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 Einstein ring radius θE Magnification ∝ Aimages /Asource 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 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 ~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?

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

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