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, …

Image credit: LSST

Image credit: LSST (not the black blob)

Images

Magnification A∝ images /Asource

Einstein ring radius θE

tE

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

Measuring masses of isolated objects

• One of the key observables is the event timescale tE

• tE is proportional to M1/2 ; typical tE for lens ~few M is ~80-100 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?

tE = θE /vang

• 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

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

Stellar-mass BH lensing

• Single stellar mass black holes should lens background source stars with tE of ~80-100 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

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 VIMOS@VLT 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)

Intermediate-mass black holes• Mass range ~102-106 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

• 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

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. 2014 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

HST EMCCD (DK1.54m) CCD (RoboNet 1m)

NGC4590/ M 68

Kains et al. 2014 (submitted)

NGC 6981 (Skottfelt et al. 2013)

CCD (DK1.54m)

NGC 6981 (Skottfelt et al. 2013)

EMCCD (DK1.54m)

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)

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

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

Lützgendorf et al. 2013