J/ApJ/827/139 Microlensing optical depth & event rates from MOA-II (Sumi+, 2016)
Possible solution of the long-standing discrepancy in the microlensing optical
depth toward the Galactic bulge by correcting the stellar number count.
Sumi T., Penny M.T.
<Astrophys. J., 827, 139-139 (2016)>
=2016ApJ...827..139S 2016ApJ...827..139S (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way ; Gravitational lensing ; Surveys
Keywords: Galaxy: bulge; gravitational lensing: micro
Abstract:
We find that significant incompleteness in stellar number counts
results in a significant overestimate of the microlensing optical
depth τ and event rate per star per year Γ toward the
Galactic bulge from the first two years of the MOA-II survey. We find
that the completeness in red clump giant (RCG) counts fRC decreases
proportional to the galactic latitude b, as
fRC=(0.63±0.11)-(0.052±0.028)xb, ranging between 1 and 0.7 at
b=-6°~-1.5°. The previous measurements using all sources by
difference image analysis (DIA) by MACHO and MOA-I suffer the same
bias. On the other hand, the measurements using an RCG sample by
OGLE-II, MACHO, and EROS were free from this bias because they
selected only the events associated with the resolved stars. Thus, the
incompleteness both in the number of events and stellar number count
cancel out. We estimate τ and Γ by correcting this
incompleteness. In the central fields with |l|<5°, we find
Γ=[18.74±0.91]x10-6exp[(0.53±0.05)(3-|b|)]/star/yr and
τ200=[1.84±0.14]x10-6exp[(0.44±0.07)(3-|b|)] for the 427
events with tE≤200 days using all sources brighter than
Is≤20mag. Our revised all-source τ measurements are about
2σ smaller than the other all-source measurements and are
consistent with the RCG measurements within 1σ. We conclude that
the long-standing problem on discrepancy between the high τ with
all-source samples by DIA and low τ with RCG samples can probably
be explained by the incompleteness of the stellar number count. A
model fit to these measurements predicts
Γ=4.60±0.25x10-5/star/yr at |b|~-1.4° and
-2.25°<l<3.75° for sources with I≤20, where the future space
mission, Wide Field Infrared Space Telescope, will observe.
Description:
We use the same data set as Sumi+ (2013, J/ApJ/778/150), which used
the data taken in the 2006 and 2007 seasons by the MOA-II survey, with
the 1.8m MOA-II telescope located at the Mt. John University
Observatory, New Zealand.
The centers of the 22 Galactic bulge (GB) fields of the MOA-II survey
are listed in Table 1. 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. Each field is divided into 80 subfields
and each subfield is individually calibrated using the red clump giant
(RCG) feature in each subfield CMD more precisely. The number of
subfields used in the final analysis is 1536 in total and also given
in Table 1 for each field. The coordinates and other properties of the
subfields are listed in Table 4.
The OGLE (Udalski 2003AcA....53..291U 2003AcA....53..291U) also conducts a microlensing
survey toward the GB with the 1.3m Warsaw telescope at the Las
Campanas Observatory in Chile. The median seeing is about 1.3 arcsec.
Most observations are taken in the standard Kron-Cousin I band with
occasional observations in the Johnson V band.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table4.dat 145 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/ApJ/778/150 : Microlensing events toward the Bulge from MOA-II (Sumi+, 2013)
J/A+A/549/A147 : Abundances of microlensed Bulge stars. V. (Bensby+, 2013)
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/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://ogle.astrouw.edu.pl/ogle4/ews/ews.html : OGLE-IV early warning system
Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- Field The field-chip-subfield identifier
11- 18 F8.4 deg GLON Galactic longitude
20- 26 F7.4 deg GLAT Galactic latitude
28- 30 I3 --- Nsub [26/115] Number of subfields (1)
32- 39 I8 --- Nstar Number of stars in subfields (2)
41- 43 I3 --- Nev [2/118] Number of events in subsfields (2)
45- 52 F8.6 10-6 tau [0.1/8.6] Microlensing optical depth
in 1e-6 (3)
54- 61 F8.6 10-6 s_tau [0.09/4.2] The 68% symmetric error in tau
63- 70 F8.6 10-6 e_tau Lower 68% error in tau
72- 79 F8.6 10-6 E_tau Upper 68% error in tau
81- 87 F7.4 10-6/yr Gamma [2/43] Event rate per star per year
in 1e-6
89- 95 F7.4 10-6/yr s_Gamma [1/26] The 68% symmetric error in Gamma
97-103 F7.4 10-6/yr e_Gamma Lower 68% error in Gamma
105-111 F7.4 10-6/yr E_Gamma Upper 68% error in Gamma
113-120 F8.4 deg-2/yr Gammad [2/164] Event rate per square degree per year
122-128 F7.4 deg-2/yr s_Gammad [4/98] The 68% symmetric error in Gammad
130-137 F8.4 deg-2/yr e_Gammad Lower 68% error in Gammad
139-145 F7.4 deg-2/yr E_Gammad Upper 68% error in Gammad
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Note (1): Used for estimating tE and ε within 1.0 degree around
the subfields.
Note (2): Used for estimating tau, Gamma, Gammad within 1.0 degree around
the subfield
Note (3): Averaged within 1.0 degree around the subfield.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 28-Nov-2016