrophys.
Space Sci. 29, 331.
Buff, J. 1975, IAU Circular No. 2788.
Davidson, K. 1973, Nature Phys. Sci. 246, 1.
Evans, W. D., Belian, R. D., and Conner, J. P. 1970, Ap. J. (Letters)
1159, L57.
Giacconi, R., Murray, S., Gursky, H., Kellogg, E., Schreier, E.,
Matilsky, T., Koch, D., and Tanenbaum, H. 1974, Ap. J. Suppl. No.
237, 27, 37.
Gursky, H. et al. 1975, IAU Circ. No. 2778.
Holt, S. S. 1975, Proc. Cospar Symp. Fast Transients R- and Gamma-Kay
Astronomy, Astrophys. Space Sci. (in press).
Holt, S. S., Boldt, E. A., Kaluzienski, L. J., and Serlemitsos, P. J.
1975, Nature (in press).
Holt, S. S., Boldt, E. A., Serlemitsos, P. J., Murray, S. S., Giacconi,
R., Kellogg, E. M., and Matilsky, T. A. 1974, Ap. J. (Letters) 188, L97.
Ho'At, S. S., Kaluzienski, L. J., Boldt, E. A. and Serlemitsos, P. J.
1975, in preparation.
Ives, J., Sanford, P. W., and Bell-Burnell, S. J. 1975, Nature 245, 578.
Kaluzienski, L. J., Holt, S. S., Boldt, E. A., and Serlemitsos, P. J.
1975, Nature (in press).
Matilsky, T. A., Giacconi, R., Gursky, H., Kellogg, E. M., and Tanenbaum,
H. D. 1972, Ap. J. (Letters) 1^74, L53.
Pounds, K. A. 1974, IAU Circ. No. 2729.
Pounds, K. A., Cooke, B. A., Ricketts, M. J., Turner, M. J., Page, C.,
and Watson, M. 1975, M. N. R. A. S. (in press).
Pringle, J. E. 1973, Nature 243, 90.
Rose, W. K. 1968, Ap. J. 152, 245.
Silk, J. 1973, Ap. J. '18,1 747.
Van Horn, H. M, and Hansen, C. J. 1974, Ap. J. 191, 479.
PRECEDING PAGE BLANK NOT FTLMEY
aFIGURE CAPTIONS
1.	 The 3-6 keV X-ray Light curve of A1524-62. 	 The points are data
from the ARM accumulated over 3-7 orbits (- 4.5-10.5 hours).
	 The
solid trace is a representation of the SSI data displayed in
Figure 2, normalized to the natural ASM ordinate using the response
i
of both experiments to the Crab Nebula (1.4 cm -2sec -1 in the 3-6
•` keV band).	 The shaded regions indicate where the source is out of
the field of view of the ASM. 	 SSI data in addition to those
taken over deyv 314-337 consist of a single measurement on day 433
(displayed in the Figure), and an upper limit of 0.07 cm-2sec-1
on day 294 (not displayed).
2.	 The detailed history of the precursor peak from the SSI.
	
The lower
trace is the sum of the 1.2-5.8 keV (1E) and 2.4-19.8 keV (RE)
count rates, and the upper trace is their ratio.
	 The Crab Nebula
has a total SSI count rate of 510 see-1.
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L. J. KALUZIENSKI, S. S. BOLT, G. A. BOLDT and P. J. SERLEMITSOS,
Code 661, Goddard Space Flight Center, Greenbelt, Maryland 20771
G. EADIE, K. A. POUNDS, M. J. RICKETTS and M. WATSON, Department of
Physics, University of Leicester, Leicester LE1 7R11, United Kingdom


The light curve of a transient X-ray source

The light curve of a transient X-ray source

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