1. Exploring Black Hole Demographics with Microlensing
Noé Kains
(STScI)
with Kailash Sahu, Annalisa Calamida, Josh Sokol, Jay Anderson, Stefano Casertano, Dan Bramich, Roberto Figuera Jaimes, Armando Arellano Ferro, Jesper Skottfelt, …

2. Image credit: LSST

3. Image credit: LSST (not the black blob)

4. 
Images
Einstein ring radius θE
Magnification ∝ Aimages /Asource tE

5. What can we do with microlensing?
Need
Probability of microlensing taking place at a given time is ~10-6 so dense stellar environments are better (e.g. Galactic Bulge, clusters)
Lots of observations with good time resolution (depending on science)
Decent spatial resolution
Science
Historically: dark matter probing (e.g. MACHO/EROS collaborations)
Exoplanets, especially cool rocky exoplanets (out of reach of other methods, e.g. Beaulieu et al. 2006, Gaudi et al. 2009, Kains et al. 2013a)
Brown dwarfs
Stellar physics: stellar atmospheres, single object mass measurements
Black hole populations

6. Measuring masses of isolated objects
tE = θE /vang
One of the key observables is the event timescale tE
tE is proportional to M1/2 ; typical tE for lens ~few M is ~ days.
But the timescale is a degenerate function of source velocity, lens mass, and lens/ source distances
Rearranging this: mass is a function of Einstein ring radius θE and lens-source parallax πLS
How to determine those parameters to obtain a mass measurement?

7. The amplitude of the astrometric shift scales with the lens mass
In addition to magnification, microlensing produces an astrometric shift due to asymmetric images
The amplitude of the astrometric shift scales with the lens mass
Signature astrometric pattern as the event unfolds
Measuring this allows us to determine θE
Parallax is fitted from the light curve so requires good time resolution and high-precision photometry
Alcock et al. 1995

8. Single stellar-mass black holes
Since stars > 20 M end their lives as BH, there should be ~108 BH in in the MW (e.g. Sahu et al. 2012)
Many should be isolated:
Single stars (~1/3 of those stars)
Wide binaries
Merged close binaries during supernova explosions
No definite single BH detection so far
BH (and neutron star) mass measurements from binary systems are a biased sample
Microlensing is a great method to address this:
Single object detections
Mass distribution

9. Stellar-mass BH lensing
Single stellar mass black holes should lens background source stars with tE of ~ days
No blending from the lens
The astrometric shift produced by a lens of ~few M is of the order of ~few mas
This can be routinely measured from HST observations

10. 2 HST projects
‘Detecting and measuring the masses of stellar remnants’ – PI: K. Sahu
4 ACS + 8 WFC3/UVIS fields, monitoring ~1.5-2 million stars in total
Each field observed every 2 weeks, 8 months/ year for 3 years
HST observations to measure astrometric shifts
Ground-based observations with to get parallax: every 3-4 days (PI: M. Zoccali)
Also, HST follow-up of long-duration events from ground-based microlensing survey teams (OGLE/ MOA), PI: K. Sahu
Some promising candidates
Also lots of other science to be done with the data (e.g. Calamida et al. 2014)

11. Intermediate-mass black holes
Mass range ~ M
Seeds for SMBH formation
Motivation for studying IMBH
M-σ relation
(Silk & Rees 1998,
also Sadoun & Colin 2012 for GC)
Extrapolate down to IMBH masses  range of σ of globular clusters / dwarf galaxies
Lützgendorf et al. 2013

12. Observational evidence
Ultra-luminous X-ray sources in stellar clusters (e.g. Soria et al. 2011, Farrell et al. 2009, Maccarone et al. 2007)
Low-mass SMBH in NGC 4395 = IMBH? (Peterson et al. 2005)
Dynamics of globular clusters (e.g. Lützgendorf et al. 2013, Feldmeier et al. 2013)
No unambiguous detection yet
Clues on IMBH populations would shed light on BH growth, how SMBH form, and how galaxies form
Microlensing could be a good way to probe the existence of IMBH in GC

13. LIMBO: A project to search for IMBH
Monitoring 32 GC cores, 6 months/ year with 1 observations/ night with an EMCCD camera at the Danish 1.54m telescope in La Silla (with MiNDSTEp consortium, GC project PI: Kains)
EMCCD enables us to obtain high-resolution images of crowded GC cores (Kains et al submitted, Skottfelt et al. 2013)
Many very short (~0.1s) exposures freeze turbulence  ~diffraction-limited resolution
No saturated stars
Can do various things with data cubes depending on target science
Combine with difference image analysis to obtain high-precision photometry (no need to throw away images i.e. not Lucky Imaging)
Difficulty: understanding properties of resulting images

14. HST EMCCD (DK1.54m) CCD (RoboNet 1m) NGC4590/ M 68
Kains et al (submitted)

15. NGC 6981 (Skottfelt et al. 2013)
CCD (DK1.54m)

16. NGC 6981 (Skottfelt et al. 2013)
EMCCD (DK1.54m)

17. What do we search for?
The astrometric shift produced by an IMBH could be several 10s of mas, which is easily detectable from the ground
We get the distance to the lens for “free”, since the IMBH resides in the GC core, so the most important part is to measure θE to get a mass measurement
Search for lensing signature in the photometry (in progress), as well as “blind” astrometric shift searches (in the future)
Source stars both in the cluster (cluster self-lensing) and background stars (important for target selection)

18. LIMBO science
Long-baseline, high-precision time-series  lots of science that can be done with data (variable stars/ asteroseismology, e.g. Kains et al. 2012, 2013b, 2014)
Detection  great!
Non-detections could allows us to place limits on presence of IMBH in those GC (cf. cool exoplanets mass functions e.g. Cassan et al. 2012)
Difficulties
Lensing probabilities are low, events are very long (at least a few hundred days)  long-term project
Need a good model to predict event rates to compare with our rates of (non-)detections

19. Summary
Microlensing is a great method to measure and constrain isolated black holes masses unambiguously
3 projects underway:
IMBH project will take a few more years of data before results come out, but lots of other science on the way
2 HST projects: some promising BH candidates, watch out for results over the next year or 2

20. Lützgendorf et al. 2013