[email protected]
Spectral analysis of MXB 1728-34
with XMM-Newton
E. Egron (1), T. Di Salvo (2), L. Burderi (1), et al.
(1) Dipartimento di Fisica, Università degli Studi di Cagliari, Monserrato, Italy; (2) Dipartimento di Scienze Fisiche ed Astronomiche, Palermo, Italy;
This work was supported by the Initial Training Network ITN 215212: Black Hole Universe funded by the European Community
We have analysed an XMM-Newton observation of the Low Mass X-ray Binary and atoll source MXB 1728−34 on 2002 October 3rd. The source was in a low luminosity
state during the XMM-Newton observation, corresponding to a bolometric X-ray luminosity of 5 x 10e36 ergs/s for a distance of 5.1 kpc. The 1−11 keV X-ray spectrum
of the source, obtained combining data from all the five instruments on-board XMM-Newton is well fitted by a Comptonized continuum. Evident residuals are present at
6−8 keV which are ascribed to the presence of a broad iron emission line. This feature can be equally well fitted by a broad Gaussian, by a relativistically smeared line
or by a self-consistent, relativistically smeared, reflection model. In the hypothesis that the iron line is produced by reflection from the inner accretion disk, we can
infer important information on the physical parameters of the system, such as the inner disk radius, Rin ~ 18 Rg, and the inclination of the system with respect to the
line of sight, 44° < i < 60°.
The iron line as a reflection component
The broad iron line in the energy range 6.4−7 keV has
been detected in many X-ray systems such as AGN and
X-ray binary systems containing either a black hole or a
neutron star. It could be produced from irradiation
by hard X-rays of cold matter in the accretion disk.
When a hard X-ray photon hits the accretion disk, it is
subject to different interactions such as Compton
scattering, photoelectric absorption. The fluorescent iron
line is produced to re-adjust the iron atom or ion after
one of the two K-shell electrons is ejected following
photoelectric absorption of an X-ray photon.
The continuum
10 1
Fig. 3. Top panel: EPIC-pn (black),
MOS1 (red), MOS2 (green), RGS1
(cyan), RGS2 (blue) data points of
MXB 1728−34 in the range 1−11 keV.
Bottom panel: Residuals (Data
−Model) in units of sigmas for the
continuum reported in Table 1.
10 0
10!1
5
0
Egron et al.: XMM-Newton observation of MXB 1728-34
Fig. 2.Parameter
Top panel: EPIC-pn (black), MOS1 (red), MOS2 (green),
ValueRGS1
(cyan),
NHRGS2
(×1022(blue)
cm−2 )data points of MXB 1728–34 in the
2.4range
± 0.11−11
keV.
Bottom
panel: Residuals (Data-Model) in unit of0.59
sigmas
for the
4 kT
± 0.02
seed (keV)
continuum
reported in Table 2. Data have been rebinned
graphical
kT e (keV)
2.74for
± 0.04
purposes.
τ
16.5 ± 0.2
Norm
9.54 ± 0.02
Flux 2.0−10.0 keV (pn)
8.06
We
tried
an alternative
model 8.17
for the
iron features
(see D’Ai
Flux
2.0−10.0
keV (MOS)
(MOS1)
- 8.07 (MOS2)
et al.Flux
2005)
usingkeV
two(RGS)
absorption
edges
(instead
of an
emission
1.0−2.0
0.186
(RGS1)
- 0.185
(RGS2)
χ2 are
(d.o.f.)
line)Total
which
found at 7.50 keV (τ ∼ 0.06) and 1732
8.49 (903)
keV (τ ∼
Note:
model used
to fit theand
continuum
cons*phabs*compTT.
The
0.06),The
associated
to mildly
highly isionized
iron, respectively.
−10
absorbed
flux isfor
in units
of is
101519/899
erg cm−2(which
s−1 . has to be compared
The χ2 /d.o.f.
this fit
Normalized counts s!1 keV!1
!5
Table 2. Best fitting parameters of the continuum emission for
the XMM-Newton pn, MOS1,
MOS2, RGS1 and RGS2 spectra
2
10 of
Energy (keV) 5
MXB 1728–34.
3
! (") Normalized counts s!1 keV!1
3
10!2
! (")
Results Egron et al.: XMM-Newton observation of MXB 17
10 1
Table 1. Best fitting parameters of
10 0
the continuum emission. The
model used is const*phabs*
10!1
compTT.
The aborbed flux is in units
of 10e-10 erg cm-2 s-1.
! (")
Normalized counts s!1 keV!1
n et al.: XMM-Newton observation of MXB 1728–34
96) v.12.5.1. All unlevel for one intersimultaneously the
btained from all the
ration ranges of the
EPIC-pn, MOS1 and
y range 2.4−11 keV,
fferent cross calibrao account by includfactors were fixed to
nts.
Comptonized comk 1994), modified at
ic absorption modtions of Balucinskaw He cross-section
ndances of Anders &
(d.o.f.) correspond903. We then tried to
) to improve the fit.
o be not statistically
he blackbody in our
continuum emission
Fig. 2. The line profile results from the superposition of
different effects: Doppler shifts (double-horns due to the
approaching matter, blueshift, and receeding matter,
redshift, respectively), relativistic beaming enhancing the
blue peak, and gravitational redshift because of the
strong gravitational field close to the compact object.
(Mueller, 2007, Proceedings of Science)
Fig. 1. Top panel: The three main components of the Xray emission from an accreting black hole (top) and a
plausible geometry of the accretion flow in the hard
spectral state (bottom).
(Marat Gilfanov,2009, arXiv:0909.2567)
10!2
10 1
10 0
6 Continuum
3
0
!3
!6
6 Model 3
3
0
!3
!6
6 Model 5
3
0
!3
!6
3
be ascribed to the r
during the XMM-N
Also the bolometr
plies a bolometric
s−1 , corresponding
LEdd = 2.5 × 1038
Paradijs & McClin
the disk is expected
in the low/hard sta
compact object, an
less important. We
ponent is just too w
the results obtained
tion model. This in
bly truncated far fr
a relatively high va
the line of sight es
ther reduce the disk
component in the h
ometry around the
Different models to fit the residuals
! (")
! (")
n excess was present
probably indicating
5
e fit was improved
0
odelled by a simple
red at 6.6 keV with
!5
to 1489/901 that we obtained fitting the iron line with a Gaussian
fit gave a χ2 /d.o.f. =
5
10
(Model 1) or to 1463/899 that we obtained fitting the iron line
Acknowledgements. Th
provement of the fit, cameras was restricted to the energy range 2.4−11 keV, and to
2
10
Energy (keV)
with a diskline (Model 2)). Therefore this model (called Model 4
Energy (keV) 5
rameters).
1−2 keV for RGS1 and RGS2. The different cross calibrations of
ITN 215212: Black Hol
in Table 3) gives a worse fit of the iron features than the previous
five instruments were taking into account by including noran at 6.6 keV with the
ones.
factors in the model normalizing factors. These factors Fig. 3. Top pannel: EPIC-pn (black), MOS1 (red), MOS2 (green), RGS1
better result with a malizing
Top panel:
EPIC-pn
data
pointsofofMXB
MXB
1728–34ininthe
therange
range 2.4−11
Fig. 4.Fig.
Top3.panel:
EPIC-pn
data
points
1728-34
In order to test the consistency of the broad iron line with a (cyan), RGS2 (blue) data points with error bars of MXB 1728–34 in the
fixed to 1 for pn and kept free for the other instruments.
rtainties on the outer were
reflection component, we fitted the data using reflion, a self- range 1−11 keV. Bottom pannels: Residuals (Data-Model) in unit of
2.4−11 keV.
Bottom
panels: Residuals
(Data-Model)
in unit
of sigmas
keV.
Bottom
panels:
Comparison
of
the
residuals
(Data
−
Model)
in units of
We firstreflection
fit the continuum
with
a
thermal
Comptonized
comf the system if they consistent
sigma
for
the
continuum
reported
in
Table
2.
Data
have
been
rebinned
model
including
both
the
reflection
conline
3. Relativistic line
continuum
model
Table 2, aforrelativistic
Model 3 including
a References
ponent
using
the
compTT
model
(Titarchuk
1994),
modified
at
1000 Rg and the 1.
in- Gaussian
for
graphical
purposes.
sigmasfor
forthe
the
continuum,
for reported
Model 3inincluding
line (relline),
and
tinuum and the corresponding discrete features (Ross & Fabian
low
energy
by
the
interstellar
photoelectric
absorption
mody dip in its ligtcurve,
E = 2005),
6.57 ±
0.05 keV
E = 6.43 ± 0.07 keV
relativistic
line
(relline), and
for Model
5 using a (reflion).
relativistic reflection
Diskline
in addition
to a thermally comptonized2.
continuum
mod- profile
for
Model
5
using
a
relativistic
reflection
component
by
phabs
using photoelectric
cross-sections
of
Balucinskae fit corresponds to elled
elled
with
nthComp,
by
Zdziarski
et
al.
(1996),
and
extended
component (reflion), respectively. Data have been rebinned for graphi- Anders, E. & Grev
Index = 2.8 ± 0.1
σ
=
0.6
keV
(frozen)
E
=
6.45
±
0.06
keV
&
McCammon
(1992)
with
a
new
He
cross-section
2
s (the F-test gives a Church
by Zycki et al. (1999)), instead of compTT. The χ /d.o.f. was
−4
53,
197
based
Yan
et al.the
(1998)
and standard
abundances
of Anders
cal
purposes.
t 10 ). This modelχ²/dof
i
=
60°
(frozen)
=on1489/901
Betor
=
-2.8
±
0.1
1555/901,
without
inclusion
of
relativistic
smearing.
Since
1. Gaussian line: Improvement of the fit with respect to the best fit
2
Grevesse
(1989).
The
χ
/degrees
of
freedom
(d.o.f.)
corref the inner radius of &
the iron line was found to be significantly broad in ithe
previous
Arnaud,
K.
A.
199
Rin
=
19
+3
−4
Rg
=
60°
(frozen)
account
the
emissivity
in
the
best
way,
the
model
contains
two
to this 1,
fit2,was
1732/903 .smearing
We then
continuum (Δχ² = 243 for the addition of 2 parameters).
ravitational radius). sponding
models (Model
andunacceptably
3), we addedlarge,
the relativistic
Analysis
Softwa
emissivity
the emissivity
changes from
χ²/dof
= 1464/899
a blackbody
component
(bbodyofmodel)
to=improve
energy range 7−10 tried
18 +3
Rg index. The radius where
usingtotheadd
rdblur
component.
The addition
thisRin
component
to −6
2.
Diskline
profile:
The
profile
appears
broad
and
possibly
asymmetric.
We
the
The constitutes
temperatureModel
of the5blackbody
ata0.70
keV, Index1 to Index2, and the Index2 were frozen at 400 Rg and 3
detected.
(Fe
XXV−XXVI),
respectively.
Although
the
fitting
with
two
the fit.
model
in Table 3was
andfound
ledχto
χ2 /d.o.f.
Balucinska-Church
²/dof
=
1463/899
ascribe
this
shape
to
Doppler
and
relativistic
effects
in
the
inner
part
of the
respectively.
We
also
fixed
the
outer
radius
of
the
disk
and
the
seems The
to be
very high
compared
to addition
the different
model for a relativis- which
= 1466/899.
decrease
of the
χ2 for the
of thevalues
relairon
edges
cannot
be
excluded
yet,
we
think
that
the
most
probL699
in the previous
sourceof(Di
Salvo et al. inclination of the system to the same values than in the previous
2
Dauser et al. 2010), obtained
tivistic smearing
was ∆χpapers
= 89 about
for thethis
addition
2 parameters
accretion disk, which is coherent with a disk-reflection scenario.
2
2
model.
The
χ
/d.o.f.
obtained
in
these
conditions
is
1464/899.
2000;
D’Ai
et
al.
2005),
in
average
at
0.55
keV.
The
χ
/d.o.f.
account all the rela- (corresponding to an F-test probability of chance improvement
able
explanation
is
that
a
couple
of
edges
may
mimic,
given
the
Basinska,
E. M., Le
3.
Relativistic
line:
The
results
are
perfectly
consistent
with
the
diskline
This
model
(Model
3)
estimates
the
inner
radius
of
the
disk
R
∼
−12
decreased
to
the
value
1679/902.
However
an
F-test
estimates
a
in
4. Two
absoption
5. Self reflection model
pact object. To take of ∼ 10
). Assuming
that iron has edges
a solar abundance−7and freezrelatively low statistics, the shape of a broad iron line. We favor
& Marshall, F. J
of chance
of improvement
the fit ofvalue)
10 , and
thus the
we 19 Rg .
the model contains probability
ing the emissivity
betor
index to -2.7 of
(standard
model.
Betor
=
-2.7
(frozen)
E₁ =ξthe
7.5LXkeV
notparameter
to include
our where
model.LThe
values
of
e emissivity changes decided
the interpretation
ofComparison
the iron feature
asthe
a broad
emission
line,
ionization
= blackbody
/(nr2 ) toin660,
is
the
ionBhattacharyya,
S. &
We tried an alternative model for the iron features (see D’Ai
X
4.
Two
absorption
edges:
with
results
obtained
by
D’Ai
et
al
the
parameters
of
the
continuum
emission
are
reported
in
Table
= 20 +29edges
−6 (instead
Rg
frozen at 400 Rg and izing X-ray luminosity,
n is±the
electron density in the reflector, et al. 2005) using Rin
two absorption
of an emission
τ = 0.06
0.01
since
thisnot
gives
agood
slightly
better
fit ofprevious
the XMM-Newton
spec-favor
Cackett,
E. M., Mil
s of the disk and the 2.
(2006).
The
fit
is
as
as
with
the
models.
We
in
the
and r the distance of the reflector to the emitting central source, line) which are found at 7.50
(τ
∼
0.06)
and
8.49
(τ
∼
0.06),
i > 44°
E₂
=
8.5
keV
excess
was
present
in
the
residuals
between
5.5
and
8
used in the diskline the An
trum
and
very
reasonable
values
of
the
reflection
parameters.
inner radius has been estimated again to be 18 Rg and a lower associated to mildly and highly ionized iron . The χ2 /d.o.f. for
674,
415
interpretation
of
the
iron
feature
as
a
broad
emission
line.
◦
indicating
the was
presence
features. this fit is 1519/899 (which
ξ = 660
(frozen)
ditions is 1464/899. keV,
limit probably
to the inclination
angle
foundof
to iron
be 44discrete
.
we can
compare to 1489/901 that we
τ
=
0.06
±
0.01
The equivalent
hydrogen
column
inferred from
the Galactic
Claret,
A., Goldwu
s way are perfectly We are going to fit several models which are reported in the Table obtained fitting the iron line with a Gaussian (Model 1) or with
5.
Self
reflection
model:
Relativistic
distorsions
are
required
to
take
into
χ²/dof = 1463/900
22
−2
χ²/dof = 1519/899
kline model. In par- 3.
photoelectric
absorption
component,
N
∼
2.8
×
10
cm
, is D’Ai, A., Iaria, R.
1463/899 that we obtained fitting the iron line with a diskline
H of 2 parameters).
(Δχ²
=
89
for
the
addition
account
the
broad
line
The fit was improved by adding a broad-iron K line, mod- (Model 2)). So this model (called Model 4) seems to be less
ates the inner radius 4. Discussion
in agreement with typical values for this source (NH ∼ 2.6 −
Proceedings, 797
elled by a simple Gaussian line (Model 1) at 6.6 keV with the good than the previous ones.
We performed
MXB
22
−2
sigma
parametera spectral
frozen atanalysis
0.6 keV.ofThe
fit 1728–34
gave a χ2observed
/d.o.f. =
2.7
×
10
cm
, (Di Salvo et al. 2000; Piraino et al. 2000; Dauser, T., Wilms
2 3rd in the 1−11 keV energy
by
XMM-Newton
on
2002
October
In
order
to
test
the
consistency
of
the
broad
iron
line
with
a
1489/901 (corresponding to a ∆χ = 243 for the addition of two
−30absorbed
range. The and
bestanfitestimation
continuumof model
consists
of10an
ser/research/relline/
reflection component, we fitted the data using reflion, a selfD’Ai
et
al.
2005)).
The
Comptonized
component
can
be
pro2010,
ArXiv
e-p
parameters
the
F-test
of
3
×
).
The line profile can be equally well fitted using
a relativistic line or a self-consistent reflection model. The inclination angle of the system with respect to the line of
We substituted the Gaussian at 6.6 keV with a diskline profile consistent reflection component including both the reflection
duced by inverse Compton scattering from relatively hot elec- den Herder, J. W.,
2
continuum
and
discrete
features
(Ross
&
Fabian
2005),
in
adobtained ato
χ /d.o.f.
= 1463/899
Due to theis
large
sightandiswefound
be >
44°, .which
inunceragreement with the fact that this source does not show any dips in its lightcurve (which implies i < 60°). The inner radius of
trons (kT e ∼ 3.3 keV using the compTT model or kT e ∼ 4.8 keV
A&A, 365, L7
tainties on the outer radius of the disk and on the inclination of dition to a thermally comptonized continuum model nthComp
the accretion
disk
is free,
estimated
100
the etneutron
star center.
means that the using
disk the
would
be truncated
quite
far from
the neutron
star Di
surface.
model from
by Zdziarski
al. (1996), extended
by Zycki etThat
al.
the system if they
were let
we have frozenat
the25
first−one
at km
nthComp
model)
off
soft
photons
(kT
∼
0.7
keV
Salvo, T., D’Ai,
2
seed
◦
(1999))
instead
of
compTT.
The
χ
/d.o.f.
=
1555/901.
Because
1000 Rag and
the inclination
at 60 (the
source does not show any
Usually
soft
blackbody
broad band X-ray spectra of LMXBs, interpreted as using
emitted
by the model
accretion
This model).
component
is not
◦is required to fit the
the compTT
or thedisk.
nthComp
The seed
pho-significantly
Di Salvo, T., Iaria,
dip in its ligtcurve, implying i < 60 ). The improvement of the the iron line was found as a broad line in the previous models
2
(Model
1 and
Model 2), the relativistic
taken into
fit corresponds
to ∆χ
= 26 for the addition
of two parameters
detected
in the
XMM-Newton
spectrum.
This
is in
agreement
with aeffects
diskwere
truncated
relatively far from
the
neutron
star. are compatible with coming from the
tons
for
Comptonization
542, 1034
account
using
the
rdblur
component.
The
addition
of
this
com(the F-test gives a probability of chance improvement of about
neutron star surface. In fact we can estimate the size of the emit- Fabian, A. C., Iwa
the Model 5 and led to χ2 /d.o.f.
10−4 ). This model (called Model 2) gives an estimation of the in- ponent to the model constitutes
−12
2
=
1466/899
(F-test
∼
10
). Assuming that iron has solar abunner radius of the disk Rin ∼ 18 Rg (GM/c ). We searched for an
ting region of the soft photons, using the formula given by in
2000, PASP, 112
dance
and
freezing
the
betor
index
to
-2.7
(standard
value)
and
absorption edge in the energy range 7−10 keV, but no statiscally
X-ray Binaries
2Energy View of Accreting Objects: AGN and
Nikolaos,
Crete,
Greece 5-14
October
’t Zand
et al. (1999). For this, one Agios
assumes
that the
bolometFabian,
A. 2010
C., Re
the ionization parameter ξ = LHigh
X /(nr ) at 660, where LX is the
significant edge was detected.
We also used a new model called relline1 (Dauser et al., ionizing X-ray luminosity, n is the density in the refector, and
ric luminosity of the soft photons is equal to the correspondMNRAS, 729