The unusual host galaxy of the BL Lac object PKS 1413+135

The BL Lacertae object PKS 1413+135 is associated with a disk dominated galaxy which heavily absorbs the BL Lac nucleus at optical and X-ray wavelengths. It has been argued whether this galaxy is actually the host galaxy of PKS 1413+135 or whether the BL Lac is a background QSO, gravitationally lensed by the apparent host galaxy. We have obtained deep high resolution H-band images of this unusual BL Lac object using the UKIRT IRCAM3. Our observations show that the BL Lac nucleus is centered within < 0.05 arcsec of the galaxy. Based on this result we assess the probability for the lensing scenario and come to the conclusion that the disk galaxy is indeed the host of PKS 1413+135. The galaxy shows peanut-shaped isophotes, suggesting the presence of a central bar which is a common feature of AGNComment: accepted for publication in MNRAS, 5 pages with 2 Postscript figure

2XMM J083026+524133: The most X-ray luminous cluster at redshift 1

In the distant universe X-ray luminous clusters of galaxies are rare objects. Large area surveys are therefore needed to probe the high luminosity end of the cluster population at redshifts z >= 1. We correlated extended X-ray sources from the second XMM-Newton source catalogue (2XMM) with the SDSS in order to identify new clusters of galaxies. Distant cluster candidates in empty SDSS fields were imaged in the R and z bands with the Large Binocular Telescope. We extracted the X-ray spectra of the cluster candidates and fitted thermal plasma models to the data. We determined the redshift 0.99 +-0.03 for 2XMM J083026+524133 from its X-ray spectrum. With a bolometric luminosity of 1.8 x 10^45 erg/sec this is the most X-ray luminous cluster at redshifts z >= 1. We measured a gas temperature of 8.2 +- 0.9 keV and and estimate a cluster mass M(500) = 5.6 x 10^14 M(solar). The optical imaging revealed a rich cluster of galaxies.Comment: New version, as accepted for publication in Astronomy & Astrophysic

An absorption event in the X-ray lightcurve of NGC 3227

We have monitored the Seyfert galaxy NGC 3227 with the Rossi X-ray Timing Explorer (RXTE) since January 1999. During late 2000 and early 2001 we observed an unusual hardening of the 2-10 keV X-ray spectrum which lasted several months. The spectral hardening was not accompanied by any correlated variation in flux above 8 keV. We therefore interpret the spectral change as transient absorption by a gas cloud of column density 2.6 10^23 cm^-2 crossing the line of sight to the X-ray source. A spectrum obtained by XMM-Newton during an early phase of the hard-spectrum event confirms the obscuration model and shows that the absorbing cloud is only weakly ionised. The XMM-Newton spectrum also shows that ~10% of the X-ray flux is not obscured, but this unabsorbed component is not significantly variable and may be scattered radiation from a large-scale scattering medium. Applying the spectral constraints on cloud ionisation parameter and assuming that the cloud follows a Keplerian orbit, we constrain the location of the cloud to be R~10-100 light-days from the central X-ray source, and its density to be n_H~10^8cm^-3, implying that we have witnessed the eclipse of the X-ray source by a broad line region cloud.Comment: 5 pages, 6 figures, accepted for publication in MNRAS letter

A Rapid X-ray Flare from Markarian 501

We present X-ray observations of the BL Lacertae (BL Lac) object Markarian 501 (Mrk 501), taken with the Rossi X-ray Timing Explorer in 1998 May as part of a multi-wavelength campaign. The X-ray light curve shows a very rapid flare in which the 2-10 keV flux increased by ~60% in <200 seconds. This rapid rise is followed by a drop-off in the 2-10 keV flux of ~40% in <600 seconds. The 10-15 keV variation in this flare is roughly a factor of two on similar time-scales. During the rise of the flare, the 3-15 keV spectral index hardened from 2.02 +/- 0.03 to 1.87 +/- 0.04, where it remained during the decay of the flare. This is the fastest variation ever seen in X-rays from Mrk 501 and among the fastest seen at any wavelength for this object. The shift in the energy at which the spectral power peaks (from 30 keV during the flare) is also among the most rapid shifts seen from this object. This flare occurs during an emission state (2-10 keV flux approximately 1.2e-10 erg cm^-2 s^-1) that is approximately 25% of the peak flux observed in 1997 April from this object but which is still high compared to its historical average X-ray flux. The variations in the hardness ratio are consistent with the low energy variations leading those at high energies during the development and decay of the flare. This pattern is rare among high frequency peaked BL Lac objects like Mrk 501, but has been seen recently in two other TeV emitting BL Lacs, Mrk 421 and PKS 2155-304. The hard lag is consistent with a flare dominated by the acceleration time-scale for a simple relativistic shock model of flaring.Comment: 11 pages, 3 figures, accepted for publication in Astrophys. J. Letter

The Meinunger "Nicht Rote" Objects

Four high-latitude slow variable stars have been noted by Meinunger (1972) as "nicht rote" ("not red") objects and thus curious. We have previously reported (Margon & Deutsch 1997) that one of these objects, CC Boo, is in fact a QSO. Here we present observations demonstrating that the remaining three are also highly variable active galactic nuclei. The most interesting object of the four is perhaps S 10765 (= NGP9 F324-0276706), which proves to be a resolved galaxy at z=0.063. Despite the rapid and large reported variability amplitude (~1.6 mag), the spectrum is that of a perfectly normal galaxy, with no emission lines or evident nonthermal continuum. We also present new spectroscopic and photometric observations for AR CVn, suggested by Meinunger to be an RR Lyrae star despite its very faint magnitude (=19.4). The object is indeed one of the most distant RR Lyrae stars known, at a galactocentric distance of ~40 kpc.Comment: Accepted for publication in Publications of the Astronomical Society of the Pacific, Volume 111, January 1999; 14 pages including 4 figures and 1 tabl

X-ray spectral variability of the Seyfert galaxy NGC 4051

We report on the X-ray spectral variability of the Seyfert 1 galaxy NGC 4051 observed with the Rossi X-ray Timing Explorer (RXTE) during a 1000 day period between May 1996 and March 1999. The spectra were obtained as part of monitoring observations and from two long observations using the RXTE Proportional Counter Array (PCA). During the monitoring period the 2-10 keV flux of NGC 4051 varied between 10E-12 and 7x 10E11 (cgs). We re-analysed RXTE PCA observations from a distinct low state in May 1998 using the latest background and detector response models. The RXTE and BeppoSAX observations of NGC 4051 during the low state show a very hard spectrum with a strong unresolved fluorescence line. This emission, probably due to reflection from a molecular torus, is likely to be constant over long time-scales and is therefore assumed as an underlying component at all flux states. By subtracting the torus component we are able to determine the spectral variability of the primary continuum. In the variable component we observe a strong anti-correlation of X-ray flux and spectral hardness in the PCA energy band. We show that the changes in hardness are caused by slope variability of the primary power law spectrum rather than by changing reflection or variable photoelectric absorption. The primary spectral index varies between Gamma=1.6 for the faintest states and Gamma=2.3 during the brightest states, at which level the spectral index approaches an asympotic value. We find that the response of the flux of the 6.4 keV iron fluorescence line to changes in the continuum flux depends on the timescale of the observation. The profile of the line is very broad and indicates an origin in the innermost regions of the accretion disk.Comment: accepted for publication in MNRA

XMM-Newton observations of the BL Lac MS 0737+7441

We report on the XMM-Newton observations of the BL Lac object MS 0737.9+7441 during the performance verification phase. A simple power-law fit provides an adequate description of the integrated spectrum in the 0.2-10 keV energy band. The photon index is slightly steeper in the EPIC pn data with 2.38+-0.01 compared to the EPIC MOS data (2.28+-0.01). The difference is most probably due to the present uncertainties in the calibration of the EPIC MOS and EPIC pn data sets. We report evidence for intrinsic absorption in the distant BL Lac above the Galactic column N_H,Gal=3.2*10^20 cm^-2 which is N_H,fit(z=0.315)= (2.70+-0.20)*10^20cm^-2 in the EPIC pn data and N_H,fit(z=0.315)= (3.25+-0.25)*10^20cm^-2 in the EPIC MOS data assuming neutral gas and solar abundances. The flux variations are found to be of the order of 10 %.Comment: 4 pages, 4 Figures, accepted for publication in the special A&A Letters issue for XMM-Newto