Central Bureau for Astronomical Telegrams
IAUC 5139: TRANSIENT X-RAY SOURCE NEAR MX 0836-42; LOCAL GROUP DWARF GALAXY IN Tuc
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Circular No. 5139
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
Telephone 617-495-7244/7440/7444 (for emergency use only)
TWX 710-320-6842 ASTROGRAM CAM EASYLINK 62794505
MARSDEN or GREEN@CFA.BITNET MARSDEN or GREEN@CFAPS2.SPAN
TRANSIENT X-RAY SOURCE NEAR MX 0836-42
F. Makino and the Ginga Team, Institute of Space and Astronautical
Science, telex: "A transient x-ray source was discovered in
the vicinity of MX 0836-42. The x-ray flux observed with the All-
Sky Monitor (ASM) aboard Ginga was < 50 mCrab on Nov. 21, and was
about 100 mCrab on Nov. 22 and 23. The pointing observation with
the Large Area Proportional Counter (LAC) was conducted on Nov. 27
and 28, when the x-ray flux was about 50 mCrab and the power-law
spectrum had a photon index of 0.1 with exponential cut-off. A
pulsation was discovered at an apparent heliocentric period of 12.328
+/- 0.001 s (at Nov. 27.422 UT). The pulse profile is double-peaked
and the relative intensity of each peak changed with increasing
energy. Two x-ray bursts were observed on Nov. 27.91774 and 28.30505.
The decay time of the bursts was about 20 s and they exhibited cooling
in the decay phase; the rise time was about 5 s and peak temperature
was 2.4 keV. But we cannot conclude that both the burst and
pulsation are from the same source, because of the large field-of-
view of the LAC (1 deg x 2 deg FWHM) in which MX 0836-42 (Cominsky
et al. 1978, Ap.J. 224, 46) and GRS 0831-429 (IAUC 5122) are located.
Linear scanning with the LAC showed slightly wider spread than
single source response. The line positions determined by two-source
fitting intersect the error boxes of MX 0836-42 and GRS 0831-429.
The flux of the GRS 0831-429 component was 3.5 times higher than
that of the MX 0836-42 component and the former spectrum was harder
than the latter. Our tentative conclusion is that MX 0836-42 is a
burster and GRS 0831-429 is an x-ray pulsar. However, single source
hypotheses cannot be rejected because of large chi-squared value due
to variability in both cases. Optical and/or infrared observation
is urged. We are planning further observation with Ginga."
LOCAL GROUP DWARF GALAXY IN TUCANA
R. J. Lavery, Mount Stromlo Observatory, writes: "I have
discovered a new dwarf galaxy in Tucana at R.A. = 22h38m27s.3, Decl. =
-64 41'03" (equinox 1950.0; l = 323.0, b = -47.4), which is resolved
into stars in CCD images and is a likely new Local Group member.
The system is highly flattened, being 2'.5 x 5' in size, and the
initial classification is dE5. A V-I CM diagram, which shows a
probable horizontal branch, yields a preliminary distance modulus of
22.3 +/- 0.5. CCD photometry gives an absolute magnitude of R =
-7.7 +/- 0.5 and B-R = +1.1 +/- 0.1."
1990 November 29 (5139) Daniel W. E. Green
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