Column description for the catalogue file SF_DRQ16.fits (fits table format). This is the data file used in the publication "Ensemble X-ray variability of optically selected QSOs: dependence on black hole mass and Eddington ratio" by Georgakakis et al. to deterine the structure function of DRQ16 QSOs. This data file is read by the python script SF.py to determine the structure function of the QSO population. 

SDSS_NAME: Identification String listed in the Sloan Digital Sky Survey Quasar Catalog sixteenth data release (DR16Q) 

obsid_1: the XMM-Newton Observation Identification Number with which the QSO overlaps. This is the 1st epoch observation of the pair used to determine the structure function. 

# First Epoch X-ray Photometry (XMM-Newton)

EP1_CTS: photon counts in the 0.2-2keV energy band extracted at the position of the QSO in the first epoch observation. The counts are from the EPIC cameras onboard XMM-Newton and are extracted from the RAPIDXMM database. 

EP1_BKG: background expectation value in the 0.2-2keV energy band  at the position of the QSO in the first epoch observation. The background estimate is for the EPIC cameras onboard XMM-Newton and is extracted from the RAPIDXMM database. 

EP1_EXP: the exposure time in the 0.2-2keV energy band extracted at the position of the QSO in the first epoch observation multiplied by the Encircled Energy Fraction and the Energy Conversion Factor. The exposure time is for the EPIC cameras onboard XMM-Newton and is extracted from the RAPIDXMM database. The Energy Conversion Factor assumes a power-law spectral energy distribution with photon index Gamma=1.9 and Galactic Extinction in the direction of the QSO.

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obsid_2: the XMM-Newton Observation Identification Number with which the QSO overlaps. If blank the measurement is from eROSITA. This is the 2nd epoch observation of the pair used to determine the structure function.

# Second Epoch X-ray Photometry (XMM-Newton or eROSITA)

EP2_CTS: XMM-Newton or eROSITA photon counts. In the case of XMM-Newton the counts are extracted in the 0.2-2keV energy band at the position of the QSO in the second epoch observation. The counts are from the EPIC cameras onboard XMM-Newton and are extracted from the RAPIDXMM database. In the case of eROSITA the counts are extracted from the 0.2-2.3keV energy band using the APETOOL task of eSASS. 

EP2_BKG: XMM-Newton or eROSITA background level. In the case of XMM-Newton the background is extracted in the 0.2-2keV energy band at the position of the QSO in the second epoch observation. The background counts are for the EPIC cameras onboard XMM-Newton and are extracted from the RAPIDXMM database. In the case of eROSITA the background is estimated in the 0.2-2.3keV energy band using the APETOOL task of eSASS. 

EP2_EXP: the exposure time in the 0.2-2keV energy band extracted at the position of the QSO in the first epoch observation multiplied by the Encircled Energy Fraction and the Energy Conversion Factor. The exposure time is for the EPIC cameras onboard XMM-Newton and is extracted from the RAPIDXMM database. The Energy Conversion Factor assumes a power-law spectral energy distribution with photon index Gamma=1.9 and Galactic Extinction in the direction of the QSO.

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RA: Optical Right Ascension of the DRQ16 QSO 

DEC: Declination of the DRQ16 QSO

Z_FIT: Redshift of the DRQ16 QSO

LOGMBH: Black Hole Mass estimate for the DRQ16 QSO taken from Wu & Shen (2022)

LOGLEDD_RATIO: Eddington ratio estimate for the DRQ16 QSO taken from Wu & Shen (2022

ZWARNING: SDSS spectroscopic pipeline warning flag listed in the DR16Q catalogue

SN_MEDIAN_ALL: SDSS spectrum signal-to-noise ratio listed in the DR16Q catalogue

DTREST: rest-frame time interval between the 1st and 2nd epoch observation in units of days