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Studies of the faint X-ray source populations
in the SMC
University of Crete, Greece
Harvard-Smithsonian Center for Astrophysics
Vallia Antoniou
In collaboration with:
Andreas Zezas (CfA), Despina Hatzidimitriou (UoC)
362 Galactic Foreground Cluster
Why do weNGCobserve
the Small
Magellanic
Cloud
?
Why do we observe
the Small
Magellanic
Cloud?
 2nd nearest star-forming galaxy (~60kpc)
 Low interstellar absorption
47 Tuc
 Well determined
 metallicity (Z~0.2Z◉)
 stellar populations (e.g. Harris & Zaritsky, 2004; Gardiner & Hatzidimitriou,
1992)
* young (~ 8-30Myr): in the center
* intermediate (< 500Myr): drop rapidly in larger distances
E
* old (~ 2-10Gyr): in a fairly regular spheroid extending to the
outer regions of the SMC
Anglo-Australian Observatory/Royal Obs.Edinburgh (UK Schmidt plates by David Malin)
N
XRBs in the SMC
 large population of HMXBs
Be-XRBs: most numerous sub-class
 population associated with recent SF
 Classification of different type of sources (e.g. Be/SG - XRBs)
understand the connection between SF and XRB formation
 Number statistics of these different classes
 Luminosity functions
study the faint end of the luminosity distribution of XRBs &
compare it with the LF of other galaxies
X-ray study of the SMC
Chandra observations
XMM-Newton observations
FIELD 2
FIELD 3
FIELD 5
FIELD 7
FIELD 1
FIELD 6
FIELD 6
FIELD 4
FIELD 3
FIELD 5
Chandra observations
 122 sources (@ 3 level)
FIELD 3
 Lx ~ 4 x 1033 erg s-1
(0.7-10keV)
FIELD 5
FIELD 7
(Zezas et al., in prep.)
FIELD 6
FIELD 4
 15 pulsars in our fields
3 (out of 15) detected in
our survey
(Edge et al., 2004)
XMM-Newton observations
 144 sources (@ 3 level)
1033
 Lx ~ 3.4 x
(0.5-12keV)
erg
FIELD 2
s-1
NO detections in
XMM Field-5 due to
high background
(1 SSS; Orio et al. 2007)
(Antoniou et al., in prep.)
FIELD 6
FIELD 1
 3 pulsars in our fields :
FIELD 3
1 detected also in our survey
FIELD 5
1 detected without pulsations
(Lx ~ 3.2 x 1034 erg s-1)
1 not detected at all
Online compilation of SXPs
(Coe; last update: June 2007)
SFH of our Chandra fields
42 Myr
Harris & Zaritsky, 2004
0.01
42 Myr
0.01
422 Myr
422 Myr
FIELD 3
FIELD 5
FIELD 7
27 Myr
0.01
168 Myr
42 Myr
FIELD 6
0.01
FIELD 4
42 Myr
0.01
6.7 Myr 422 Myr
422 Myr
SFH of our XMM-Newton fields
0.01
0.01
67 Myr
FIELD 2
17 Myr
0.01
FIELD 1
668 Myr
FIELD 6
11 Myr
FIELD 3
0.01
67 Myr
422 Myr
Harris & Zaritsky, 2004
Optical study of the SMC
 OGLE-II survey
(Optical Gravitational Lensing Experiment; Udalski et al., 1998)
 BVI photometric data for ~2.2M stars
(down to B~20, V~20.5, I~20mag; ~80% completeness at these limits)
 Astrometric accuracy ~0.7”, photometric errors <0.01mag
 Coverage of our Chandra survey ~70%, XMM-Newton survey <40%
 MCPS survey
(Magellanic Clouds Photometric Survey; Zaritsky et al., 2002)
 UBVI photometric data for ~5M stars
(significant incompleteness below V~20)
 Less accurate astrometric & photometric solutions in crowded fields than
OGLE-II
 Coverage of our Chandra/XMM-Newton surveys ~100%
Optical counterparts of our Chandra sources
15.5 Myr
The most likely optical counterpart (113 Chandra sources) :
9 without counterpart
 42 with single counterpart
 62 with multiple matches

…with 89 not previously known!!!
27.5 Myr
49.0 Myr
87.1 Myr
154.9 Myr
275.4 Myr
Chance coincidence probability for bright sources ~ 19%
(Vo < 18.5, (B-V)o < -0.11)
o 10 new candidate Be-XRBs
o 2 new candidate HMXBs
o consistent results with previous classifications in all cases of overlap
(18 in total; all Be-XRBs)
Antoniou et al., in prep
Optical counterparts of our XMM-Newton
sources
The most likely optical counterpart (133 XMM-Newton sources):
 11 without counterpart
 43 with single counterpart
 79 with multiple matches
Chance coincidence probability for bright sources ~ 2%
(Vo < 18.5, (B-V)o < -0.11)
Antoniou et al., in prep
The largest existing sample of Be-XRB
optical spectra

Obtained ~100 excellent quality spectra with the 2dF
spectrograph (AAT)

First results confirmed all of the Be-XRB tentative
classifications based on the CMD

52 Be-XRBs (Chandra sources) have high quality optical spectra
Hatzidimitriou et aL., in prep.

Total number of Be-XRBs in our Chandra fields = 57
(52 spectroscopic + 5 photometric classification)
Number of Be-XRBs in each Chandra field
SF peak
@
42 Myr
422 Myr
# of
# of
pulsars Be-XRBs
3
5
FIELD 3
SF peak
@
# of
pulsars
# of
Be-XRBs
27 Myr
168 Myr
4
5
Compilation of Be-XRBs
(Liu et al. 2005)
+
our new Be-XRBs
(Antoniou et al., in prep.)
SF peak
@
# of
pulsars
# of
Be-XRBs
42 Myr
422 Myr
7
16
FIELD 5
FIELD 7
FIELD 6
SF peak
@
# of
pulsars
# of
Be-XRBs
FIELD 4
42 Myr
422 Myr
1
7
SF peak
@
6.7 Myr
42 Myr
422 Myr
# of
# of
pulsars Be-XRBs
4
24
Number of Be-XRBs in each XMM-Newton field
SF peak
@
# of
pulsars
# of
Be-XRBs
67 Myr
17 Myr
2
8
FIELD 2
SF peak
@
# of
pulsars
# of
Be-XRBs
668 Myr
0
1
FIELD 1
FIELD 6
SF peak
@
# of
pulsars
# of
Be-XRBs
11 Myr
0
11
Compilation of Be-XRBs (Liu et al. 2005)
+
our new Be-XRBs (Antoniou et al., in prep.)
FIELD 3
SF peak
@
# of
pulsars
# of
Be-XRBs
67 Myr
422 Myr
1
13
Normalizing the XRB population to the SFR
• Study the Be-XRBs with respect to their related stellar populations
N(Be-XRBs)/N(OB)
• Minimize age effects or variations due to SFR differences for
populations of different ages
* our candidate SMC Be-XRBs + compilation of MCs HMXBs
(Liu et al. 2005)
* OB stars from MCPS
(Zaritsky et al. 2001)
X-ray source populations as a function of age
McSwain & Gies, 2005
Comparison with the Milky Way
 sample of Be-XRBs (Lx 1034erg/s, within 10kpc of the Sun) :
- compilations of MCs & MW HMXBs (Liu et al. 2005, 2006)
- our candidate SMC Be-XRBs
 OB stars :
- Chandra fields (MCPS; Zaritsky et al. 2001)
- Galactic (Reed 2001)
 Be-XRBs ~2 times more common in the SMC when compared to the MW
There is still a residual excess that can NOT be accounted for by the
difference in the SF rate
Difference in solar & SMC metallicity (0.2Z): Dray 2006 predict a
factor of ~3 higher numbers
Summary
 We present the largest census of Be-XRBs in the SMC so far based
on a combination of Chandra, XMM-Newton, and optical data
 Find a peak of Be-XRBs at ages of ~ 40-60 Myr, and possible
evidence for variation within this age range
 Find an excess of Be-XRBs in the SMC with respect to the MW
In the future:
Extend the analysis to lower luminosities using the Chandra deep
observations
 IMACS - Magellan analysis:
* Identify optical counterparts for currently unidentified sources
* Derive accurate SFH
 Follow-up spectroscopically the candidate counterparts without
spectra
Identify the counterparts for most of the X-ray sources
and probe the connection with the SFH of the SMC in more detail