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most black hole x - ray binaries are transients . they spend years to decades in a very low state , and then increase in luminosity over a few days by as much as eight orders of magnitude . the outbursts
last for weeks to months before the source returns to quiescence .
quiescent black hole binaries ( hereafter qbhbs ) , have optical / ir emission dominated by the companion star , which can be used to precisely determine the orbital parameters , including the mass of the black hole ( see @xcite for details and a discussion of potential errors ) .
but there is also emission from the residual accretion flow and from jets , which plays an important role in understanding the changes in accretion state during the outburst . finally , the inferred demographics of x - ray binaries , which have assumed a heightened importance in the dawning era of gravitational wave astronomy ( @xcite ) , depend critically on the duration of quiescence .
observational astrophysicists commonly employ a variety of metaphors in explaining the importance of their work .
accordingly i will organize the rest of this paper by means of three of the more common metaphors , whose literal manifestations are shown in figure 1 .
i will take as an example the prototypical black hole x - ray transient a0620 - 00 ( hereafter a0620 ) , which underwent a huge outburst in 1975 ( @xcite ) reaching a peak flux of over 50 crabs . and
has been in quiescence ever since .
a `` smoking gun '' is an observation that confirms something that was already suspected to be true . in the case of qbhbs
, the confirmed truth is that the accreting object in many x - ray binaries is a black hole .
the companion stars which dominate the optical / ir light in qbhbs have optical absorption spectra similar to those of ordinary main sequence or giant stars . by applying traditional binary star techniques to these stars , the orbital parameters of the system can be robustly obtained . in many cases ,
the mass of the compact accreting star can be shown to be greater than @xmath0 , the upper limit on the mass of a neutron star .
such objects are sometimes referred to as `` dynamically confirmed black hole candidates '' .
the radial velocity curve of the companion can be used to determine the mass function @xmath1 , which is a strict lower limit on the mass of the compact object .
the first mass function greater than @xmath0 was obtained by @xcite for a0620 ; since then better data have been obtained for a0620 ( @xcite ) and over a dozen other systems have been observed ( @xcite ) .
the radial velocity curves are beautifully sinusoidal , as expected from tidally circularized systems , and there is little doubt that they represent the true motion of the companion star .    to obtain a mass , rather than a mass function , for the compact object
, information about the orbital inclination @xmath2 must be obtained .
this is generally done by studying the ellipsoidal variations of the companion star , caused by the tidally distorted shape of the companion , which results in changes in the cross - sectional area of the star as a function of orbital phase .
the interpretation of such observations requires accounting for the contribution of the accretion flow to the observed flux ( e.g @xcite for a0620 ) .
when this is done , the derived binary parameters fix the geometric size of the companion quite precisely , and the observed colors and absorption spectrum fix the temperature
. therefore the intrinsic luminosity of the companion is well - determined , leading to robust determinations of the distance .
one relevant case is that of gro j1655 - 40 , which shows very clearly defined ellipsoidal variations ( @xcite , @xcite ) .
the well - determined physical size and temperature of the companion require a distance of @xmath3 kpc .
an alternative distance of @xmath4 kpc ( @xcite ) would require the companion star to underfill its roche lobe by a factor of 2 , which would result in a nearly spherical star that can not produce the observed ellipsoidal variations .
the lower distance estimate is based on the assumption that the observed spectral type implies a star with mass and radius similar to those of isolated field stars .
but the evolution and surface gravity of a star losing mass through roche lobe overflow differ dramatically from those of isolated stars , and the observed masses and radii of qbhb companions differ from field stars of similar spectral type in many well - studied systems , including a0620 ( @xcite ) and v4641 sgr ( @xcite ) , as well as gro j1655 - 40 .
a  rosetta stone " is an observation that connects two hitherto disparate phenomena .
qbhbs may provide such a connection between accretion physics and jet outflows .        during the transition from quiescence to outburst and back ,
x - ray binaries change not only the amount of accretion , but the nature and state of the accretion flow .
these so - called `` state changes '' have been the subject of considerable recent work . the general consensus ( see @xcite for a review ) is that there is a low - hard state , with a power - law x - ray spectrum and radio emission from a jet , and a high - soft state , in which the x - ray luminosity is dominated by a thermal accretion disk , which quenches the jet .
there are also a variety of intermediate states .
the presence of a jet in the low - hard state has led to a scenario in which the accretion in that state is in the form of a radiatively inefficient radial flow , which allows the formation of of a jet ( @xcite ) .
but it should be noted that the empirical support for such accretion - jet scenarios rests on the radio / x - ray correlation .
the possibility that there might be a `` universal '' correlation in which @xmath5 , and , with suitable correction for the mass of the accretor , extends to the agn , generated considerable excitement ( @xcite ) .
observations of a0620 ( @xcite ) held down the low luminosity end of this correlation nicely . however , further investigation revealed multiple correlations and branches , and a recent reobservation of a0620 has shown that the source has moved in the @xmath6 plane in a direction perpendicular to the general correlation ( see figure 2 ) .
so the connection between accretion and jet physics is not yet as empirically well - founded as one might have hoped .
the `` tip of the iceberg '' is an object or observation which implies the existence of a large unseen population .
the current set of observed x - ray transients is certainly the tip of the iceberg of many currently unobserved galactic black hole binaries .
x - ray transients are generally discovered during their brief outbursts , which requires that the outburst occur at a time when it can be observed .
since x - ray astronomy is only @xmath7 years old , objects that have not gone into outburst during that time can not have been identified .
many objects have been observed in outburst only once , which implies the existence of objects that havent gone into outburst at all .
if such objects are common , the number of qbhbs in the galaxy will scale with the typical outburst recurrence time , a quantity for which there is currently no empirical constraint .
theoretical arguments can be made regarding the outburst recurrence time based on assumptions associated with the disk instabilities responsible for the outbursts ( e.g. @xcite ) .
however the variable recurrence timescales displayed by objects that have multiple outbursts demonstrate that we still have much to learn about these instabilities .
but we now have decades of monitoring of qbhbs , and we can begin to see how they evolve as the disk builds up toward its next outburst .
recently , a decade - long brightening trend has been found in nova muscae 1991 ( @xcite ) .
it is also notable that the passive quiescent state in a0620 ( @xcite ) has disappeared over the past ten years in favor of the brighter active quiescent state .
we therefore examined all of the quiescent data from a0620 accumulated since 2003 .
as can be seen in figure 3 , the initial finding was very encouraging  however when data from 2015 - 16 were included , the inferred increase diminishes dramatically ( although it is still significantly @xmath8 ) . as in many situations
when studying quasi - periodic phenomena , it is important to beware mistaking short term trends for periodic cycles or long - term evolution .
i discuss some of what is known and unknown about the behavior of black hole binary systems in the quiescent accretion state . 
 quiescence is important for several reasons : 1 ) the dominance of the companion star in optical and ir wavelengths allows the binary parameters to be robustly determined  as an example , we argue that the longer proposed distance to the x - ray source gro j1655 - 40 is correct ; 2 ) quiescence represents the limiting case of an extremely low accretion rate , in which both accretion and jets can be observed ; 3 ) understanding the evolution and duration of the quiescent state is a key factor in determining the overall demographics of x - ray binaries , which has taken on a new importance in the era of gravitational wave astronomy .