Download L. Zampieri - Astrophysics Group of the University of Crete

Low metallicity and
UltraLuminous X-ray Sources
Luca Zampieri
INAF-Astronomical Observatory of Padova
M. Mapelli (Universita` di Milano Bicocca)
E. Ripamonti (Universita` di Milano Bicocca)
S. Bressan (INAF-Astronomical Observatory of Padova)
M. Colpi (Universita` di Milano Bicocca)
T. Roberts (Durham University)
Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ
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Outline
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Possible evidences of preferential location of ULXs in low
metallicity environments
Metallicity of the nebula around NGC 1313 X-2
Results of the statistical analysis of a sample of 66 galaxies
observed in X-rays with SFR and metallicities measurements
A fraction of ULXs may originate from massive stars in low
metallicity environments
Conclusions
Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ
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ULXs
ULXs are pointlike, offnuclear X-ray sources
in nearby galaxies with
L >> Ledd for 1 Msun
(L>1.0e39 erg/s)
Most compelling evidence of
binary nature provided by
detection of a modulation in
the X-ray light curve of two
ULXs, interpreted as the
orbital period:
M82 X-1  P~62 days
(Kaaret et al. 2006a,b)
NGC5408X-1  P~115 days
(Strohmayer 2010; but see
poster by Foster et al.)
X-ray flux
variability,
Lx-spectral
variability
X-ray and
optical orbital
modulations
X-ray
spectra:
similar to
BHC spectra
High
L end
of the XLF
of XRBs
ULXs
X-ray binaries
with massive
donors
QPOs in the
power
spectrum
Often
in young
stellar
environ.
Detection of
stellar optical
counterparts
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ULXs: some
recent advancements
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HLX1 in ESO243−49 (Farrell et al. 09), with an inferred isotropic
L~1.0e42 erg/s, interpreted as first unambiguous identification of an
IMBH (Godet et al. 09; Webb et al. 10; Wiersema et al. 10).
But interpretation of the X-ray spectra not unique (Soria et al. 2010)
Jet-inflated bubble around a powerful microquasar in the galaxy NGC
7793, of size and energy content comparable to those around ULXs
(Pakull, Soria & Motch 10; Soria et al. 10)
Optical modulation of 6 days
in NGC 1313 X-2 (Liu et al.
09)
* Significance is low (~3
sigma; Impiobato et al. 10)
* System consistent only with
a 12-15 Msun main sequence
donor dumping matter on a
50-100 Msun BH (Patruno &
Zampieri 10)
Impiombato et al. (2010)
Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ
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ULXs: some
recent advancements
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X-ray quasi-periodic oscillations (QPOs) at frequencies of 3-4 mHz
from a transient ULX in M82 (Feng & Kaaret 10)
Curvature in high S/N X-ray spectra  low temperature (T~5-10
keV) optically thick (tau~3-5) coronae (Gladstone et al. 09)
Critical review of available Mbh estimates  Mbh >> 100 Msun not
required for the majority of bright and persistent ULXs (Zampieri &
Roberts 09)
Emerging picture:
* Most ULXs are accreting X-ray binaries with massive companions
in a rather peculiar state ('ultraluminous state')
* Some may host IMBHs, but most of them do not require BHs this
massive  However, not necessarily all ULXs contain stellar-mass
(<10-20 Msun) BHs
Definitive answer from measurement of dynamical mass function of
ULX counterparts (Roberts et al. 10; Motch et al. 10)
Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ
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Preferential location of ULXs in
low metallicity environments?
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Subsample of galaxies with
X-ray pointed observations
(ROSAT, Chandra, XMM)
from the Catalog of
Neighboring Galaxies
(Karachentsev et al. 04)
Specific ULX frequency
decreases with increasing
host galaxy mass indicating
that smaller, lower metallicity
systems have more ULXs per
unit mass (Swartz et al.
2008; Walton et al., in prep.)
Swartz et al. (2008)
Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ
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Preferential location of ULXs in
low metallicity environments?
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(+) Measurement of the metallicity of the stellar environment also
attempted (NGC 4559 X-7, Cropper et al. 2004; NGC 1313 X-2,
Grise’ et al. 08)  subsolar abundance
(+) Optical spectrum of the nebula of Ho II X-1  Z~0.1 Zsun
(Pakull & Mirioni 02), but XMM-Newton RGS spectrum  Z~0.6
Zsun (Goad et al. 06)
(-) Analysis of high signal-to-noise ratio XMM spectra of a sample of
14 ULXs, trying to determine the oxygen abundance from the
detection of K-shell photoionization edges  solar abundance
(Winter et al. 2007)
Simulated EPIC-pn spectra fitted with same input model with zero oxygen+edge
systematically overestimate the abundance (Pintore & Zampieri 10)
Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ
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Metallicity of the nebula
around NGC 1313 X-2
Comparison with a grid of photoionization models computed with
Cloudy (Ferland 96)
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Agreement with
metallicity of HII
regions in NGC 1313
(Ryder 93; Walsh &
Roy 97; Hadfield &
Crowther 07)
Z=0.15-0.5 Zsun
Ripamonti et al. (2010)
Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ
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Galaxies observed in X-rays with
measurements of SFR and metallicity
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Statistical analysis of a sample of galaxies hosting ULXs plus a
number of galaxies of the Local Group (excluded ellipticals)
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Mean Z = 0.22+/-0.11
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66 galaxies
observed in X-rays
with SFR and Z
measurements
SFR  average of
available estimates
Z from intensity of
emission lines of
HII regions
(calibration of
Pilyugin & Thuan
05)
Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ
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Galaxies observed in X-rays with
measurements of SFR and metallicity
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NULX
Strong correlation between Nulx and SFR (e.g. Grimm et al.
2003; Mineo et al. 2010)  Nulx ~ A*SFR
A=1.20+/-0.2
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Mapelli et al. (2010)
There is marginal
(~2 sigma)
evidence of
different
normalizations
between high-red
(Z>0.2 Zsun) and
low-black (Z<0.2
Zsun) metallicity
galaxies
Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ
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Galaxies observed in X-rays with
measurements of SFR and metallicity
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Is there a residual dependence on Z after subtracting off the
dominant effect of SFR?
Fit of Nulx/SFR vs Z using a power-law wrt a fit with a constant:
improvement significant at the 96% confidence level
Mapelli et al. (2010)
Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ
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ULXs forming in
low metallicity environments
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Besides SFR, metallicity may be a further ingredient playing a
role in forming and/or triggering the ULX phenomenon: How
to explain this connection?
Try to explan it within the framework of BH formation from
the direct collapse of massive stars in low-metallicity
environments:
If a massive star retains an envelope more massive than ~30-40 Msun at
the time of explosion, it may collapse directly to form a BH (Zampieri 02;
Heger et al. 03)
The BH mass is comparable to the final star mass: Mbh>30 Msun (e.g.
Fryer 99; Fryer & Kalogera 01; Belczynski et al. 10)  massive (stellarremnant) BH
Lower Z stars tend to have more massive envelopes because mass loss is
less efficient  Low Z environment may produce more and bigger BHs
Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ
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ULXs forming in
low metallicity environments
Zampieri & Roberts (2009)
Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ
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ULXs forming in
low metallicity environments
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60 Msun He
core (WR star)
from 120 Msun
star (Bressan,
Marigo, Girardi
et al. 10)
Bressan et al. (2010)
A possibly independent piece of evidence: Dynamical mass of IC 10
X-1  23-33 Msun (Prestwich et al. 07; Silverman & Filippenko 08)
Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ
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ULXs forming in
low metallicity environments
mmax=120 Msun  maximum stellar mass
mprog(Z)  minimum MS mass to form a BH through direct collapse
from Portinari et al. (98) and Belczynski et al. (10)
mprog(0.3 Zsun)=80 Msun; mprog(0.01 Zsun)=60 Msun
Distribution of BHs  IMF (Kroupa 2001) above mprog
mmin=0.08 Msun  minimum stellar mass
tco=10 Myr  characteristic lifetime of a binary companion (typical of
a 15 Msun donor)
Nbh  number of massive BHs that have an active phase
Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ
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ULXs forming in
low metallicity environments
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Cartwheel: Nulx explained with a reasonable production efficiency
and fraction of star-forming mass ending up in BHs (Mapelli et al. 09)
Linear fit is statistically consistent with the observed correlation
Mapelli et al. (2010)
Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ
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Conclusions
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Marginal statistical evidence of Z dependence in ULX formation:
More, and more accurate measurements needed
A fraction of ULXs (most of the the bright tail) may be originating
from the direct collapse of massive stars formed in low-metallicity
environments
Binary evolution at low Z? See Vicky’s talk
Z dependence possibly connected to:
* Evolution of massive stars in low Z environments
* Relative strengths of the ULX-HMXB formation pathways (Linden
et al. 10)
Hydrodynamics of direct BH formation
* How much energy/mass is ejected?
* How symmetric is it? What is the typical natal kick?
Low Metallicity and ULXs - Crete – Oct 14, 2010 - LZ
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