SpectralIndex.h

Classes

SpectralIndex -- Models the spectral variation with a spectral index (full description)

class SpectralIndex: public SpectralModel

Interface

Public Members
SpectralIndex()
SpectralIndex(const MFrequency& refFreq, Double exponent = 0.0)
SpectralIndex(const SpectralIndex& other)
virtual ~SpectralIndex()
SpectralIndex& operator=(const SpectralIndex& other)
virtual ComponentType::SpectralShape type() const
const Double& index() const
void setIndex(const Double& newIndex)
virtual Double sample(const MFrequency& centerFrequency) const
virtual void sample(Vector<Double>& scale, const Vector<MFrequency::MVType>& frequencies, const MFrequency::Ref& refFrame) const
virtual SpectralModel* clone() const
virtual uInt nParameters() const
virtual void setParameters(const Vector<Double>& newSpectralParms)
virtual Vector<Double> parameters() const
virtual void setErrors(const Vector<Double>& newSpectralErrs)
virtual Vector<Double> errors() const
virtual Bool fromRecord(String& errorMessage, const RecordInterface& record)
virtual Bool toRecord(String& errorMessage, RecordInterface& record) const
virtual Bool convertUnit(String& errorMessage, const RecordInterface& record)
virtual Bool ok() const
See Also
ConstantSpectrum - Models the spectrum as constant

Description

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Prerequisite

Synopsis

This class models the spectral variation of a component with a spectral index.

This class like the other spectral models becomes more useful when used through the SkyComponent class, which incorperates the flux and spatial variation of the emission, or through the ComponentList class, which handles groups of SkyComponent objects.

A spectral index is the exponent in a power law model for the variation flux with frequency. It is mathematically is defined as:

  (nu / nu_0)^alpha
Where:
nu_0
is the reference frequency
alpha
is the spectral index
nu
is the user specified frequency

As with all classes derived from SpectralModel the basic operation of this class is to model the flux as a function of frequency. This class does not know what the flux is at the reference frequency. Instead the sample functions return factors that are used to scale the flux and calculate the amount of flux at a specified frequency.

Besides the reference frequency this class has one parameter; the spectral index. This parameter can be set & queried using the general purpose parameters functions or the class specific index functions.

This class also contains functions (toRecord & fromRecord) which perform the conversion between Records and SpectralIndex objects. These functions define how a SpectralIndex object is represented in glish. The format of the record that is generated and accepted by these functions is:

    c := [type = 'spectral index',
          frequency = [type = 'frequency',
                       refer = 'lsr',
                       m0 = [value = 1, unit = 'GHz']
                      ],
          index = 0.7
         ]
    
The frequency field contains a record representation of a frequency measure and its format is defined in the Measures module. Its refer field defines the reference frame for the direction and the m0 field defines the value of the reference frequency. The parsing of the type field is case insensitive. The index field contains the spectral index.

Example

These examples are coded in the tSpectralModel.h file.

Example 1:

In this example a SpectralIndex object is created and used to calculate the flux at a number of frequencies.
  SpectralIndex siModel;
  siModel.setRefFrequency(MFrequency(Quantity(1.0, "GHz")));
  siModel.setIndex(1.0, Stokes::I);  
  siModel.setIndex(0.5, Stokes::Q);  
  siModel.setIndex(0.5, Stokes::U);  
  siModel.setIndex(-1.0, Stokes::V);
  const Flux<Double> LBandFlux(1.0, 1.0, 1.0, 1.0);
  const MVFrequency step(Quantity(100.0, "MHz"));
  MVFrequency sampleFreq = siModel.refFrequency().getValue();
  Flux<Double> sampleFlux;
  cout << "Frequency\t I-Flux\t Q-Flux\t U-Flux\t V-Flux\n";
  for (uInt i = 0; i < 11; i++) {
    sampleFlux = LBandFlux.copy();
    sampleFlux.convertPol(ComponentType::LINEAR);
    sampleFlux.convertUnit(Unit("WU"));
    siModel.sample(sampleFlux,
   	             MFrequency(sampleFreq, siModel.refFrequency().getRef()));
    cout << setprecision(3) << sampleFreq.get("GHz")
         << "\t\t " << sampleFlux.value(0u).re
         << "\t " << sampleFlux.value(1u).re
         << "\t " << sampleFlux.value(2u).re
         << "\t " << sampleFlux.value(3u).re
         << " " << sampleFlux.unit().getName() << endl;
    sampleFreq += step;
  }

Motivation

A Spectral Index frequency variation is the most widely used model in radio astronomy. In particular the NFRA package 'newstar' uses it extensively.

To Do

Member Description

SpectralIndex()

The default SpectralIndex has a reference frequency of 1 GHz in the LSR frame and a spectral index of zero. As such it is no different from the ConstantSpectrum class (except slower).

SpectralIndex(const MFrequency& refFreq, Double exponent = 0.0)

Construct a SpectralIndex with specified reference frequency and exponent.

SpectralIndex(const SpectralIndex& other)

The copy constructor uses copy semantics

virtual ~SpectralIndex()

The destructor does nothing special.

SpectralIndex& operator=(const SpectralIndex& other)

The assignment operator uses copy semantics.

virtual ComponentType::SpectralShape type() const

return the actual spectral type ie., ComponentType::SPECTRAL_INDEX

const Double& index() const
void setIndex(const Double& newIndex)

set/get the spectral index.

virtual Double sample(const MFrequency& centerFrequency) const

Return the scaling factor that indicates what proportion of the flux is at the specified frequency. ie. if the centreFrequency argument is the reference frequency then this function will always return one. At other frequencies it will return a non-negative number.

virtual void sample(Vector<Double>& scale, const Vector<MFrequency::MVType>& frequencies, const MFrequency::Ref& refFrame) const

Same as the previous function except that many frequencies can be sampled at once. The reference frame must be the same for all the specified frequencies. Uses a customised implementation for improved speed.

virtual SpectralModel* clone() const

Return a pointer to a copy of this object upcast to a SpectralModel object. The class that uses this function is responsible for deleting the pointer. This is used to implement a virtual copy constructor.

virtual uInt nParameters() const
virtual void setParameters(const Vector<Double>& newSpectralParms)
virtual Vector<Double> parameters() const
virtual void setErrors(const Vector<Double>& newSpectralErrs)
virtual Vector<Double> errors() const

return the number of parameters. There is one parameter for this spectral model, namely the spectral index. So you supply a unit length vector when using these functions. Otherwise an exception (AipsError) may be thrown.

virtual Bool fromRecord(String& errorMessage, const RecordInterface& record)
virtual Bool toRecord(String& errorMessage, RecordInterface& record) const

These functions convert between a Record and a SpectralIndex. These functions define how a SpectralIndex object is represented in glish and this is detailed in the synopsis above. These functions return False if the record is malformed and append an error message to the supplied string giving the reason.

virtual Bool convertUnit(String& errorMessage, const RecordInterface& record)

Convert the parameters of the spectral index object to the specified units. Only one field of the supplied record is used, namely 'index'. This field is optional as the spectral index is a unitless quantity. If the index field is specified it must have the empty string as its value. This function always returns True unless the index field is specified and does not contain an empty string.

virtual Bool ok() const

Function which checks the internal data of this class for consistant values. Returns True if everything is fine otherwise returns False.