Creating Time Series¶
Initializing a Time Series¶
The first type of time series that we will look at here is a TimeSeries
object, which can be used for a time series which samples a continuous variable
at discrete, instantaneous times. Initializing a TimeSeries object can be done
in the same ways as initializing a Table object (see Data Tables), but additional arguments related to the times should be
specified.
Evenly Sampled Time Series¶
The most convenient way to construct an evenly sampled TimeSeries is to
specify the start time, the time interval, and the number of samples:
>>> from astropy import units as u
>>> from astropy.timeseries import TimeSeries
>>> ts1 = TimeSeries(time_start='2016-03-22T12:30:31',
... time_delta=3 * u.s,
... n_samples=5)
>>> ts1
<TimeSeries length=5>
time
Time
-----------------------
2016-03-22T12:30:31.000
2016-03-22T12:30:34.000
2016-03-22T12:30:37.000
2016-03-22T12:30:40.000
2016-03-22T12:30:43.000
The time keyword argument can be set to anything that can be passed to the
Time class (see also Time and Dates) or Time objects
directly. Note that the n_samples argument is only needed if you are not
also passing in data during initialization (see Passing Data During
Initialization).
Arbitrarily Sampled Time Series¶
To construct a sampled time series with samples at arbitrary times, you can
pass multiple times to the time argument:
>>> ts2 = TimeSeries(time=['2016-03-22T12:30:31',
... '2016-03-22T12:30:38',
... '2016-03-22T12:34:40'])
>>> ts2
<TimeSeries length=3>
time
Time
-----------------------
2016-03-22T12:30:31.000
2016-03-22T12:30:38.000
2016-03-22T12:34:40.000
You can also specify a vector Time object directly as the time= argument,
or a vector TimeDelta argument or a quantity array to the time_delta=
argument.:
>>> TimeSeries(time_start="2011-01-01T00:00:00",
... time_delta=[0.1, 0.2, 0.1, 0.3, 0.2]*u.s)
<TimeSeries length=5>
time
Time
-----------------------
2011-01-01T00:00:00.000
2011-01-01T00:00:00.100
2011-01-01T00:00:00.300
2011-01-01T00:00:00.400
2011-01-01T00:00:00.700
Initializing a Binned Time Series¶
The BinnedTimeSeries can be used to represent time series where each entry
corresponds to measurements taken over a range in time — for instance, a light
curve constructed by binning X-ray photon events. This class supports equal-size
or uneven bins, and contiguous and non-contiguous bins. As for TimeSeries,
initializing a BinnedTimeSeries can be done in the same ways as initializing a
Table object (see Data Tables), but additional
arguments related to the times should be specified as described below.
Equal-Sized Contiguous Bins¶
To create a binned time series with equal-size contiguous bins, it is sufficient to specify a start time as well as a bin size:
>>> from astropy.timeseries import BinnedTimeSeries
>>> ts3 = BinnedTimeSeries(time_bin_start='2016-03-22T12:30:31',
... time_bin_size=3 * u.s, n_bins=10)
>>> ts3
<BinnedTimeSeries length=10>
time_bin_start time_bin_size
s
Time float64
----------------------- -------------
2016-03-22T12:30:31.000 3.0
2016-03-22T12:30:34.000 3.0
2016-03-22T12:30:37.000 3.0
2016-03-22T12:30:40.000 3.0
2016-03-22T12:30:43.000 3.0
2016-03-22T12:30:46.000 3.0
2016-03-22T12:30:49.000 3.0
2016-03-22T12:30:52.000 3.0
2016-03-22T12:30:55.000 3.0
2016-03-22T12:30:58.000 3.0
Note that the n_bins argument is only needed if you are not also passing in
data during initialization (see Passing Data During Initialization).
Uneven Contiguous Bins¶
When creating a binned time series with uneven contiguous bins, the bin size can
be changed to give multiple values (note that in this case n_bins is not
required):
>>> ts4 = BinnedTimeSeries(time_bin_start='2016-03-22T12:30:31',
... time_bin_size=[3, 3, 2, 3] * u.s)
>>> ts4
<BinnedTimeSeries length=4>
time_bin_start time_bin_size
s
Time float64
----------------------- -------------
2016-03-22T12:30:31.000 3.0
2016-03-22T12:30:34.000 3.0
2016-03-22T12:30:37.000 2.0
2016-03-22T12:30:39.000 3.0
Alternatively, you can create the same time series by giving an array of start times as well as a single end time:
>>> ts5 = BinnedTimeSeries(time_bin_start=['2016-03-22T12:30:31',
... '2016-03-22T12:30:34',
... '2016-03-22T12:30:37',
... '2016-03-22T12:30:39'],
... time_bin_end='2016-03-22T12:30:42')
>>> ts5
<BinnedTimeSeries length=4>
time_bin_start time_bin_size
s
Time float64
----------------------- -----------------
2016-03-22T12:30:31.000 3.0
2016-03-22T12:30:34.000 3.0
2016-03-22T12:30:37.000 2.0
2016-03-22T12:30:39.000 3.0
Uneven Non-Contiguous Bins¶
To create a binned time series with non-contiguous bins, you can either specify an array of start times and bin widths:
>>> ts6 = BinnedTimeSeries(time_bin_start=['2016-03-22T12:30:31',
... '2016-03-22T12:30:38',
... '2016-03-22T12:34:40'],
... time_bin_size=[5, 100, 2]*u.s)
>>> ts6
<BinnedTimeSeries length=3>
time_bin_start time_bin_size
s
Time float64
----------------------- -------------
2016-03-22T12:30:31.000 5.0
2016-03-22T12:30:38.000 100.0
2016-03-22T12:34:40.000 2.0
Or in the most general case, you can also specify multiple times for
time_bin_start and time_bin_end:
>>> ts7 = BinnedTimeSeries(time_bin_start=['2016-03-22T12:30:31',
... '2016-03-22T12:30:33',
... '2016-03-22T12:30:40'],
... time_bin_end=['2016-03-22T12:30:32',
... '2016-03-22T12:30:35',
... '2016-03-22T12:30:41'])
>>> ts7
<BinnedTimeSeries length=3>
time_bin_start time_bin_size
s
Time float64
----------------------- ------------------
2016-03-22T12:30:31.000 1.0
2016-03-22T12:30:33.000 2.0
2016-03-22T12:30:40.000 1.0
Adding Data to the Time Series¶
The above examples show how to initialize TimeSeries objects, but these do not
include any data aside from the times. There are different ways of adding data,
as with the Table class.
Passing Data During Initialization¶
It is possible to pass data during the initialization of a TimeSeries
object, as for Table objects. For instance:
>>> ts8 = BinnedTimeSeries(time_bin_start=['2016-03-22T12:30:31',
... '2016-03-22T12:30:34',
... '2016-03-22T12:30:37',
... '2016-03-22T12:30:39'],
... time_bin_end='2016-03-22T12:30:42',
... data={'flux': [1., 4., 5., 6.] * u.mJy})
>>> ts8
<BinnedTimeSeries length=4>
time_bin_start time_bin_size flux
s mJy
Time float64 float64
----------------------- ----------------- -------
2016-03-22T12:30:31.000 3.0 1.0
2016-03-22T12:30:34.000 3.0 4.0
2016-03-22T12:30:37.000 2.0 5.0
2016-03-22T12:30:39.000 3.0 6.0
Adding Data After Initialization¶
Once a TimeSeries object is initialized, you can add columns/fields to it as
you would for a Table object:
>>> from astropy import units as u
>>> ts1['flux'] = [1., 4., 5., 6., 4.] * u.mJy
>>> ts1
<TimeSeries length=5>
time flux
mJy
Time float64
----------------------- -------
2016-03-22T12:30:31.000 1.0
2016-03-22T12:30:34.000 4.0
2016-03-22T12:30:37.000 5.0
2016-03-22T12:30:40.000 6.0
2016-03-22T12:30:43.000 4.0
Adding Rows¶
Adding rows to TimeSeries or BinnedTimeSeries can be done using the
add_row() method, as for Table and QTable. This
method takes a dictionary where the keys are column names:
>>> ts8.add_row({'time_bin_start': '2016-03-22T12:30:44.000',
... 'time_bin_size': 2 * u.s,
... 'flux': 3 * u.mJy})
>>> ts8
<BinnedTimeSeries length=5>
time_bin_start time_bin_size flux
s mJy
Time float64 float64
----------------------- ----------------- -------
2016-03-22T12:30:31.000 3.0 1.0
2016-03-22T12:30:34.000 3.0 4.0
2016-03-22T12:30:37.000 2.0 5.0
2016-03-22T12:30:39.000 3.0 6.0
2016-03-22T12:30:44.000 2.0 3.0
If you want to be able to skip some values when adding rows, you should make sure that masking is enabled — see Masking Values in Time Series for more details.