arXiv subjects

H. Steinle

Publications and source records attributed to H. Steinle.

17 recordsLinked to original sources

arXiv2017

Cosmology with XMM galaxy clusters: the X-CLASS/GROND catalogue and photometric redshifts

The XMM Cluster Archive Super Survey (X-CLASS) is a serendipitously-detected X-ray-selected sample of 845 galaxy clusters based on 2774 XMM archival observations and covering approximately 90 deg$^2$ spread across the high-Galactic latitude ($|b|>20$ deg) sky. The primary goal of this survey is to produce a well-selected sample of galaxy clusters on which cosmological analyses can be performed. This article presents the photometric redshift followup of a high signal-to-noise subset of 266 of these clusters with declination $δ<+20$ deg with GROND, a seven channel ($grizJHK$) simultaneous imager on the MPG 2.2m telescope at the ESO La Silla Observatory. We use a newly developed technique based on the red sequence colour-redshift relation, enhanced with information coming from the X-ray detection to provide photometric redshifts for this sample. We determine photometric redshifts for 236 clusters, finding a median redshift of $z=0.39$ with an accuracy of $Δz = 0.02 (1+z)$ when compared to a sample of 76 spectroscopically confirmed clusters. We also compute X-ray luminosities for the entire sample and find a median bolometric luminosity of $7.2\times10^{43} \mathrm{erg\ s^{-1}}$ and a median temperature 2.9 keV. We compare our results to the XMM-XCS and XMM-XXL surveys, finding good agreement in both samples. The X-CLASS catalogue is available online at http://xmm-lss.in2p3.fr:8080/l4sdb/.

astro-ph.CO↗

arXiv2010

The absence of sub-minute periodicity in classical T Tauri stars

Classical T Tauri stars (CTTS) are young, late-type objects, that still accrete matter from a circumstellar disk. Analytical treatments and numerical simulations predict instabilities of the accretion shock on the stellar surface. We search for variability on timescales below a few minutes in the CTTS TW Hya and AA Tau. TW Hya was observed with SALTICAM on the Southern African Large Telescope (SALT) in narrow-band filters around the Balmer jump. The observations were performed in slit mode, which provides a time resolution of about 0.1 s. For AA Tau we obtained observations with OPTIMA, a single photon-counting device with even better time resolution. Small-scale variability typically lasts a few seconds, however, no significant periodicity is detected. We place a 99 % confidence upper limit on the pulsed fraction of the lightcurves. The relative amplitude is below 0.001 for TW Hya in the frequency range 0.02-3 Hz in the 340 nm filter and 0.1-3 Hz in the 380 nm filter. The corresponding value for AA Tau is an amplitude of 0.005 for 0.02-50 Hz. The relevant timescales indicate that shock instabilites should not be seen directly in our optical and UV observations, but the predicted oscialltions would induce observable variations in the reddening. We discuss how the magnetic field could stabilise the accretion shock.

astro-ph.SR↗

arXiv2009

Centaurus A at Hard X-rays and Soft Gamma-rays

Centaurus A, at a distance of less than 4 Mpc, is the nearest radio-loud AGN. Its emission is detected from radio to very-high energy gamma-rays. Despite the fact that Cen A is one of the best studied extragalactic objects the origin of its hard X-ray and soft gamma-ray emission (100 keV < E < 50 MeV) is still uncertain. Observations with high spatial resolution in the adjacent soft X-ray and hard gamma-ray regimes suggest that several distinct components such as a Seyfert-like nucleus, relativistic jets, and even luminous X-ray binaries within Cen A may contribute to the total emission in the MeV regime that has been detected with low spatial resolution. As the Spectral Energy Distribution of Cen A has its second maximum around 1 MeV, this energy range plays an important role in modeling the emission of (this) AGN. As there will be no satellite mission in the near future that will cover this energies with higher spatial resolution and better sensitivity, an overview of all existing hard X-ray and soft gamma-ray measurements of Cen A is presented here defining the present knowledge on Centaurus A in the MeV energy range.

astro-ph.HE↗

arXiv2005

New optical polarization measurements of the Crab pulsar

The Crab nebula and its pulsar have been observed for about 3 hours with the high-speed photo-polarimeter OPTIMA in January 2002 at the Calar Alto 3.5m telescope. The Crab pulsar intensity and polarization are determined at all phases of rotation with higher statistical accuracy than ever. Therefore, we were able to separate the so-called 'off-pulse' phase emission (with an intensity of about 1.2% compared to the main peak, assumed to be present at all phases) from the pulsed emission and show the 'net' polarization of the pulsed structures. Recent theoretical results indicate that the measured optical polarization of the Crab pulsar is similar to expectations from a two-pole caustic emission model or a striped pulsar wind model.

astro-ph↗

arXiv2004

Search for fast correlated X-ray and optical variability in Cir X-1 and XTE J1746-321

Coordinated observations X-ray+optical observations of two southern X-ray binaries, the black hole candidate XTE J1746-321 and the neutron star accreter Cir X-1 (a `microquasar') are reported. With a photon counting optical photometer on the 1.9m telescope at Sutherland, South Africa and the PCA detector on RXTE, 4h each of simultaneous data were obtained on XTE J1746 and Cir X-1. Cir X-1 showed no X-ray variability at the 2% level, XTE J1746 was variable at 5-7% with a 5Hz QPO. Cross-correlation yielded no correlated signals on either source, to a level of 1%. A problem with a recently published orbital ephemeris of Cir X-1 is pointed out.

astro-ph↗

arXiv2003

BeppoSAX Observations of Centaurus A: the Hard Continuum and the Iron Line Feature

The radio galaxy Centaurus A was observed by the BeppoSAX satellite five times from 1997 to 2000. From July 6 1999 to August 17 1999, the source was also simultaneously pointed by COMPTEL on-board of the satellite CGRO. Centaurus A has a complex spectrum with multiple extended components and a strongly absorbed (N_H\sim 10^{23}$ cm${-2}) nucleus well fitted by a power law (Γ\sim 1.8) which bends at high energies. When the BeppoSAX and COMPTEL observations are combined together, an exponential cutoff with e-folding energy \sim 1000 keV gives an adequate description of the spectral steepening. A complex feature in emission at 6-7 keV is resolved into two Fe components, one narrow cold line and an ionized line centred at 6.8 keV. Significant variations have been observed in the iron feature, with the less prominent ionized line seemingly being the only one responsible for them: its variations do not appear to correlate with the strength of the continuum. The high energy cutoff and the Fe feature suggest the presence of an accretion flow in the Centaurus A nucleus. However the absence of a significant reflection, the narrowness of the cold line as well as the lack of correlation between the continuum and 6.8 keV line variations disfavour a standard cold/ionized thin disk (at least in the inner regions). A more plausible configuration might be a hot thick optically thin accretion flow surrounded by material with different opacities. Finally, we note that high energy break observed by BeppoSAX and COMPTEL could be also reasonably explained by Inverse Compton radiation from a jet. If this is the case, a structured jet with outer slow layers surrounding a beamed inner region is necessary to explain the strong Fe feature observed by BeppoSAX.

astro-ph↗

arXiv2002

The Soft Gamma-Ray Spectral Variability of Cygnus X-1

We have used observations of Cygnus X-1 from the Compton Gamma-Ray Observatory (CGRO) and BeppoSAX to study the variation in the MeV gamma-ray emission between the hard and soft spectral states, using spectra that cover the energy range 20 keV up to 10 MeV. These data provide evidence for significant spectral variability at energies above 1 MeV. In particular, whereas the hard X-ray flux decreases during the soft state, the flux at energies above 1 MeV increases, resulting in a significantly harder gamma-ray spectrum at energies above 1 MeV. This behavior is consistent with the general picture of galactic black hole candidates having two distinct spectral forms at soft gamma-ray energies. These data extend this picture, for the first time, to energies above 1 MeV. We have used two different hybrid thermal/non-thermal Comptonization models to fit broad band spectral data obtained in both the hard and soft spectral states. These fits provide a quantitative estimate of the electron distribution and allow us to probe the physical changes that take place during transitions between the low and high X-ray states. We find that there is a significant increase (by a factor of ~4) in the bolometric luminosity as the source moves from the hard state to the soft state. Furthermore, the presence of a non-thermal tail in the Comptonizing electron distribution provides significant constraints on the magnetic field in the source region.

astro-ph↗

arXiv2001

Gamma-Ray Spectral Variability of Cygnus X-1

We have used observations from CGRO to study the variation in the MeV emission of Cygnus X-1 between its low and high X-ray states. These data provide a measurement of the spectral variability above 1 MeV. The high state MeV spectrum is found to be much harder than that of the low state MeV spectrum. In particular, the power-law emission seen at hard X-ray energies in the high state spectrum (with a photon spectral index of 2.6) is found to extend out to at least 5 MeV, with no evidence for any cutoff. Here we present the data and describe our efforts to model both the low state and high state spectra using a hybrid thermal/nonthermal model in which the emission results from the Comptonization of an electron population that consists of both a thermal and nonthermal component.

astro-ph↗

arXiv2000

The COMPTEL instrumental line background

The instrumental line background of the Compton telescope COMPTEL onboard the Compton Gamma-Ray Observatory is due to the activation and/or decay of many isotopes. The major components of this background can be attributed to eight individual isotopes, namely 2D, 22Na, 24Na, 28Al, 40K, 52Mn, 57Ni, and 208Tl. The identification of instrumental lines with specific isotopes is based on the line energies as well as on the variation of the event rate with time, cosmic-ray intensity, and deposited radiation dose during passages through the South-Atlantic Anomaly. The characteristic variation of the event rate due to a specific isotope depends on its life-time, orbital parameters such as the altitude of the satellite above Earth, and the solar cycle. A detailed understanding of the background contributions from instrumental lines is crucial at MeV energies for measuring the cosmic diffuse gamma-ray background and for observing gamma-ray line emission in the interstellar medium or from supernovae and their remnants. Procedures to determine the event rate from each background isotope are described, and their average activity in spacecraft materials over the first seven years of the mission is estimated.

astro-ph↗

arXiv2000

A High Sensitivity Measurement of the MeV Gamma-Ray Spectrum of Cygnus X-1

The Compton Gamma-Ray Observatory (CGRO) has observed the Cygnus region on several occasions since its launch in 1991. The data collected by the COMPTEL experiment on CGRO represent the most sensitive observations to date of Cygnus X-1 in the 0.75-30 MeV range. A spectrum accumulated by COMPTEL over 10 weeks of observation time shows significant evidence for emission extending out to several MeV. We have combined these data with contemporaneous data from both BATSE and OSSE to produce a broad-band gamma-ray spectrum, corresponding to the low X-ray state of Cygnus X-1, extending from 50 keV up to approximately 5 MeV. Although there is no evidence for any broad line-like emissions in the MeV region, these data further confirm the presence of a hard tail at energies above several hundred keV. In particular, the spectrum at MeV energies can be described as a power-law with a photon spectral index of $α$ = -3.2, with no evidence for a cutoff at high energies. For the 200 keV to 5 MeV spectrum, we provide a quantitative description of the underlying electron spectrum, in the context of a hybrid thermal/non-thermal model for the emission. The electron spectrum can be described by a thermal Maxwellian with a temperature of $kT_e$ = 86 keV and a non-thermal power-law component with a spectral index of $p_e$ = 4.5. The spectral data presented here should provide a useful basis for further theoretical modeling.

astro-ph↗

arXiv2000

A bright X-ray transient towards NGC 5128 (Centaurus A)

During five ROSAT HRI observations in July 1995, a bright X-ray source (designated 1RXH J132519.8-430312) was detected 2.5' south-west of the nucleus of NGC 5128 (Centaurus A) at the outer regions of the elliptical galaxy. At that time it was the brightest point source in the ROSAT HRI field-of-view. All other ROSAT observations made before and after these observations show no trace of the source suggesting that this is a highly variable object. Chandra observations of Cen A made in September 1999 show a source at the ROSAT position which may be the quiescent counterpart of the transient. It is very probable that the transient is located in NGC 5128. Given the existence of a transient source that close to the direction of NGC 5128, all observations (at least in soft X-rays) of Cen A, with instruments of low spatial resolution, must take the presence and variability of this object into account.

astro-ph↗

arXiv2000

The Spectral Variability of Cygnus X-1 at MeV Energies

In previous work, we have used data from the first three years of the CGRO mission to assemble a broad-band $γ$-ray spectrum of the galactic black hole candidate Cygnus X-1. Contemporaneous data from the COMPTEL, OSSE and BATSE experiments on CGRO were selected on the basis of the hard X-ray flux (45--140 keV) as measured by BATSE. This provided a spectrum of Cygnus X-1 in its canonical low X-ray state (as measured at energies below 10 keV), covering the energy range from 50 keV to 5 MeV. Here we report on a comparison of this spectrum to a COMPTEL-OSSE spectrum collected during a high X-ray state of Cygnus X-1 (May, 1996). These data provide evidence for significant spectral variability at energies above 1 MeV. In particular, whereas the hard X-ray flux {\it decreases} during the high X-ray state, the flux at energies above 1 MeV {\it increases}, resulting in a significantly harder high energy spectrum. This behavior is consistent with the general picture of galactic black hole candidates having two distinct spectral forms at soft $γ$-ray energies. These data extend this picture, for the first time, to energies above 1 MeV.

astro-ph↗

arXiv2000

A large high-energy gamma-ray flare from the blazar 3C 273

The Compton Gamma-Ray Observatory (CGRO) experiments EGRET and COMPTEL observed the Virgo sky region continuously for 7 weeks between December 10, 1996 and January 28, 1997. The prominent quasar 3C~273 was found to be the brightest source in gamma-rays and was significantly detected by EGRET and COMPTEL. The EGRET experiment observed a time-variable flux at energies above 100 MeV, which reached in a 2-week flaring period (December 30, 1996 to January 14, 1997) its highest flux level observed during the CGRO-era. COMPTEL, however, does not observe obvious time variability at energies below ~30 MeV contemporaneous to EGRET. In particular, no flare was observed, indicating that this outburst is solely a high-energy (>100 MeV) phenomenon. The energy spectrum between 3 MeV and 10 GeV is well represented by a simple power-law model. Below 3 MeV a spectral turnover is indicated. Performing spectral analysis for different time periods, we found evidence for a spectral hardening during the flaring period, which is consistent with the flare occurring mainly at the higher energies and with its absence at COMPTEL energies of a few MeV. This may be interpreted as an indication that the emission in the EGRET energy range is dominated by a different radiation mechanism than the MeV emission. We argue that the most likely mechanism for the high-energy flare is inverse-Compton scattering of reprocessed accretion-disk radiation.

astro-ph↗

arXiv1999

COMPTEL MeV Observations of the TeV Sources Markarian 421 and Markarian 501

The COMPTEL experiment aboard the COMPTON Gamma-Ray Observatory (CGRO) has observed the prominent TeV-blazars Mkn 421 and Mkn 501 many times between the start of its mission in April '91 and December '98. This paper reports first COMPTEL results from mainly time-averaged (CGRO Cycles) data. No evidence for both sources is found up to the end Cycle VI. However, the sum of all 10-30 MeV Cycle VII data shows a weak (3.2 sigma detection) MeV-source being positionally consistent with Mkn 421. During Cycle VII Mkn 421 was rather active at TeV-energies. Due to the lack of other known Gamma-ray sources in this sky region, we consider Mkn 421 as the most likely counterpart for this Gamma-ray emission. However, its connection cannot be proven by COMPTEL.

astro-ph↗

arXiv1999

COMPTEL Observations of AGN at MeV-Energies

The COMPTEL experiment aboard CGRO, exploring the previously unknown sky at MeV-energies, has so far detected 10 Active Galactic Nuclei (AGN): 9 blazars and the radio galaxy Centaurus A. No Seyfert galaxy has been found yet. With these results COMPTEL has opened the field of extragalactic Gamma-ray astronomy in the MeV-band.

astro-ph↗

arXiv1997

COMPTEL observations of the quasar PKS 0528+134 during the first 3.5 years of the CGRO mission

The COMPTEL observations of the blazar-type quasar PKS 0528+134 in the energy range 0.75 MeV to 30 MeV carried out between April 1991 and September 1994 have been analyzed. During the first two years PKS 0528+134 was most significantly detected at energies above 3 MeV. During the last year there is only evidence for the quasar at energies below 3 MeV indicating a spectral change. The time-averaged COMPTEL energy spectrum between 0.75 MeV and 30 MeV is well represented by a power-law shape. Spectra collected from different observational periods reveal different power-law shapes: a hard state during flaring observations reported by EGRET, and a soft state otherwise. The combined simultaneous EGRET and COMPTEL spectra indicate these two spectral states as well. During low intensisty gamma-ray phases no spectral break is obvious from the combined COMPTEL and EGRET measurements. For the gamma-ray flaring phases however, the combined COMPTEL and EGRET data require a spectral bending at MeV-energies. By fitting broken power-law functions the best-fit values for the break in photon index range between 0.6 and 1.7, and for the break energy between ~5 MeV and ~20 MeV. Because the flux values measured by COMPTEL below 3 MeV in both states are roughly equal, the observations would be consistent with an additional spectral component showing up during gamma-ray flaring phases of PKS 0528+134. Such a component could be introduced by e.g. a high-energy electron-positron population with a low-energy cutoff in their bulk Lorentz factor distribution. The multiwavelength spectrum of PKS 0528+134 for gamma-ray flaring phases shows that the major energy release across the entire electro-magnetic spectrum is measured at MeV-energies.

astro-ph↗

arXiv1996

COMPTEL detection of the high-energy gamma-ray source 2CG 135+01

We present an analysis of COMPTEL observations made between November 1991 and May 1994 of 2CG 135+01, a bright gamma-ray source located near the Galactic plane. At energies above 1 MeV, an excess consistent with the position of 2CG 135+01 is detected in the sum of the observations, at flux levels which are a factor of 10-100 below those published in the past. The detection significance of this excess, when the possible presence of underlying Galactic diffuse emission is neglected, is 6.6 sigma for 3 degrees of freedom. The differential photon spectrum in the 1-30 MeV energy range can be described by a power law with a spectral index of $1.95^{+0.2}_{-0.3}$. Due to the uncertainties involved in modelling the Galactic-disk diffuse emission underneath the source, the absolute flux levels must be considered uncertain by a factor of two. They are consistent with the extrapolation of the time-averaged spectrum of 2CG 135+01 measured with EGRET, thereby strengthening the identification. No significant temporal correlation between the gamma-ray emission and the monitored radio emission of the possible counterpart radio source GT 0236+610 (showing a 26.5 day modulation) is found.

astro-ph↗