Stability of the MIDI photometry: the case of Circinus

TL;DR

This paper investigates the stability of MIDI photometry for the Circinus galaxy, identifying offsets and drifts in flux measurements and analyzing their potential causes to improve calibration accuracy.

Contribution

It provides a detailed analysis of flux variations in MIDI data for Circinus, highlighting potential instrumental and observational factors affecting photometric stability.

Findings

Detected flux offsets between 2008 and 2009 data

Identified slow flux drifts correlated with hour angle

Suggested possible instrumental or observational causes

Abstract

In principle, the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Array (VLTI) should always measure the same calibrated total flux spectrum for a specific source, independent of the instrument settings and the baseline geometry. In the data on the Circinus galaxy, however, there is (a) a general offset of the flux values for 2009 and (b) a slow drift of the total fluxes at short wavelengths during two nights (2008-04-17 and 2009-04-14). The latter seems to depend on the hour angle of the observation. In this document, a more detailed analysis of these two effects is carried out and summarised. The goal is to find an explanation for these variations in the photometry.