Discovery
of
an
Orbital
Period
in
IGR
J18450‐0435
1
1
1
M.
E.
Goossens ,
A.
J.
Bird ,
S.P.
Drave
1
School
of
Physics
and
Astronomy,
University
of
Southampton,
Highfield,
SO17
1BJ,
UK
Abstract
and
IntroducAon
The
source
IGR
J18450‐0435
was
first
discovered
by
INTEGRAL
during
a
survey
of
the
SagiTarius
arm
tangent
region
carried
out
in
spring
2003
(Molkov
et
al.
2004).
In
2006
Halpern
et
al.
(2006)
noted
that
the
flaring
ASCA
source
AX
J1845.0‐0433
and
IGR
J18450‐0435
are
likely
to
be
the
same
object.
This
was
later
confirmed
by
Sguera
et
al.
(2006)
sta^ng
that
the
op^cal
counterpart
of
AX
J1845.0‐0433
is
located
0.5’
from
the
ISGRI
posi^on
of
IGR
J18450‐0435.
AX
J1845.0‐0433
was
first
discovered
during
an
ASCA
observa^on
in
1993
undergoing
several
short
flares
of
X‐ray
ac^vity
(Yamauchi
et
al.
1995).
In
1996,
Coe
et
al.
(1996)
reported
observa^ons
that
determined
the
counterpart
of
AX
J1845.0‐0433
as
a
09.5I
supergiant
star.
The
fast
X‐ray
transient
behaviour
as
well
as
its
associa^on
with
a
supergiant
star
suggests
that
IGR
J18450‐0435
is
a
member
of
the
HMXB
subclass
of
supergiant
fast
X‐ray
transients
(SFXTs)
(Negueruela
et
al.
2005).
The
most
recent
outburst
ac^vity
from
IGR
J18450‐0435
was
reported
by
Sguera
et
al.
(2006),
which
was
the
first
to
be
announced
since
its
discovery
in
1993.
Outbursts
from
SFXTs
are
quite
rare
and
long
periods
of
inac^vity
occur
so
it
is
difficult
to
iden^fy
periodici^es.
Here
we
report
the
temporal
study
of
the
SFXT
IGR
J18450‐0435
leading
to
the
discovery
of
a
5.7195±0.0007
day
periodicity.
Using
the
newly
discovered
periodicity
value,
the
orbit
geometry
of
the
system
could
be
modelled
and
its
eccentricity
could
be
constrained
to
ε≥0.03.
Moreover,
we
report
the
discovery
of
three
new
outbursts
and
discuss
how
they
relate
to
the
ones
that
are
already
known.
Orbit
Geometry
and
Size
of
the
Supergiant
Data
Set
and
Analysis
Searching
for
New
Outbursts
The
newly
found
5.7195‐day
period
was
used
to
model
We
analysed
IBIS/ISGRI
data
for
all
the
available
science
A
comprehensive
search
for
new
outbursts
was
carried
out
windows
covering
the
range
from
10th
March
2003
to
on
the
18‐60
keV
IBIS
light
curve
also
containing
the
possible
orbits
and
calculate
a
limit
on
the
size
of
the
29th
September
2010
(MJD
range
52708‐55704)
in
the
previously
men^oned
GPS
science
windows.
supergiant.
Kepler’s
equa^on
was
u^lised
to
compute
the
18‐60
keV
energy
band.
Three
dis^nc^vely
significant
outbursts
were
found.
posi^on
of
the
neutron
star
around
the
orbit
as
a
func^on
of
Moreover,
16
science
windows
of
public
Galac^c
Plane
^me.
In
order
to
find
a
limit
on
the
size
of
the
supergiant,
the
Scan
(GPS2)
from
2012
data
were
added.
approximate
radius
of
its
Roche
lobe
was
found
using
The
resul^ng
light
curve
was
filtered
so
that
individual
Eggleton’s
formula.
As
Roche
lobe
overflow
is
not
the
driving
science
windows
where
the
source
was
more
than
12
accre^on
mechanism
in
SFXTs
,
the
Roche
lobe
radius
is
also
degrees
off‐axis
and
those
with
an
exposure
^me
of
less
the
maximum
radius
the
supergiant
can
have.
Moreover,
it
than
200
s
were
excluded.
limits
the
eccentricity
of
the
orbit
as
the
posi^on
of
These
filters
resulted
in
an
IBIS
data
set
consis^ng
of
periastron
has
to
be
greater
than
the
size
of
the
Roche
lobe
4610
science
windows
with
a
total
exposure
^me
of
6.65
so
as
to
avoid
any
overflow.
Ms.
Periodicity
Analysis
As
a
test
for
periodicity,
the
Lomb‐Scargle
method
was
used
(Scargle
1982;
Lomb
1976).
A
periodic
signal
at
5.7195
days
could
be
observed
and
in
order
to
determine
the
significance
of
this
period
Monte
Carlo
simula^ons
were
produced.
200,000
itera^ons
were
carried
out
so
as
to
establish
a
99.999
per
cent
confidence
level.
A
final
bootstrapping
analysis
was
completed
and
consequently
the
error
on
the
period
was
found
to
be
±0.0007
days.
Figure
3.
IBIS/ISGRI
light
curve
of
IGR
J18450‐0435
(18‐60
keV)
during
the
observa^on
on
3rd
September
2006.
Figure
6.
Orbit
Geometry
and
loca^on
of
the
supergiant
for
eccentrici^es
ranging
from
0.1‐0.3.
The
loca^ons
of
the
respec^ve
foci
where
the
supergiant
would
be
located
are
displayed
by
the
markers
towards
the
centre
of
the
plot.
Figure
4.
IBIS/ISGRI
light
curve
of
IGR
J18450‐0435
(18‐60
keV)
during
the
observa^on
on
14th
March
2010.
Taking
into
account
both
of
these
limits
it
is
apparent
that
the
orbit
has
to
have
an
eccentricity
ε≤0.03.
The
Roche
lobe
radius
was
calculated
to
be
∼26R⊙,
which
seems
to
be
quite
a
good
limit
for
the
size
of
the
supergiant
as
it
coincides
with
what
would
be
expected
for
one
in
this
spectral
class
(Cox,
1999).
Figure
1.
Lomb‐Scargle
periodogram
for
the
IBIS/ISGRI
18‐60
keV
light
curve.
The
99.999
per
cent
significance
power
is
21.4,
calculated
using
Monte
Carlo
simula^ons.
Figure
5.
IBIS/ISGRI
light
curve
of
IGR
J18450‐0435
(18‐60
keV)
during
the
observa^on
on
20th
March
2010.
All
outbursts
of
IGR
J18450‐0435
found
to
date
(Sguera
et
al.
2006)
are
summarised
in
table
1.
Note
that
the
last
two
are
separated
by
the
orbital
period.
Figure
7.
Distance
of
neutron
star
from
supergiant
against
orbital
phase
for
eccentrici^es
ranging
from
0.1‐0.3.
The
different
line
styles
represent
the
orbits
in
Figure
6.
Compare
with
Figure
2.
Table
1.
Summary
of
INTEGRAL,
Swi$
and
ASCA
observa^ons
of
outbursts
of
IGR
J18450‐0435
where
numbers
6‐8
represent
the
newly
discovered
ones.
Figure
2.
Phase
folded
light
curve
of
IGR
J18450‐0435
(18‐60
keV).
The
same
periodicity
tes^ng
method
was
used
on
the
IBIS
light
curves
in
several
other
energy
bands
(20‐40
keV,
17‐30
keV,
30‐60
keV,
20‐100
kev,
40‐100
keV
and
100‐300
keV).
A
periodic
signal
at
5.7195
days
could
be
observed
in
the
first
four
of
these
energy
bands
confirming
the
previous
result.
The
signal
could
not
be
detected
in
the
two
higher
energy
bands
due
to
the
source
having
too
low
flux
values.
Moreover,
the
same
analysis
was
performed
on
Swin
and
RXTE
data,
however,
no
significant
periodic
signals
could
be
iden^fied.
Background
credit:
Hubble
Deep
Field,
HubbleSite
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