The INTEGRAL view of 511 keV annihilation line in our Galaxy

Transcript

The INTEGRAL view of 511 keV
annihilation line in our Galaxy
G. De Cesare, IASF-INAF, Rome
[email protected]
X-ray Astrophysics up to 511 keV, Ferrara September 14-16 2011
lunedì 12 settembre 2011
Outline
I.
First detection of positrons in cosmic rays
II.
Galactic positrons as revealed by 511 keV annihilation line
III.
A possible 511 keV emission from the microquasar 1E 1740.7-2942 in 1990
IV.
The INTEGRAL gamma-ray observatory
V.
SPI key results on the diffuse Galactic 511 keV emission
VI.
IBIS search for Galactic 511 keV point sources
VII.
Discussion
VIII. Conclusion
First detection of a positron in cosmic rays
The positron is the antiparticle of the electron. It has a positive electric charge +e (e = 1.602177 10-19
C), a spin 1/2 and the same mass as the electron (me = 511 keV/c2);
Its existence was postulated by Paul Dirac in 1928 as a consequence of the quantum theory of the
electron;
The first experimental evidence of positrons dates 1932, when ground cosmic ray positrons were
discovered by Carl D. Anderson
6 mm lead plate
positron track
The first positron track identified in a cloud chamber
First detection
The
511
ke V
annihilatio n
emission
f ro m
the
f ro m
positro ns
d irectio n
of
the
Galactic center was first discovered in the
early 1970s, by balloon-born detectors with
low energy resolution (Johnson et al. 1972)
The line was clearly identified a few years
later with high energy resolution Ge detectors
(Leventhal et. Al., 1978)
The energy distribution
Annihilation of positron can take place by direct annihilation in two opposite 511 keV photons or by
formation of a prositronium, i.e. a bound state made by an electron and a positron. The positronium can
be formed with two total spin (S) state:
S=0 para-positronium (pPs) -> two 511 keV photons
S=1 ortho-positronium (oPs) -> three photon with a continuum of energies up to 511 keV
By measuring the intensities of the 511 keV
line and of the Ps continuum one can
derive
the
fraction
of
positrons
that
annihililate by positronium formation:
I3γ
I2γ
1
∝ 2(1 − fPs ) + 2fPs
4
fPs
Leventhal 1978
3
∝ 3fPs
4
8I3γ /I2γ
=
9 + 6I3γ /I2γ
A possible 511 keV emission from microquasar 1E 1740.7-2942 in 1990
The 1 E 1740.7 microquasars has been discovered by Einstein X-ray telescope (Herz and Grindlay,
1984). Its position is about 1 degree from the Galactic center. It is reported in the INTEGRAL/
IBIS in hard X catalogue with a flux of (29.8 ± 0.1) mCrab (A.J. Bird, 2007)
During the 1990 October 13 the SIGMA telescope,
aboard the GRANAT space observatory, discovered
that this source was exhibiting a remarkable feature
in its emission spectrum (See e.g. Bouchet et al. 1991)
This transient feature appeared during a 13 hours
obser vation
and
then
possibly
in
two
further
occasions but at a less significant level.
The line is transient (13 hours) and very broad
(hudred keV)
The 511 keV feature of 1E 1740.7-2942 was not
confirmed by OSSE measures (Jung et al. 1995).
The
INTEGRAL
gamma-ray observatory
Scientific instruments on-board
INTEGRAL, the second ESA gamma-ray mission
(the first being COS-B launched in 1975) was
launched by PROTON from Baikonur in Kazakistan
on 17th October 2002.
INTEGRAL’s
scientific
spe ctros copy
with
goal
imag ing
are
met
by
fine
capabi li tie s
and
accurate positioning of the celestial sources.
SPI
and
IBIS,
the
two
gamma
ray
instrument
aboard the INTEGRAL satellite, are optimized for
spectroscopy
and
imaging,
respectively. Both
instrument are based on coded mask imaging.
SPI and IBIS capabilities
The spectrometer SPI is based on 19 high energy resolution Germanium detection units.
The imager IBIS position sensitive detector is based on two detection plane (about 20 000 detection units):
ISGRI (128 x 128 = 16384 Detection Units)
PICsIT (64 x 64 = 4096 CsI Detection Units)
SPI detectors
Energy
range
Field of
View
Energy
resolution
SPI
IBIS
20 keV - 8 MeV
15 keV - 10 MeV
16o
9o x 9o (fully
coded)
(corner to
corner)
2.33 keV @ 1.33
MeV
19o x 19o (partially
coded, 50 %)
ISGRI: 8 % at 100
keV
IBIS
PICsIT: 10 % at
1MeV
Angular
resolution
~ 2.0o
12’
Coded mask imaging
A pinhole camera concept
It not easy to focalize gamma ray photons, but imaging is also possible without any lens or mirror. The
coded mask imaging is an extension (with larger aperture) of the pinhole camera concept. In a coded
mask system it is possible to obtain for a given point source a simultaneous measure of the source and
background count rate.
A coded aperture camera working principle: a
3D scheme in the left panel, a cut view in the
rig ht panel. The white mask elements are
transparent to gamma rays, while the black ones
are opaque. The recorded count rate in each
pixel of the detection plane (shadowgram) is the
summation of contributions from each source
flux modulated by the mask.
IBIS/ISGRI map of the Galactic central region in the
20-40 keV energy range
Pro: large field of view
Con: high background
A summary
Intensity: The total rate of positrons annihilation observed observed in gamma-rays is about 2 x 1043 s-1.
Most of it comes from the bulge.
Morphology: The bulge/disk ratio of e+ annihilation is B/D ~ 1.4 About half of the disk emission can be
explained by the observed radioactivity of
26Al
(1800 keV line)
Variability: There is not evidence of 511 keV flux variability.
Spectroscopy: The emission is made by a narrow (~ keV) line and a 3-gamma ortho-positronium continuum.
The energy spectrum can be explained by positronium decay (fPs = (100 ± 2) %, Churazov et al. 2011)
Initial energy: The positrons injection energy must be less then ~ 3 MeV (otherwise the emission from inflight annihilation would exceed the observed gamma ray flux measured by COMPTEL)
A possible correlation with the LMXBs population
Some evidence of an asymmetry of the 511 keV emission along the Galactic longitude, possibly correlated with the
spacial distribution of the hard X (E > 20 keV) LMXBs detected by the imager IBIS aboard INTEGRAL, has been reported
The SPI sky map in the 511 keV electron-positron annihilation line and the sky
distribution of the hard X (E > 20 keV) LMXBs detected by IBIS (Weidenspointner et al.,
2008)
But a different analysis of the same SPI data do not confirm this asymmetry (Bouchet et
al., 2008, Churazov et al., 2011) ...
Flux profile and spectrum
SPI flux profile along the Galactic plane in 3 energy
band (511 keV band in the center), obtained with a
simple scan (“light bucket imaging”). The gray bars show
the positions of a series of scans perpendicular tio the
Galactic plane. (Churazov et. al 2011).
Broad 30-1000 keV spectrum of the Galactic Bulge emission
measured by SPI (Churazov et. al 2011).
Possible astrophysical sources
Basically, any model has to explain how generate 2 x 1043 s-1 positrons with an initial energy of the order
of MeV and why they are concentrated in the Galactic bulge. I mention here some possible solutions:
Radioactivity from stellar nucleosynthesis: positrons can be emitted by the β+ decay of instable nucley, that
are produced either hydrostatically in massive stars or explosively in novae or SN explosions.
Astrophysically important positron
emitting radioactivities
X-ray binaries and microquasars: e+e- pairs can be created either in the hot inner accretion disk, in the Xray corona surrounding the disk, or at the base of the jet. If jets consist of e+ e- plasma or e- p plasma, e
+ e- pairs can be also created by the jet interaction with the ISM or with the companion stars.
The supermassive black hole in our Galaxy (Sgr A*): this source was proposed in the 1980s on the basis of
the variability found in the HEAO-3 data. This variability was not confirmed by the subsequent measures.
Therefore this model requires that (1) the positrons have time to file the bulge before annihilate and (2) that
the Sgr A* activity was much higher in the past then now.
Dark matter: Positrons from dark matter decays have been discussed as a possibility, mainly because a
rather spheroidal, symmetric, bulge-centered annihilation emission morphology.
The data sample and analysis
With our sample, we reach a 10 Ms exposure in the Galactic center region. The data set consists of all
the IBIS data accumulated until April 2007. In addition, our sample includes all the Core program data
until April 2008. All the data that we have reduced correspond to 39413 IBIS pointings each lasting
about 2000 s., i.e. a total observing time of about 80 Ms. The IBIS pointings are also called Science
Windows (ScWs).
The IBIS data consists in a large set of small
Data Unit, named Science Windows (ScWs). Each
ScW, associated to one IBIS pointing, corresponds
to one observation of ~2000 s. A sky map is
obtained by a mosaic of n ScWs.
The data reduction is CPU time consuming: to
increase sensitivity a large sample is needed. Our
data sample consists in about 5 years of data,
which corresponds to
The exposure map for the IBIS all sky 511 keV data
analysis. The data set consists in about 5 year of
observations. The exposure obtained in the Galactic
bulge (yellow and white colors) is 10 Ms (De Cesare,
2011)
~ 40 000 ScWs, each one
has some cost in term of computing time.
The data analysis at 511 keV requires calibration:
we need to evaluate the energy resolution,
background count rate, and the effective area.
the
Physical Monte Carlo simulation
This tool is used to estimate the IBIS detector effective area and sensitivity at 511 keV
An IBIS Monte Carlo simulation with a 60 keV
The flowchart of IBIS Monte Carlo. The
code is based on the CERN GEANT library
to
repro duce
gamma-rays
the
with
passive materials.
interaction
IBIS
of
detectors
the
and
mono-energetic
beam,
simulating
on-axis
point source. As expected, the ISGRI detector
co u nt
map
( le f t
p a n e l) ,
the
s o - c a l le d
shadowgram, reproduces the IBIS mask. After
decoding the detector map, the point source is
shown in the right panel.
an
Instrumental background long term monitoring and detector effective area
The 511 keV line sensitivity basically depends on the background count rate and on the effective area.
ISGRI background count rate vs revolution
Solar activity
Simulating an on-axis source with the calibrated Monte Carlo, we estimate the following peak
effective area at 511 keV:
AIef f = (27.1 ± 0.1) cm2
The 511 keV line sensitivity: a first estimation
The sensitivity is taken to mean as the minimum flux that a source can have with a appreciable
chance to be detected with a given exposure. Thus I define the sensitivity as the flux detected at 2
sigma of significance level.
We measure: Count = Source + Background
Then: Source = Count - Background
Assuming B >> S, the source signal to noise is given by (Goldwurm et al., 2003):
Signal
Reconstructed Source Count
S×T
√
=
=!
#S×
N oise
T otal Count
(S + B)T
"
T
B
Using the background measure and the effective area (MC estimated), for an exposure T = 10 Ms we
estimate for IBIS/ISGRI a 2 sigma sensitivity of the order of 10-4 ph cm-2 s-1
The instrument gamma-ray background is mainly due to the cosmic interaction with the the satellite and the upper
atmosphere. At the beginning of the INTEGRAL observation, we measure for ISGRI a background count rate of:
I
B511
keV = (7.85 ± 0.06)counts/s rev. 50
If one assumes a source with a 511 keV flux F = 10-3 ph cm-2 s-1 (the total flux measure by SPI) the source count is S
= F x Aeff = 0.027 counts/s. The source to background ratio is S/B = 0.34 %.
The 511 keV line sensitivity: Monte Carlo simulation
A simulation of a 511 keV point source as seen by
IBIS/ISGRI during a pointing in the early (rev. 50),
rev. 50
rev. 600
left panel, and late (rev. 600), right panel, orbits of
the INTEGRAL satellite. The signal level of the
detected 511 keV source goes to 10 σ in the first
image to 7 σ in the second one. These images have
been obtained adding to the real data the data
generated by the IBIS Monte Carlo (one million
511 keV photons as input) and then processing the
result with the standard analysis software until the
image level.
The sensitivity with an exposure of
10 Ms estimated by MC simulation
The 511 keV line sensitivity: Crab detection
40-100 keV
40-100 keV
431-471 keV
491-531 keV
The Crab nebula detected by IBIS with an exposure of 3 Ms
The Crab nebula as detected by IBIS/
ISGRI in the soft gamma-ray domain
with a 3 Ms of exposure. Left and right
top
panel:
the
ISGRI
40-100
keV
shadowgram accumulated during one
ScW and the decoded sky image. Left
The IBIS 511 keV sensitivity as estimated with the Crab
data
bottom panel: sky mosaic in 431-471
keV (colorbar: 2 - 8 σ). Right bottom
panel:
sky
mosaic
in
491-531
keV
(colorbar: 3 - 4 σ).
The 511 keV line sensitivity map
Processing about 5 year of IBIS data I do not find in a 511 keV full sky map any
evidence of point source.
The best flux limit constraint in this map is equal to 1.6
10−4 ph cm−2 s−1 for 1 E 1740.7-2942 and GRS 1758-2258,
the two IBIS microquasars located near the center of the
Galaxy. For the sources with lower exposure, as for example
Sco X-1 that has a flux upper limit of 3.7 10−4 ph cm−2 s−1,
our observations gives a less tight constraint on the 511
keV emission.
LMXBs 511 keV upper limits
De Cesare 2011
Sky distribution of the the LMXBs detected
by
IBIS
above
20
keV. The
red
point
corresponds to the LMXBs with |b| < 10
degrees, while for the blue ones |b| > 10
degrees. A
possible
correlation
between
…
…
…
…
…
this source distribution and the SPI 511
keV
map
has
been
p o i nt e d
o ut
by
Weidenspointner et al., 2008
…
…
Microquasars 511 keV upper limits
The possibility to detect a 511 keV radiation due to the
jet positrons hitting the atmosphere of the companion
star,
in
the
case
of
misaligned
microquasars,
is
discussed by Guessoum et al. 2006.
De Cesare 2011
The Galactic Center monitoring
A possible 511 keV emission from the Galactic Center has been also monitored during the
INTEGRAL visibility periods at Ms timescale, without finding any signal. The flux upper
limit are reported here:
De Cesare 2011
In data analysis a sky map has been generated for each INTEGRAL revolution, therefore also
monitoring the IBIS 511 keV sky in day-timescale.
Discussion
Low Mass X-ray Binaries
The observed distribution of LMXRBs in the Galaxy is strongly peaked towards the central regions, similar to that
of the 511 keV emission
In terms of total energy the LMXBs population could explain the 511 keV flux
n
f=
4πd2
n = 2 × 1043 e+ s−1
d = 10 kpc
(taking into account of the positron fraction)
fSP I (511 keV ) = 10
−3
ph
cm2 s
Ee+ = 511 keV = 10−6 erg
L511 keV = 2 × 1037 erg/s
This value is 50 times lower than the total
luminosity of the LMXBs (Grimm et al., 2002)
Ltot (LM XB) = 1039 erg/s
Discussion
Positronium continuum 3-gamma emission
The proof of positrons annihilation in compact object could be found by detecting the gamma rays produced in
the 3-gamma positronium decay.
The signal depends on the positronium fraction, being maximum when all the positrons annihilate by the
positromium formation.
I3γ
I2γ
3
∝ 3fPs
4
fPs = 1
1
∝ 2(1 − fPs ) + 2fPs
4
fPs
8I3γ /I2γ
=
9 + 6I3γ /I2γ
I3γ
9
=
I2γ
2
Depending on the positronium fraction the ratio between continuum and
line goes from 0 to 9/2 We note also that the gamma ray sensitivity is
better at lower energies...therefore one possibility is to analyze all the
INTEGRAL archival data looking for some positronium component in a
galactic compact object.
Conclusions
By reducing 5 year of the IBIS imager data, I do not find any evidence of 511 keV lines feature
from BH sources. The 2 sigma flux upper in the Galactic center region, with an exposure of 10 Ms,
is estimated at a level of 1.6 x 10-4 ph cm-2 s-1. In principle it will be possible to obtain a much
lower flux limits with the application of the gamma-ray lenses in astronomy.
In particular analyzing data at day-mounts-year timescales, IBIS does not not detect any
persistent or transient 511 keV emission from 1E 1740.7-2942 (from this source a transient
emission with SIGMA was reported in 1990).
The Galactic 511 keV emission measured by SPI spectrometer (narrow line, large positronium
fraction, diffuse emission) is generally explained by positrons propagation in the interstellar
medium before they annihilate away from the birth place.
There is some evidence of an asymmetry of the 511 keV emission along the Galactic longitude,
possibly correlated with the spacial distribution of the hard X (E > 20 keV) LMXBs detected by
IBIS. I have shown in this presentation that also in terms of energy LMXRs are possible candidates.
In principle, it is possible to search for the 3 gamma continuum in Galactic BH. We note that the
intensity of this component depends on the positronium fraction, being 9/2 of the 511 keV line
intensity if all positrons annihilate by formation of positronium atoms.

The INTEGRAL view of 511 keV annihilation line in our Galaxy

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