The calibration of read-out-streak photometry in the XMM-Newton Optical Monitor and the construction of a bright-source catalogue

TL;DR

This paper calibrates read-out streak photometry for the XMM-Newton Optical Monitor to improve bright source measurements, extending the dynamic range and enabling better photometric data for bright objects in the UV spectrum.

Contribution

It introduces a calibration method for read-out streaks in XMM-OM, determining the bright-source limits and demonstrating its application to real data, including a nova.

Findings

MCP recharge time is 0.55 ms.

Bright-source limits are about 1.5 magnitudes brighter than normal photometry.

Photometry for 50% of missing UV sources in the catalog was recovered.

Abstract

The dynamic range of the XMM-Newton Optical Monitor (XMM-OM) is limited at the bright end by coincidence loss, the superposition of multiple photons in the individual frames recorded from its micro-channel-plate (MCP) intensified charge-coupled device (CCD) detector. One way to overcome this limitation is to use photons that arrive during the frame transfer of the CCD, forming vertical read-out streaks for bright sources. We calibrate these read-out streaks for photometry of bright sources observed with XMM-OM. The bright source limit for read-out streak photometry is set by the recharge time of the MCPs. For XMM-OM we find that the MCP recharge time is 0.55 ms. We determine that the effective bright limits for read-out streak photometry with XMM-OM are approximately 1.5 magnitudes brighter than the bright source limits for normal aperture photometry in full-frame images. This translates into bright-source limits in Vega magnitudes of UVW2=7.1, UVM2=8.0, UVW1=9.4, U=10.5, B=11.5, V=10.2 and White=12.5 for data taken early in the mission. The limits brighten by up to 0.2 magnitudes, depending on filter, over the course of the mission as the detector ages. The method is demonstrated by deriving UVW1 photometry for the symbiotic nova RR Telescopii, and the new photometry is used to constrain the e-folding time of its decaying UV emission. Using the read-out streak method, we obtain photometry for 50 per cent of the missing UV source measurements in version 2.1 of the XMM-Newton Serendipitous UV Source Survey (XMM-SUSS 2.1) catalogue.