Information on Service 'Gaia DR3 RP/BP (XP) Monte Carlo sampled test table'

[Use this service from your browser]

Further access options are discussed below

This is a re-publication the Gaia DR3 RP/BP spectra in the IVOA Spectral Data Model. It presents the continous spectra in sampled form, using a Monte Carlo scheme to decorrelate errors, elaborated in this resource's reference URL. The underlying tables are also available for querying through TAP, which opens some powerful methods for mass-analysing the data.

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 test table'

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

u.uT⟩ = 1

holds (x denotes the expectation), we can re-write the covariance matrix using a Cholesky decomposition into LLT as

C =  LLT  =  Lu.uTLT  =  Lu.uTLT  =  x.xT

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:

This service is not published. This can mean that it was deemed too unimportant, for internal use only, or is just a helper for a published service. Equally likely, however, it is under development, abandoned in development or otherwise unfinished. Exercise caution.

Other services provided on the underlying data include:

Spatial Coverage

Input Fields

The following fields are available to provide input to the service (with some renderers, some of these fields may be unavailable):

NameTable Head DescriptionUnitUCD
ssa_location Location ICRS location of aperture center deg pos.eq

Default Output Fields

The following fields are contained in the output by default. More fields may be available for selection; these would be given below in the VOTable output fields.

NameTable Head DescriptionUnitUCD
dec Dec (ICRS) Barycentric Declination in ICRS at epoch J2016.0 deg pos.eq.dec;meta.main
location_dec Dec N/A deg pos.eq.dec;meta.main
location_ra RA N/A deg pos.eq.ra;meta.main
phot_bp_mean_mag Mag BP Mean magnitude in the integrated BP band. This is computed from the BP-band mean flux applying the magnitude zero-point in the Vega scale. To obtain error estimates, see phot_bp_mean_flux_over_error. mag phot.mag;em.opt.B
phot_g_mean_mag m_G Mean magnitude in the G band. This is computed from the G-band mean flux applying the magnitude zero-point in the Vega scale. To obtain error estimates, see phot_g_mean_flux_over_error. mag phot.mag;em.opt;stat.mean
phot_rp_mean_mag Mag RP Mean magnitude in the integrated RP band. This is computed from the RP-band mean flux applying the magnitude zero-point in the Vega scale. To obtain error estimates, see phot_rp_mean_flux_over_error. mag phot.mag;em.opt.R
ra RA (ICRS) Barycentric Right Ascension in ICRS at epoch J2016.0 deg pos.eq.ra;meta.main
source_id Source Id Gaia DR3 unique source identifier. Note that this *cannot* be matched against the DR1 or DR2 source_ids. Note id N/A meta.id;meta.main
spectrum Sampled Spectrum Merged BP/RP spectrum from Gaia DR3, using a local Monte Carlo technique for conversion from the upstream continuous form N/A N/A

VOTable Output Fields

The following fields are available in VOTable output. The verbosity level is a number intended to represent the relative importance of the field on a scale of 1 to 30. The services take a VERB argument. A field is included in the output if their verbosity level is less or equal VERB*10.

NameTable Head DescriptionUnitUCD Verb. Level
phot_g_mean_mag m_G Mean magnitude in the G band. This is computed from the G-band mean flux applying the magnitude zero-point in the Vega scale. To obtain error estimates, see phot_g_mean_flux_over_error. mag phot.mag;em.opt;stat.mean 1
phot_bp_mean_mag Mag BP Mean magnitude in the integrated BP band. This is computed from the BP-band mean flux applying the magnitude zero-point in the Vega scale. To obtain error estimates, see phot_bp_mean_flux_over_error. mag phot.mag;em.opt.B 1
phot_rp_mean_mag Mag RP Mean magnitude in the integrated RP band. This is computed from the RP-band mean flux applying the magnitude zero-point in the Vega scale. To obtain error estimates, see phot_rp_mean_flux_over_error. mag phot.mag;em.opt.R 1
source_id Source Id Gaia DR3 unique source identifier. Note that this *cannot* be matched against the DR1 or DR2 source_ids. Note id N/A meta.id;meta.main 1
ra RA (ICRS) Barycentric Right Ascension in ICRS at epoch J2016.0 deg pos.eq.ra;meta.main 1
dec Dec (ICRS) Barycentric Declination in ICRS at epoch J2016.0 deg pos.eq.dec;meta.main 1
accref Product key Access key for the data N/A meta.ref.url;meta.dataset 1
ssa_location Location ICRS location of aperture center deg pos.eq 5
ssa_dateObs Date Obs. Midpoint of exposure (MJD) d time.epoch 5
ssa_timeExt Exp. Time Exposure duration s time.duration 5
ssa_length Length Number of points in the spectrum N/A N/A 5
accsize File size Size of the data in bytes byte N/A 11
preview Preview URL of a preview for the dataset N/A meta.ref.url;meta.preview 15
ssa_dstitle Title A compact and descriptive designation of the dataset. N/A meta.title;meta.dataset 15
ssa_creatorDID C. DID Dataset identifier assigned by the creator N/A meta.id 15
ssa_pubDID P. DID Dataset identifier assigned by the publisher N/A N/A 15
ssa_cdate Proc. Date Processing/Creation date N/A time;meta.dataset 15
ssa_pdate Pub. Date Date last published. N/A N/A 15
ssa_bandpass Bandpass Bandpass (i.e., rough spectral location) of this dataset; this should be the most appropriate term from the vocabulary http://www.ivoa.net/rdf/messenger. N/A instr.bandpass 15
ssa_cversion C. Version Creator assigned version for this dataset (will be incremented when this particular item is changed). N/A meta.version;meta.dataset 15
ssa_targname Object Common name of object observed. N/A meta.id;src 15
ssa_targclass Ob. cls Object class (star, QSO,...; use Simbad object classification http://simbad.u-strasbg.fr/simbad/sim-display?data=otypes if at all possible) N/A src.class 15
ssa_redshift z Redshift of target object N/A src.redshift 15
ssa_targetpos Obj. pos Equatorial (ICRS) position of the target object. N/A pos.eq;src 15
ssa_snr SNR Signal-to-noise ratio estimated for this dataset N/A stat.snr 15
ssa_aperture Aperture Angular diameter of aperture deg phys.angSize;instr.fov 15
ssa_specmid Mid. Band Midpoint of region covered in this dataset m em.wl;instr.bandpass 15
ssa_specext Bandwidth Width of the spectrum m em.wl;instr.bandwidth 15
ssa_specstart Band start Lower value of spectral coordinate m em.wl;stat.min 15
ssa_specend Band end Upper value of spectral coordinate m em.wl;stat.max 15
ssa_csysName Sys System RA and Dec are given in N/A N/A 15
ssa_spectralucd UCD(spectral) UCD of the spectral column in the spectra served; when you have wavelengths, use em.wl for vacuum wavelengths, em.wl;obs.atmos for air wavelengths. N/A meta.ucd 15
ssa_spectralunit unit(spectral) Unit of the spectral column N/A meta.unit 15
ssa_fluxucd UCD(flux) UCD of the flux column N/A meta.ucd 15
ssa_fluxunit unit(flux) Unit of the flux column N/A meta.unit 15
ssa_publisher Publisher Publisher of the datasets included here. N/A meta.curation 15
ssa_creator Creator Creator of the datasets included here. N/A N/A 15
ssa_collection Collection A short handle naming the collection this spectrum belongs to. N/A N/A 15
ssa_instrument Instrument Instrument or code used to produce these datasets N/A meta.id;instr 15
ssa_datasource Src Method of generation for the data (one of survey, pointed, theory, custom, artificial). N/A N/A 15
ssa_creationtype Using Process used to produce the data (archival, cutout, filtered, mosaic, projection, spectralExtraction, or catalogExtraction) N/A N/A 15
ssa_reference Ref. URL or bibcode of a publication describing this data. N/A meta.bib.bibcode 15
ssa_fluxStatError Err. flux Statistical error in flux N/A stat.error;phot.flux.density;em 15
ssa_fluxSysError Sys. Err flux Systematic error in flux N/A stat.error.sys;phot.flux.density;em 15
ssa_fluxcalib Calib Flux Type of flux calibration (ABSOLUTE, CALIBRATED, RELATIVE, NORMALIZED, or UNCALIBRATED). N/A N/A 15
ssa_binSize Spect. Bin Bin size in wavelength m em.wl;spect.binSize 15
ssa_spectStatError Err. Spect Statistical error in wavelength m stat.error;em 15
ssa_spectSysError Sys. Err. Spect Systematic error in wavelength m stat.error.sys;em 15
ssa_speccalib Calib. Spect. Type of wavelength calibration N/A meta.code.qual 15
ssa_specres Spec. Res. Resolution (in meters of wavelength) on the spectral axis m spect.resolution;em.wl 15
ssa_spaceError Err. Spc Statistical error in position deg stat.error;pos.eq 15
ssa_spaceCalib Calib. Spc Type of calibration in spatial coordinates N/A meta.code.qual 15
ssa_spaceRes Res. Spc Spatial resolution of data deg pos.angResolution 15
mime Type MIME type of the file served N/A meta.code.mime 20
ssa_model Model Data model name and version N/A N/A 20
ssa_timeSI [Time] Time conversion factor in Osuna-Salgado convention. N/A N/A 20
ssa_spectralSI [Spectral] Spectral conversion factor in Osuna-Salgado convention N/A N/A 20
ssa_fluxSI [Flux] Flux/magnitude conversion factor in Osuna-Salgado convention N/A N/A 20
ssa_dstype Data type Type of data (spectrum, time series, etc) N/A N/A 20
owner Owner Owner of the data N/A N/A 25
embargo Embargo ends Date the data will become/became public a N/A 25
ssa_region Coverage Rough coverage based on location and aperture N/A N/A 30

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).