Information on Service 'Gaia DR3 MC sampled XP spectra SSA'

For a list of all services and tables belonging to this service's resource, see Information on resource 'Gaia DR3 RP/BP (XP) Monte Carlo sampled spectra'

Service Documentation

In Gaia's DR3, most BP/RP spectra come in “continuous” form only, that is, as coefficients of Gauss-Hermite polynomials. These can be turned into a “sampled” representation using GaiaXPy; however, since the errors are given in the form of covariance matrices for the polynomial coefficients, the errors in the resulting spectra are strongly correleated, which can sometimes result in artefacts in the signal.

To get approximately decorrelated errors and hence sampled spectra usable with less caution, we apply a scheme of Monte Carlo-sampling different realisations from the error model of the coefficients. Specifically, given the covariance matrix C defined through the Xp_coefficient_errors and Xp_coefficient_correlations column in the DR3 xp_continuous_mean_spectrum table, and noting that for a unit normal-distributed random variable u

holds (⟨x⟩ denotes the expectation), we can re-write the covariance matrix using a Cholesky decomposition into LL^T as

Hence, x = Lu is a realisation of the errors satisfying the covariance matrix. To come up with a sampled spectrum, we now draw (in this case) 10 samples of the coefficients and have GaiaXPy convert them to a sampled spectrum.

The source code we used for that is dr3_to_mcsampled.py.

To be on the conservative side of the resolution and the bandwidth, and also to keep storage requirements modest, we have chosen a relatively rough grid over the optical band, that is, bins of 10 nm over the spectral range between 400 and 800 nm.

Overview

You can access this service using:

• form -- an interface for web browsers through an HTML form
• ssap.xml -- a standard SSAP interface as defined by the IVOA to access spectral or time-series data; SSAP clients use https://dc.zah.uni-heidelberg.de/gaia/s3/ssa/ssap.xml? to access the service

Copyright, License, Acknowledgements

This data is derived from public Gaia DR3 data. Please take note of ESAC's guide on how to acknowledge and cite Gaia results.

VOResource XML (that's something exclusively for VO nerds)

Note id

For the contents of Gaia DR3, the source ID consists of a 64-bit integer, least significant bit = 1 and most significant bit = 64, comprising:

• a HEALPix index number (sky pixel) in bits 36 - 63; by definition the

  smallest HEALPix index number is zero.

• a 3-bit Data Processing Centre code in bits 33 - 35; for example

  MOD(source_id / 4294967296, 8) can be used to distinguish between sources initialised via the Initial Gaia Source List by the Torino DPC (code = 0) and sources otherwise detected and assigned by Gaia observations (code > 0)

• a 25-bit plus 7 bit sequence number within the HEALPix pixel in bits 1

  to 32 split into:

  • a 25 bit running number in bits 8 - 32; the running numbers are

    defined to be positive, i.e. never zero (except in the case of forced empty windows)

  • a 7-bit component number in bits 1 - 7

This means that the HEALpix index level 12 of a given source is contained in the most significant bits. HEALpix index of 12 and lower levels can thus be retrieved as follows:

• HEALpix level 12 = source_id / 34359738368
• HEALpix level 11 = source_id / 137438953472
• HEALpix level 10 = source_id / 549755813888
• HEALpix level n = source_id / 2^35 * 4^(12 - level).