# 5.1 aux_qso_icrf2_match

This table has an entry for all sources in the auxiliary QSO solution matched to the ICRF2 sources and passing all quality filters discussed in the corresponding documentation.

Columns description:

solution_id : Solution Identifier (long)

The data in the MDB will be described by means of a ”Solution identifier” parameter. This will be a numeric field attached to each table row that can be used to unequivocally identify the version of all the subsystems that where used in the generation of the data as well as the input data used. Each DPC generating the data will have the freedom to choose the Solution identifier number, but they must ensure that given the Solution identifier they can provide detailed information about the ”conditions” used to generate the data: versions of the software, version of the data used…

source_id : Unique source identifier (long)

A unique single numerical identifier of the source.

For the contents of Gaia DR1, which does not include Solar System objects, the source ID consists 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 2-bit Data Processing Centre code in bits 34 - 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 - 32 split into:

• a 25 bit running number in bits 8 â 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 â 7

• one spare bit in bit 33

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).

Additional details can be found in the Gaia DPAC public document Source Identifiers — Assignment and Usage throughout DPAC (document code GAIA–C3–TN–ARI–BAS–020) available from http://www.cosmos.esa.int/web/gaia/public-dpac-documents

ref_epoch : Reference epoch (double, Time[Julian Years])

Reference epoch to which the astrometic source parameters are referred, expressed as a Julian Year in TCB.

ra : Right ascension (double, Angle[deg])

Barycentric right ascension $\alpha$ of the source in ICRS at the reference epoch ref_epoch

ra_error : Standard error of right ascension (double, Angle[mas])

Standard error $\sigma_{\alpha*}\equiv\sigma_{\alpha}\cos\delta$ of the right ascension of the source in ICRS at the reference epoch ref_epoch.

dec : Declination (double, Angle[deg])

Barycentric declination $\delta$ of the source in ICRS at the reference epoch ref_epoch

dec_error : Standard error of declination (double, Angle[mas])

Standard error $\sigma_{\delta}$ of the declination of the source in ICRS at the reference epoch ref_epoch

ra_dec_corr : Correlation between right ascension and declination (float, Dimensionless[see description])

Correlation between right ascension and declination, in dimensionless units [-1:+1]

phot_g_mean_mag : G-band mean magnitude (double, Magnitude[mag])

Mean magnitude in the G band. This is computed from the G-band mean flux applying the magnitude zero-point in the Vega scale.

astrometric_priors_used : Type of prior used in in the astrometric solution (int)

Type of prior used in the astrometric solution:

• 0: No prior used

• 1: Galaxy Bayesian Prior for parallax and proper motion

• 2: Galaxy Bayesian Prior for parallax and proper motion relaxed by factor 10

• 3: Hipparcos prior for position

• 4: Hipparcos prior for position and proper motion

• 5: Tycho2 prior for position

• 6: Quasar prior for proper motion

The Galaxy Bayesian Prior is defined in (2), where it is denoted $\sigma_{\varpi,F90}$ (for the parallax) and $\sigma_{\mu,F90}={\cal R}\sigma_{\varpi,F90}$, with ${\cal R}=10$ yr${}^{-1}$ (for proper motion). The Galaxy Bayesian Prior relaxed by a factor 10 is $10\sigma_{\varpi,F90}$ and $10\sigma_{\mu,F90}$, respectively.

For Gaia DR1 the only types of priors used are 2 (for the secondary data set), 3 (for the Hipparcos subset of the primary data set), or 5 (for the non-Hipparcos subset of the primary data set). Type 6 was used for internal calibration purposes and alignment of the reference frame.

icrf2_match : ICRF2 designation of the source matched to this Gaia source (string)

ICRF2 designation of the source matched to this Gaia source.

rot_flag : Flag indicating how this source was used to fix the orientation of the reference frame of the Gaia DR1 solution (int)

Flag indicating how this source was used to fix the orientation of the reference frame of the Gaia DR1 solution:

• 0: Not used

• 1: Only right ascention is used

• 2: Only declination is used

• 3: Both right ascention and declination are used