Research Projects

My recent research is mainly concerned with sudies of X-ray and optical properties of AGN. I participated in the work on the `Hamburg/RASS catalogue of optical identifications' (s. Bade et al., 1998, A&AS 127).
During my Ph.D. studies, I analysed an X-ray survey of the field HS 47.5/22 in Ursa Maior, and performed and reduced optical follow-up observations. The field is part of the Hamburg Quasar Survey (HQS). Its central 11.5 deg² contain an optically selected quasar sample the completeness of which I determined.
The X-ray survey consists of 48 overlapping pointings with the PSPC detector on board the ROSAT satellite. Its main purpose is the selection of new AGN, particularly below the limit of the HQS Schmidt plates. These should allow a comparison between X-ray selected and optically selected AGN samples drawn from the same area.

I performed source detection both on individual pointings and on the merged data. The latter required some adjustments of existing EXSAS procedures, which were developed for single pointing analysis, and the development of new ones. Cooperating with the EXSAS authors proved highly valuable.
The net exposure time could be increased to more than 20000s in the central part of the field by merging. This allows a detailed analysis of the hard (0.5 - 2.0 keV) band, and the detection of hitherto invisible sources with extreme spectra. Other sources, however, were lost through the addition of parts of the detector field of view with different sensitivities. Both source detection modes are therefore necessary to detect all X-ray sources.
The final catalogue contains 574 sources. A comparison of the log N - log S curve with previous work showed it to be complete down to fluxes of 3.4 ^. 10^-14ergs/cm²/s in the total ROSAT band.

Besides published catalogues of various kinds, I used mainly the HQS Schmidt plates for source identification which allowed object classification down to a magnitude m_b = 18^m.5, and a recognition of blue objects (i.e. AGN candidates) a magnitude deeper. I performed optical spectroscopy and imaging with the 3.5m telescope at Calar Alto/Spain. The X-ray catalogue and the identifications are published (Molthagen et al., 1997, A&AS 126).

The X-ray observations were made over a period of more than two years and are thus a good base for variability studies. 71 X-ray sources (36 AGN) could be investigated, and 48 showed variability, or indications thereof, between their detections in different pointings. 27 sources were sufficiently bright to allow the pointings to be split in search of short time changes. Among AGN, the fraction of variable sources is higher: 30 of 36 show changes. The observed variations are, however, moderate. In all but one cases the changes were of a factor <5 in several hours or more.

The completeness of the X-ray selected AGN sample is determined by the identification status of the catalogue; the current limit is f_x = 2 ^. 10^-13 ergs/cm²/s. 25 AGN with 0.1 < z < 2 are above that threshold. Evolution plays a noticeable role in that range, so that a binning in z had to be introduced. I could create the X-ray luminosity function for two bins containing 10 and 14 AGN, respectively. The low number of objects prohibited any analysis beyond the recognition of a good agreement with the literature.
The complete optical sample contains even fewer objects, so that even an attempt to create the luminosity function was impossible.

I studied some special objects in detail. The high redshift quasar PC 0953+4749 (z = 4.457) is located in a part of HS 47.5/22 with a net exposure time > 15000 s. It is not detected at X-rays, but I determined upper limits for its X-ray flux which are consistent with other high redshift quasars (Molthagen et al. 1995, A&A 295).
One of the newly identified quasars shows deep absorption lines in its optical spectrum. It is as yet unclear whether these are genuine Broad Absorption Lines or associated absorption (Molthagen et al., 1996, AG Abstract Series).
The QSO RX J0947.0+4721 shows an extremely soft X--ray spectrum. It is also the above mentioned exception which shows strong and rapid X-ray variations. The shortest detected doubling timescale is 11.9^h, and the largest amplitude change is a factor 5.7. Much stronger and faster variations are indicated. The X-ray and optical properties of this object are very similar to narrow line Seyfert 1 galaxies, albeit with higher redshift and luminosity than most of them (Molthagen et al., 1998, A&A 331). New ROSAT HRI observations in search of short time and/or large amplitude variations have been performed. They brought no new results, in particular, no further giant amplitude variations were detected.

Besides X--ray emission of quasars, I have studied the object G79.29+0.46, a wind blown bubble around a Luminous Blue Variable, in particular the ISO observations. I got an introduction into the ISOPHOT data analysis with the IDL based PIA software package at the Max-Planck-Institut für Astronomie in Heidelberg (Germany). The ISO Spectrometer Data Centre people (Max Planck-Institut für extraterrestrische Physik Garching/Germany) taught me how to analyze the LWS data with ISAP. As PIA, IASP is an IDL based package. I have installed both in Hamburg, and was responsible for maintenance and the introduction of new users.

My work at the Max-Plamck-Institute consisted of scientific work and service, for example an EXSAS procedure which converts ROSAT detector coordinates into sky coordinates. During my second year at MPE, I worked at the PANTER test facility. Here, mirrors and cameras for the coming X-ray satellites ABRIXAS and XMM were calibrated. I worked at performing and analyzing these tests as well as in the documentation.