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, …
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
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