Sky & Telescope - September 2019 - 58

SEPTEMBER 2019 OBSERVING
Going Deep

γ Sct

HD 169869
H

HD 169633

these objects are, let's take a look at
some of them.
One of the brightest X-ray sources
in the sky is located in Cygnus, almost
halfway from Sadr (Gamma Cygni)
to Albireo (Beta Cygni), near Eta
(η) Cygni. Discovered in 1964 by a
sounding rocket equipped with Geiger
counters, Cygnus X-1 is the visually
brightest of our sample, thanks to its
donor companion, an 8.9-magnitude
q SCORPIUS X-1 The first-ever X-ray source
identified outside of our solar system, Sco X-1
lies about 81/3 ′ southeast of a pair of 9th- to
10th-magnitude stars.

HD 146950

HD 146935

Sco X-1

58

S E P TE M B E R 2 019 * SK Y & TELESCOPE

O9.7 blue supergiant. I've caught it in
20×80 binoculars, and it's easy to spot
in most any telescope.
Next brightest for us is LS 5039 at
magnitude 11.3. It can be found 45′
southwest of 4.7-magnitude Gamma (γ)
Scuti, in the southwestern corner of the
Shield. You might spot it in 25×100 binoculars, but the higher magniﬁcation
of a telescope is useful in this crowded
ﬁeld. In 2006 I used my 32-inch f/4
reﬂector to explore the dense star ﬁeld
in which it lies - I noted no other detail
than its stellar appearance. LS 5039
resides 8,200 light-years from us, about
a third of the way to the galactic center.
It's one of only a few galactic microquasars known to emit very-high-energy
gamma rays. Coupled with a jet pointed
directly toward Earth, this has earned it
the moniker "microblazar," akin to the
better-known extragalactic blazars.
Scorpius X-1 is the brightest source
of X-rays in the sky after our Sun. Discovered in 1962, it was the ﬁrst X-ray
object identiﬁed outside of our solar
system (see S&T: Aug. 2019, p. 14). Its
output at high-energy wavelengths is
quite variable. The mass of the neutron
star that powers its jets is most likely
the canonical value of 1.4 solar masses,
while the relatively small companion

LS 5039

star has been estimated to be around
half that. Sco X-1 lies some 9,000 lightyears away in the northern reaches of
the Scorpion - look for it slightly less
than 5½° northeast of Grafﬁas (Beta
Scorpii). The visible companion shines
at a moderately bright magnitude of
12.2, and can be easily spotted in a 4to 5-inch scope.
V4641 Sgr, a variable star in Sagittarius, was long thought to be the closest
black hole to Earth at 1,600 light-years.
But new data revised the minimum
distance to some 24,000 light-years, or
about 15 times more distant, and some
estimates place it even farther away.
This means its jets are possibly the fastest measured to date in our galaxy. It's
located almost 2° west of the top star
of the Teapot's dome, 3rd-magnitude
Lambda Sagittarii, and about 11/3°
southwest of the magniﬁcent globular
cluster M28. At magnitude 13.7, it's one
of the fainter microquasar companion
stars we can see, made more difﬁcult by
its residence within the dense, central
portion of the Milky Way. I managed to
ferret it out using my 32-inch scope at
the 2006 Okie-Tex Star Party.
This journey brings us back to
SS 433. Though relatively dim at magnitude 14.2, it's visible in an 8-inch

POSS-II / CA LTECH / STSCI / PA LO M A R OBSERVATORY (2)

LS 5039 You'll need to tease LS 5039 out of its
crowded ﬁeld. Look for it some 45′ southwest of
Gamma Sct and 3 3/4 ′ south-southeast of HD 169633.
Sky & Telescope - September 2019

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