Table of contents
- Thread content
- How to generate a Spectral Energy Distribution
- Displaying your own spectrum together with archival spectra
- Plot manipulation
- Fitting
- Theoretical models
- Access to line databases
This recipe shows how to generate and display a Spectral Energy Distribution
with VOSpec,
combining spectra from various observatories in different energy bands,
as well as user-produced spectra uploaded from the local disk. It also
illustrates the usage of simple fitting tools in
VOSpec, as well as the access
to theoretical models and absorption/emission line
databases, which have been incorporated in
Virtual Observatory servers around the world.
A video showing a typical VOSpec
session is available here.
- Go to the VOSpec start page.
The following applet appears
- Write the source name (NGC3516 in the example below)
in the Target, or
the source coordinates in the Ra/Dec widget, and
click Go. a Server Selector panel appears.
- Click on
the SSA servers handle (SSA stands for "Simple
Spectral Access"):
a list of the available spectral data databases appears.
each item of this list can be expanded into a list of
the observatory servers,
whence spectra can be downloaded
- Select the mission(s) you are interested in (in the example below:
ISO, IUE, HST and SDSS), and click Select:
The selected servers are searched for available spectra of the chosen
source. If no spectrum is available for a given server, a red warning
appears at the bottom of the
main VOSpec
display panel: No results for Sloan Digital Sky Survey Simple Spectral
Access.
- In the bottom sub-panel of the main
VOSpec window,
select the spectra you want to display by following
the same procedure as in the Server Selector (Step 4. above),
and click Display. The selected spectra are retrieved
from their server, and displayed as follows:
VOSpec allows you to
display your own spectra alongside spectra extracted from
databases. In order to do that, follow these steps:
- click on
- select the spectrum you wish to upload with the file browser window. A
Spectrum Display Data panel appears
VOSpec tries and interpret the
units of the spectrum (in this case Hz on the frequency axis and Jansky on the
flux density axis). If this interpretation step fails (either because no
units are explicitly indicated, or because unknown units are
being used), the corresponding widgets can be edited. In this
case the user must specify the correct dimensional equation for the units
being used in the Dimeq. A set of pre-defined units is available
from the
Predefined units menu
- click on Accept. The spectrum
(in the example below an XMM-Newton EPIC fluxed spectrum of 3C273)
will be displayed as in the following figure
[note that
the quantity being actually displayed is energy (in keV),
although the X-axis label says Wavelength]:
A few functions are available in VOSpec
to modify the output format:
- Change of the units on the X- and Y-axis, by clicking on the
Wave Unit and Flux Unit menus
- Swap from linear to logarithmic coordinates by clicking
on the checkbox besides the unit selection menus
- Define a redshift in the RedShift widget.
After clicking go, the energy (wavelength) axis is redshifted by an
amount 1+z, where z is the value written
in the widget. Note: No K-correction is applied!
- Change the plot style of each datasets, by modifying the
menu items in the Graphic Mode sub-panel
A few basic fitting functions (Gaussian, blackbody,
polynomial) are available in VOSpec.
We show hereafter an example of a simple Gaussian line fit
- Select a wavelength/energy range in the main
VOSpec display panel
which optimally
encompasses the Gaussian line, by
clicking with the mouse left button, and dragging the mouse
over it
- once you are happy with the
X-axis selection, click on
 .
The following panel appears:
- click on Generate.This start the calculation
of the best-fit model. Upon completion two things happen:
- the best fitting parameters appear, together with a "quality-of-the-fit"
estimator
- the best fit profile is superposed to the observed
spectrum
VOSpec allows you to
access theoretical models available through registry queries to
VO servers in the world
for
comparison with the observed spectra. The list of available
theoretical model servers can be accessed from the Server Selector
panel, by clicking on the Theoretical Spectral Service handle
The selection of the server/model is done following the same steps
as for the observational data. This prompts a input parameter selection
interface, which depends on the model being selected. Once uploaded
into VOSpec, the
theoretical model datasets can be displayed using exactly the same
functionalities as the observational datasets. The button 
allows you to "anchor" a theoretical model to the value of the
observed Spectral Energy Distribution at a given wavelength/energy
An on-line line identification facilities is built in
VOSpec. It allows you
to display the closest emission/absorption line
to a given wavelength/energy from databases accessible through VO registry
queries. Let's assume that you have displayed
the line-rich ISO spectra of NGC7072. To know which
transitions are associated with the lines you observe, you have to:
- click on
- select a region around you line of interest by
dragging the mouse while pushing its left button
- in the Line Data Model panel the lines are listed
which belong
to the ISO line catalogue and whose transition energies lay in the
interactively defined energy interval.
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