J/ApJ/778/150 Microlensing events toward the Bulge from MOA-II (Sumi+, 2013)
The microlensing event rate and optical depth toward the Galactic Bulge
from MOA-II.
Sumi T., Bennett D.P., Bond I.A., Abe F., Botzler C.S., Fukui A.,
Furusawa K., Itow Y., Ling C.H., Masuda K., Matsubara Y., Muraki Y.,
Ohnishi K., Rattenbury N., Saito TO., Sullivan D.J., Suzuki D.,
Sweatman W.L., Tristram P.J., Wada K., Yock P.C.M., (The MOA Collaboration)
<Astrophys. J., 778, 150 (2013)>
=2013ApJ...778..150S 2013ApJ...778..150S
ADC_Keywords: Gravitational lensing ; Milky Way ; Surveys
Keywords: Galaxy: bulge; gravitational lensing: micro;
stars: luminosity function, mass function
Abstract:
We present measurements of the microlensing optical depth and event
rate toward the Galactic Bulge (GB) based on two years of the MOA-II
survey. This sample contains ∼1000 microlensing events, with an
Einstein radius crossing time of tE≤200 days in 22 bulge fields
covering ∼42deg2 between -5°<l<10° and -7°<b←1°.
Our event rate and optical depth analysis uses 474 events with
well-defined microlensing parameters. In the central fields with
|l|<5°, we find an event rate of
Γ=[2.39±1.1]e[0.60±0.05](3-|b|)x10-5/star/yr and an
optical depth (for events with tE≤200days) of
τ200=[2.35±0.18]e[0.51±0.07](3-|b|)x10-6 for the 427
events, using all sources brighter than Is≤20mag. The distribution
of observed fields is centered at (l,b)=(0.°38, -3.°72). We
find that the event rate is maximized at low latitudes and a longitude
of l~1°. For the 111 events in 3.2deg2 of the central GB
at |b|≤3.°0 and 0.°0≤l≤2.°0, centered at
(l,b)=(0.°97, -2.°26), we find
Γ=4.57-0.46+0.51x10-5/star/yr and
τ200=3.64-0.45+0.51x10-6. We also consider a red clump
giant (RCG) star sample with Is<17.5, and we find that the event
rate for the RCG sample is slightly lower than but consistent with the
all-source event rate. The main difference is the lack of long
duration events in the RCG sample due to a known selection effect. Our
results are consistent with previous optical depth measurements, but
they are somewhat lower than previous all-source measurements, and
slightly higher than previous RCG optical depth measurements. This
suggests that the previously observed difference in optical depth
measurements between all-source and RCG samples may largely be due to
statistical fluctuations. These event rate measurements toward the
central GB are necessary to predict the microlensing event rate and to
optimize the survey fields in future space missions such as Wide Field
Infrared Space Telescope.
Description:
The data set used in this analysis is the same set used by Sumi et al.
(2011Natur.473..349S 2011Natur.473..349S). That is, it was taken in the 2006 and 2007
seasons by the MOA-II survey, with the 1.8m MOA-II telescope located
at the Mount John University Observatory, New Zealand. The images were
taken using the custom MOA-Red wide-band filter, which is equivalent
to the sum of the standard Kron/Cousins R and I bands.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 87 1536 MOA-II subfields with Galactic coordinates, the
numbers of source stars and events, and the mean
timescale and efficiencies for the all-source sample
table3.dat 87 1536 MOA-II subfields with Galactic coordinates, the
numbers of source stars and events, and the mean
timescale and efficiencies for the red clump giant
(RCG) sample
table4.dat 99 474 Microlensing events used in the optical depth and
event rate measurements
table7.dat 148 1536 Average microlensing optical depth and event rates at
the position of each subfield for the all-source
sample
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See also:
J/ApJS/216/12 : OGLE-III GB microlensing events (Wyrzykowski+, 2015)
J/ApJ/711/L48 : 2008 OGLE Bulge microlensing alerts (Cohen+, 2010)
J/A+A/454/185 : EROS-2 microlensing parameters (Hamadache+, 2006)
J/ApJ/636/240 : OGLE II microlensing parameters (Sumi+, 2006)
J/ApJ/631/906 : MACHO Galactic Bulge microlensing events (Thomas+, 2005)
J/AcA/52/217 : OGLE II. VI photometry of Galactic Bulge (Udalski+, 2002)
J/AcA/51/175 : OGLE-II DIA microlensing events (Wozniak+, 2001)
J/AcA/50/1 : OGLE microlensing events in Galactic Bulge (Udalski+, 2000)
http://www.massey.ac.nz/~iabond/moa/ : Microlensing Observations in
Astrophysics (MOA) alerts home page
http://ogle.astrouw.edu.pl/ogle4/ews/ews.html : OGLE-IV early warning system
http://wise-obs.tau.ac.il/~wingspan/ : WISE events by seasons
Byte-by-byte Description of file: table[23].dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- Field Field-chip-subfield identifier
11- 17 F7.4 deg GLON [-5.3/10.6] Galactic longitude
19- 25 F7.4 deg GLAT Galactic latitude
27- 32 I6 --- N* [939/128758] Number of stars in the subfield
34 I1 --- Nev [0/5] Number of events in the subfield
36- 42 F7.4 d <tE> [6.9/54.5] Mean Einstein time scale <tE>
44- 51 F8.6 --- [0.1/0.6] Mean detection efficiency <ε>
53- 55 I3 --- Nsf [26/629] Number of subfields used (1)
57- 61 F5.1 --- NsfW [7.5/202.2] Number of weighted subfields used (2)
63- 70 I8 --- N*MA Number of stars in subfields used (1)
72- 78 I7 --- N*MAW Number of weighted stars in subfields used (2)
80- 82 I3 --- NevMA [2/118] Number of events in subfields used (1)
84- 87 F4.1 --- NevMAW [0.1/39.5] Number of events in weighted
subfields used (2)
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Note (1): For estimating <tE> and within 1.0 degree around
the subfield.
Note (2): For estimating <tE> and within 1.0 degree around
the subfield. Weighted by Gaussian with σ of 0.4degrees.
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
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1- 16 A16 --- ID Field-chip-ID identifier
18- 19 I2 h RAh [17/18] Hour of Right Ascension (J2000)
21- 22 I2 min RAm Minute of Right Ascension (J2000)
24- 29 F6.3 s RAs Second of Right Ascension (J2000)
31 A1 --- DE- [-] Sign of the Declination (J2000)
32- 33 I2 deg DEd [21/35] Degree of Declination (J2000)
35- 36 I2 arcmin DEm Arcminute of Declination (J2000)
38- 42 F5.2 arcsec DEs Arcsecond of Declination (J2000)
44- 47 I4 --- Ndat [1348/7736] Number of data points
49- 58 F10.5 d t0 Julian Date peak magnification; JD-24540000
60- 65 F6.2 d tE [0.4/157.6] Einstein radius crossing time
67- 71 F5.2 d e_tE [0.03/12] Uncertainty in tE
73- 80 F8.6 --- u0 [0.0003/0.98] Impact parameter
82- 89 F8.6 --- e_u0 [0/0.3] Uncertainty in u0
91- 94 F4.1 mag Imag [14.8/20] Source I band magnitude
96- 99 F4.2 --- chi2 [0.4/2] The χ2/degree of freedom
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Byte-by-byte Description of file: table7.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 A9 --- Field Field-chip-subfield identifier
11- 17 F7.4 deg GLON [-5.3/10.6] Galactic longitude
19- 25 F7.4 deg GLAT Galactic latitude
27- 29 I3 --- Nsf [26/115] Number of subfields used (1)
31- 38 I8 --- N* [483811/10853361] Number of stars in subfields
used (2)
40- 42 I3 --- Nev [2/118] Number of events in subfields used (2)
44- 52 F9.6 10-6 tau [0.17/11.9] Microlensing optical depth τ (3)
54- 61 F8.6 10-6 s_tau [0.09/5.8] The 68% symmetric error in tau
63- 71 F9.6 10-6 e_tau [0.07/4.4] The 68% lower limit in tau
73- 80 F8.6 10-6 E_tau [0.1/7.2] The 68% upper limit in tau
82- 88 F7.4 10-6/yr Gam [3/59.5] Event rate Γ per star
per 106year
90- 96 F7.4 10-6/yr s_Gam [2/35.5] The 68% symmetric error in Gamma
98-105 F8.4 10-6/yr e_Gam [0.003/10] The 68% lower limit in Gamma
107-113 F7.4 10-6/yr E_Gam [2.5/33.6] The 68% upper limit in Gamma
115-122 F8.4 deg-2/yr Gam.d [2.4/163.1] Event rate Γdeg2
per square degree per year
124-130 F7.4 deg-2/yr s_Gam.d [4.2/97.3] The 68% symmetric error in Gammad
132-140 F9.4 deg-2/yr e_Gam.d [0.7/99.2] The 68% lower limit in Gammad
142-148 F7.4 deg-2/yr E_Gam.d [5.6/42.2] The 68% upper limit in Gammad
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Note (1): For estimating <tE> and within 1.0 degree around the subfield.
Note (2): For estimating tau, Gamma, Gammad within 1.0 degree around
the subfield
Note (3): In 1e-6 averaged within 1.0 degree around the subfield.
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 28-Apr-2015