LombScargle.false_alarm_probability fails for Quantities

[naamach]

Trying to calculate the Lomb Scragle false alarm probability while using Quantities with units throws an error while executing fap_baluev, since the exponent argument has units.
(It works ok when using method='naive')

Using Ubuntu 18.04.3, python 3.7.4, astropy 3.1.2, numpy 1.16.2.

Minimal working example:

import numpy as np
import astropy.units as u
from astropy.timeseries import LombScargle

t = 100 * np.random.rand(10) * u.day
dy = 0.1 * u.mag
y = (np.sin(2 * np.pi * t.value) + dy.value * np.random.randn(10)) * u.mag

ls = LombScargle(t, y, dy)
frequency, power = ls.autopower()
ls.false_alarm_probability(power.max())

EDIT: Syntax highlighting

[bsipocz]

Here is the error in question:

---------------------------------------------------------------------------
UnitConversionError                       Traceback (most recent call last)
~/munka/devel/astropy/astropy/units/quantity_helper/helpers.py in get_converter(from_unit, to_unit)
     31     try:
---> 32         scale = from_unit._to(to_unit)
     33     except UnitsError:

~/munka/devel/astropy/astropy/units/core.py in _to(self, other)
    952         raise UnitConversionError(
--> 953             f"'{self!r}' is not a scaled version of '{other!r}'")
    954 

UnitConversionError: 'Unit("1 / d")' is not a scaled version of 'Unit(dimensionless)'

During handling of the above exception, another exception occurred:

UnitConversionError                       Traceback (most recent call last)
~/munka/devel/astropy/astropy/units/quantity_helper/helpers.py in helper_dimensionless_to_dimensionless(f, unit)
    146     try:
--> 147         return ([get_converter(unit, dimensionless_unscaled)],
    148                 dimensionless_unscaled)

~/munka/devel/astropy/astropy/units/quantity_helper/helpers.py in get_converter(from_unit, to_unit)
     34         return from_unit._apply_equivalencies(
---> 35                 from_unit, to_unit, get_current_unit_registry().equivalencies)
     36     except AttributeError:

~/munka/devel/astropy/astropy/units/core.py in _apply_equivalencies(self, unit, other, equivalencies)
    889             "{} and {} are not convertible".format(
--> 890                 unit_str, other_str))
    891 

UnitConversionError: '1 / d' (frequency) and '' (dimensionless) are not convertible

During handling of the above exception, another exception occurred:

UnitTypeError                             Traceback (most recent call last)
<ipython-input-1-4f85c47ebb3e> in <module>
      9 ls = LombScargle(t, y, dy)
     10 frequency, power = ls.autopower()
---> 11 ls.false_alarm_probability(power.max())

~/munka/devel/astropy/astropy/timeseries/periodograms/lombscargle/core.py in false_alarm_probability(self, power, method, samples_per_peak, nyquist_factor, minimum_frequency, maximum_frequency, method_kwds)
    625                                                    normalization=self.normalization,
    626                                                    method=method,
--> 627                                                    method_kwds=method_kwds)
    628 
    629     def false_alarm_level(self, false_alarm_probability, method='baluev',

~/munka/devel/astropy/astropy/timeseries/periodograms/lombscargle/_statistics.py in false_alarm_probability(Z, fmax, t, y, dy, normalization, method, method_kwds)
    433     method_kwds = method_kwds or {}
    434 
--> 435     return method(Z, fmax, t, y, dy, normalization, **method_kwds)
    436 
    437 

~/munka/devel/astropy/astropy/timeseries/periodograms/lombscargle/_statistics.py in fap_baluev(Z, fmax, t, y, dy, normalization)
    327     # result is 1 - (1 - fap_s) * np.exp(-tau)
    328     # this is much more precise for small numbers
--> 329     return -np.expm1(-tau) + fap_s * np.exp(-tau)
    330 
    331 

~/munka/devel/astropy/astropy/units/quantity.py in __array_ufunc__(self, function, method, *inputs, **kwargs)
    445         # consistent units between two inputs (e.g., in np.add) --
    446         # and the unit of the result (or tuple of units for nout > 1).
--> 447         converters, unit = converters_and_unit(function, method, *inputs)
    448 
    449         out = kwargs.get('out', None)

~/munka/devel/astropy/astropy/units/quantity_helper/converters.py in converters_and_unit(function, method, *args)
    164 
    165         # Determine possible conversion functions, and the result unit.
--> 166         converters, result_unit = ufunc_helper(function, *units)
    167 
    168         if any(converter is False for converter in converters):

~/munka/devel/astropy/astropy/units/quantity_helper/helpers.py in helper_dimensionless_to_dimensionless(f, unit)
    150         raise UnitTypeError("Can only apply '{}' function to "
    151                             "dimensionless quantities"
--> 152                             .format(f.__name__))
    153 
    154 

UnitTypeError: Can only apply 'expm1' function to dimensionless quantities

[pllim]

tau that is being passed into np.expm1 is:

2.0394872747734722 1 / d

where d is day.

[bsipocz]

note: this only affects the baluev and davies methods, both bootstrap and naive works, as well as the single (that is not documented).