Ultra high magnification microlensing
Masamune Oguri(University of Tokyo)

2018/2/16 PACIFIC 2018@Kiroro

Dark Matter (DM)• 1/4 of the mass of the

Universe is composedof dark matter• a lot of evidence fromCMB, clusters, galaxies,..

• but its true nature is
still unknown(ESA/Planck)

unknown particle? (WIMP, axion, …){ unknown compact objects? (PBH, …)

explain dark matter with black holes as well as observational studies to constrain theabundance and mass of black holes using microlensing observations.

Microlensing• brightening of a distant star due to gravitational
lensing by a foreground compact object (DM)(MO, Kavli IPMU news No. 38)

simulated by glafic

Microlensing simulation
image:
we observe
source:we would
observe if nolensing effect← cannot resolve
this due to lackof resolution ofobservations← observe lensing
effect via time
evolution ofbrightness ofa starAn example of microlensing• first discovery of microlensing in 1993 frommonitoring observations of >10M stars in LMC• symmetric single peak light curve indicates that
relative brightnessof a star

this is a real microlensing event

time

Alcock+ (1993)monochromatic0.0
WD-0.5KNSMFL
Evaporation

Eri II

log10 fPBH

-1.0

EROS

-1.5

-2.0

HSC
Seg I

-2.5

possible window?

-3.0
-15

-10-5

log10 (Mc /M⊙ )Carr+2017

no discovery of
WB microlensing bycompact DM→constraints on
its abundance

Planckabundance of compact DM (PBH)
relative to total DM

constraints from microlensing

0

mass of compact DM (PBH)

possible window
to explain all
dark matter bycompact DM at
M ~ 10-100 Msun(~LIGO BH masses)

Discovery of new microlensing• recently new fast transient was discoveredat the center of a massive cluster of galaxies
observed in Hubble Frontier Fields (HFF)• interpreted as a highly magnified individual starat z=1.49 (magnifications >1000)• star located very far compared with previously
known microlensing events of stars(>Gpc, compared with ≲Mpc)• this opens a new opportunity to constrain
compact DM scenarios!

MO (2015)

SN Refsdal
multiply imaged
supernova in HFF
first discovery ofresolved SN multiple
images to measurer
time delays

S1SY
SY (~17 years ago)SX
SX (~1 year later)

S2
S3
S4

S1−S4 (ongoing)

5th image detected
at the right time andplace as massmodels predict(MO 2015; Treu+ 2016)

Caustic-Crossing Event

Kelly, .., MO+, arXiv:1706.10279

DiscoveryofIcarus
Discovered
Discovered inin late
lateApril
April“RefsdalRedux”“RefsdalRedux” program
program toto searchsearch forfor reappearance
reappearance

2011
2011
SN
SNRefsdal
RefsdalReappearance
Reappearance

5ʺ5ʺ

F125W
F125W(J)(J)

Late
Late MayMay 2016
2016

Considered
Considered first
first byby (Jordi)(Jordi) Miralda-Escude
Miralda-Escude (1991)(1991)(cluster MACSJ1149)

Caustic-Crossing Event

Kelly, .., MO+, arXiv:1706.10279

Discoveryof
IcarusDiscovered
Discovered inin late
lateApril
April“RefsdalRedux”“RefsdalRedux” program
program toto searchsearch forfor reappearance
reappearance
new transient

2011
2011
SN
SNRefsdal
RefsdalReappearance
Reappearance

5ʺ
5ʺ

F125W
F125W(J)(J)

Late
Late MayMay 2016
2016

Considered
Considered first
first byby (Jordi)(Jordi) Miralda-Escude
Miralda-Escude (1991)(1991)(cluster MACSJ1149)

Caustic-Crossing Event

Kelly, .., MO+, arXiv:1706.10279

DiscoveryofIcarus
Discovered
Discovered inin late
lateApril
April“RefsdalRedux”“RefsdalRedux” program
program toto searchsearch forfor reappearance
reappearance
new transient

2011
2011
SN
SNRefsdal
RefsdalReappearance
Reappearance

5ʺ5ʺ

F125W
F125W(J)(J)

Late
Late MayMay 2016
2016

Considered
Considered first
first byby (Jordi)(Jordi) Miralda-Escude
Miralda-Escude (1991)(1991)(cluster MACSJ1149) Johan Richard modelKelly, .., MO+, arXiv:1706.10279

Light curve of Icarus
Flux (µJy)

0.2

2012
Spera15 (2=184.6)

2016
Near Peak

Spera15/3% PBH ( 2 =219.6)

260.1
270.0

54000

55000
56000
Date (MJD)

57000• gradual increase, sharp drop after the peak(asymmetric light curve shape)• different from traditional microlensing

Magnitude (AB)2008

Caustic crossing
critical curve

caustic
star (source)

source planestar (image)star (image)

image planeflux• single star crossing

a caustic→ asymmetric light curvetimeCaustic crossing
image:
we observe
source:we wouldobserve if nolensing effectsimulated by glafic← cannot resolve
this due to lackof resolution ofobservations← observe lensing
effect via time
evolution ofbrightness ofa starMiralda-Escude (1991), Diego+ (2017), Venumadhav+ (2017)

Caustic crossing in massive clusters• microlensing by a foreground compact object
in high-magnification region of a cluster• cluster potential adds non circular symmetricperturbation, producing caustics (not seen in
microlensing by an isolated lens)• gravitational lensing both by the cluster

potential and the compact object leads toultra-high magnification, making it possible
to observe individual stars even at z>1

MO, Diego, Kaiser, Kelly & Broadhurst, arXiv:1710.00148

Understanding Icarus
mass fraction of compact lens

(shaded: excluded)

910lens mass

The result is shown in Figure 2, where we fixed the bulk
velocity of the lens to v = 500 km s 1 . We find that there
are large ranges of the lens mass and the source size thatcan explain MACS J1149 LS1.
However, it is expected that the lens and source populations are not distributed uniformly in this parameax
ter space. As discussed in the next Section, the size
distribution of the source star is expected to be significantly bottom-heavy, i.e., stars with smaller radii are
more abundant than those with larger radii. The same
argument also holds for the lens mass, if we assume standard stars and stellar remnants as the lens population,
but with the minimum mass of ⇡ 0.3 M below which63)
the stellar initial mass function is truncated. Therefore,
in the allowed parameter space, the most likely set ofparameters are R ⇡ 180 R and M ⇡ 0.3 M . In thiscase, the star is magnified by a factor of ⇡ 4300 at thepeak. The result is fully consistent with the scenario thatFIG.supergiant
4: Similar isto magnified
Figure 2, butconstraint
on the FIG. 3: Constraints in the M -fp plane for MACS J1149 LS1,a blueby additional
a foreground
ICL star.
lens mass range from the event rate (see Figure 3) is included. where M is the lens mass and fp is the mass fraction of theOn the other hand, our result does not exclude thepoint mass component to the total mass. Shaded regions show
possibility that the microlensing is caused by an exoticexcluded regions from the event rate (rate, equation 70). The
populationsuchas
PBHs
withmasses
between⇠
1M
R1 = R2 because larger than 10, and in that case we small rectangular region shows the rough mass fraction andandadopt⇠ 106theM former, as long
as as
they
havevalueµt,min. low surface density
the mass range of ICL stars. Contours show the constantso thatthey satisfy
saturationcondition.
more
Plugging
in the theparametervaluesfor MACSFor
J1149
LS1, even rate in this plane. From inner to outer contours, wemassivelenses, Figure 2 indicates that the peak magnifishow contours for dN/dt = 10 2 , 10 3 , 10 4 , and 10 5 .
we have
46cation is even higher andreachupto⇠
1010.
✓ can ◆
1/2 ⇣⌘source star radius

lens mass• constraints on lens and source properties fromobserved brightness, light curve, event rates• peak magnification ≈4300 for most plausible caseIntra-Cluster Light (ICL)• diffuse light in clusters• follow DM distribution• originates from stars

that were tidally strippedfrom member galaxies• Icarus event can be fully
NASA/ESA/IAC/HFF team, STScI

explained by microlensing
due to an ICL star
(no need for compact DM)

toward the critical curve. This is because
theEinDiego, .., MO+, arXiv:1706.10281
radius of the point mass lens depends on µt as√µt , and hence for very large µt the Einstein radii
diﬀerent point mass lenses overlap, even when thewhich r
ber density of point mass lenses is small. As shown
ay-tracing simulations in [26], beyond this “satura” point thesourcebreaks
intomany
micro-images,≈√μθt E
as a result it loses its sensitivity to the source posiwith respect to the macro model caustic. Therefore,In
prac
macro model
magnification
notsurface
divergedensity Σ
low surface
density Σin fact doeshighditions,
redicted by equation (31), but saturates at a finite
Fromt
when
thenumberdensityofcompactDMe.
maximuo estimate
where
thesaturation
happens,[26]
considis very high, Einstein radii highly overlap
the opticaldepthmagnification
τ defined by decreases→ peak“Saturation’’ effect•Σ √2
τ=π ( µt θE Dol ) ,M(33)τ≳1 → saturation

which iMO, Diego, Kaiser, Kelly & Broadhurst, arXiv:1710.00148

mass fraction of compact dark matter

Constraint on compact dark matter• high number

the robust interpretation
clue may be obtained b
curves before and afterin order to obtain robu
matter from observation
ray-tracing simulations t
compact dark matter, atracing simulations are hkind light curves such codensity leads
to too strong
saturation
which cannot
explain Icarus• close window
VII.

SUMMAR

at 10-100 Msun

In this paper, we havethatconsistsof a(see also model
Inoue &Kusenko
2017;
mass of compact dark matterconvergenceand2017)
shear fi
Zumalacarregui& Seljak
ing properties of a poin
FIG. 7: Constraints on the mass (M ) and abundance (fp ) ofmagnification regions ducompact dark matter. Shaded regions show excluded regionsMO, Diego, Kaiser, Kelly & Broadhurst, arXiv:1710.00148

mass fraction of compact dark matter

Constraint on compact dark matter• high number

the robust interpretation
clue may be obtained bcurves before and afterin order to obtain robu
matter from observation
ray-tracing simulations t
compact dark matter, a
tracing simulations are hkind light curves such codensity leads
to too strong
saturation
which cannot
explain Icarus• close windowVII.

careful lightcurve
mass ofanalysis?
compact dark matter

SUMMAR

at 10-100 Msun

In this paper, we havethatconsistsof a(see also model
Inoue &Kusenko
2017;
convergenceand2017)
shear fi
Zumalacarregui& Seljak
ing properties of a poin
FIG. 7: Constraints on the mass (M ) and abundance (fp ) ofmagnification regions ducompact dark matter. Shaded regions show excluded regions

… and more events (?)
3g from the Hubble Frontier Fields. The central paneland infrared bands from HST. Two boxes within ther. These regions are shown as two inset panels on the), which are caused by the gravitational lensing of the2014

October 30 2016

January 3 2017

Lev 2017A

Lev 2016B

0.5″
LS1 / Lev 2016A

LS1 / Lev 2016A

LS1 / Lev 2016A

Fig. 4: Highly magnified stellar images located near the MACS J1149 galaxy cluster’s
CC. The left panel shows LS1 in 2014; we detected LS1 when it temporarily brightened bya factor of ≥ 3 in late-April 2016. The center panel shows the appearance of a new imagedubbed Lev 2016B on 30 October 2016. The red curve marks the location of the cluster’s
CC from the CATS cluster model (7 ). The position is consistent with the possibility that
it is a counterimage of LS1. The right panel shows candidate named Lev 2017A with≥ 4‡ significance detected on 3 January 2017. If a microlensing peak, Lev 2017A mustcorrespond to a different star.

↑ additional events near Icarus(Kelly, .., MO+, arXiv:1706.10279)← similar fast transients inanother HFF cluster(Rodney, .., MO+, arXiv:1707.02434)

Summary• new microlensing near the center of a cluster,which led to ultra high magnification of≳ 4000, was recently discovered• most distant individual star and most extreme
magnification ever observed• can place new constraints on compact DMthat close high-mass (10−100 Msun) window• active research for better understanding andtighter constraints ongoing