arXiv subjects

J. Truemper

Publications and source records attributed to J. Truemper.

30 records · Page 2Linked to original sources

arXiv1999

The Compact Central Object in Cas A: A Neutron Star with Hot Polar Caps or a Black Hole?

The central pointlike X-ray source of the Cas A supernova remnant was discovered in the Chandra First Light Observation and found later in the archival ROSAT and Einstein images. The analysis of these data does not show statistically significant variability of the source. The power-law fit yields the photon index 2.6-4.1, and luminosity (2-60)e34 erg/s, for d=3.4 kpc. The power-law index is higher, and the luminosity lower, than those observed fromvery young pulsars. One can fit the spectrum equally well with a blackbody model with T=6-8 MK, R=0.2-0.5 km, L=(1.4-1.9)e33 erg/s. The inferred radii are too small, and the temperatures too high, for the radiationcould be interpreted as emitted from the whole surface of a uniformly heated neutron star. Fits with the neutron star atmosphere models increase the radius and reduce the temperature, but these parameters are still substantially different from those expected for a young neutron star. One cannot exclude, however, that the observed emission originates from hot spots on a cooler neutron star surface. Because of strong interstellar absorption, the possible low-temperature component gives a small contribution to the observed spectrum; an upper limit on the (gravitationally redshifted) surface temperature is < 1.9-2.3 MK. Amongst several possible interpretations, we favor a model of a strongly magnetized neutron star with magnetically confined hydrogen or helium polar caps on a cooler iron surface. Alternatively, the observed radiation may be interpreted as emitted by a compact object (more likely, a black hole) accreting from a fossil disk or from a late-type dwarf in a close binary.

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arXiv1999

The ROSAT Deep Survey: III. Optical spectral properties of X-ray sources in the Lockman Hole

The ROSAT Deep Survey in the Lockman Hole contains a complete sample of 50 X-ray sources with fluxes in the 0.5-2.0 keV band larger than 5.5E-15 erg/cm2/s. Previous work has provided optical identification of 46 of the 50 X-ray sources; over 75% of the sources are AGNs (Schmidt et al. 1998). We present now the atlas of optical finding charts and the full description of the spectra, including emission line properties of the optical counterparts, which are important for the object classification. New optical/infrared observations of three of the four unidentified sources show that one source is an AGN and two sources with an unusually large ratio of X-ray to optical flux have counterparts in the K'-band suggesting that they are obscured AGNs. Furthermore, we found evidence from radio emission that the remaining unidentified source is a powerful radio galaxy (AGN). We thus obtain a 100% completeness. During the course of our optical identification work, we obtained optical spectr a of 83 field galaxies, of which 67 were narrow-emission line galaxies (NELG). We demonstrate that it is highly unlikely that a significant number of NELG are physically associated with X-ray sources.

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arXiv1999

The X-Ray Luminosity Function of Active Galactic Nuclei

We derive an X-ray luminosity function for active galactic nuclei (AGN) that accounts for the X-ray source counts in the 0.5-2.0 and 2-10 keV energy ranges, the redshift distribution of AGNs in the ROSAT Deep Survey (RDS), as well as the X-ray background (XRB) from 1-10 keV. We emphasize the role of X-ray absorption, which has a large effect on the faint end of the 2-10 keV source counts, as well as on the integrated X-ray background.

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arXiv1999

The ROSAT Deep Surveys

The ROSAT Deep Survey in the Lockman Hole is the most sensitive X-ray survey performed to date, encompassing an exposure time of 200 ksec with the PSPC and 1.2 Msec with the HRI. The source counts reach a density of ~1000/deg2 at a limiting flux of ~1E-15 erg/cm2/s. At this level 70-80% of the 0.5-2 keV X-ray background is resolved into discrete sources. Because of the excellent HRI positions, 83 X-ray sources with fluxes (0.5-2 keV) above 1.2E-15 erg/cm2/s could be optically identified so far utilizing deep optical CCD images, NIR photometry and Keck spectroscopy. Only 11 sources above this flux limit remain unidentified. The majority of objects turned out to be active galactic nuclei (AGN) with minority contributions of clusters of galaxies, stars and some individual galaxies. These deep pencil beam data together with a number of shallower ROSAT surveys define the source counts over six orders of magnitude in flux and provide a unique tool of unprecedented quality to study the cosmological evolution of AGN, which can easily provide the bulk of the extragalactic X-ray background and could give an important contribution to the total background light in the universe.

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arXiv1998

X-ray Pulsations from the Central Source in Puppis A

There are several supernova remnants which contain unresolved X-ray sources close to their centers, presumably radio-quiet neutron stars. To prove that these objects are indeed neutron stars, to understand the origin of their X-ray radiation, and to explain why they are radio-quiet, one should know their periods and period derivatives. We searched for pulsations of the X-ray flux from the radio-quiet neutron star candidate RX J0822-4300 near the center of the Puppis A supernova remnant observed with the ROSAT PSPC and HRI. A standard timing analysis of the separate PSPC and HRI data sets does not allow one to detect the periodicity unequivocally. However, a thorough analysis of the two observations separated by 4.56 yr enabled us to find a statistically significant period $P\simeq 75.3$ ms and its derivative $\dot{P}\simeq 1.49\times 10^{-13}$ s s$^{-1}$. The corresponding characteristic parameters of the neutron star, age $τ=P/(2\dot{P})=8.0$ kyr, magnetic field $B=3.4\times 10^{12}$ G, and rotational energy loss $\dot{E}=1.4\times 10^{37}$ erg s$^{-1}$, are typical for young radio pulsars. Since the X-ray radiation has a thermal-like spectrum, its pulsations may be due to a nonuniform temperature distribution over the neutron star surface caused by anisotropy of the heat conduction in the strongly magnetized crust.

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arXiv1998

The ROSAT BRIGHT survey: I. Identification of an AGN sample with hard ROSAT X-ray spectra

The ROSAT Bright Survey (RBS) aims to completely optically identify the more than 2000 brightest sources detected in the ROSAT all-sky survey at galactic latitudes |b| > 30 degr (excluding LMC, SMC, Virgo cluster). This paper presents a subsample of 66 bright point-like ROSAT survey sources with almost hard PSPC spectra, the hardness ratio HR1 is > 0.5 for most of the sources. The subsample could be nearly completely identified by low-resolution optical spectroscopy with the following breakdown into object classes: 31 Seyfert galaxies, 22 BL Lac candidates, 5 clusters of galaxies 1 cataclysmic variable, and 5 bright stars. Only one object remained unidentified and one X-ray source was a spurious detect ion. The redshift distribution peaks around 0.06 for the Seyferts and around 0.13 for the BL Lac candidates. Observations with medium spectral resolution were obtained for most of the new S eyfert galaxies. A large fraction (20 objects) are type 1 Seyfert galaxies, the other fraction includes Seyfert galaxies of type 1.5 - 1.8 (5 objects), two LINERs, and 4 possible narrow-line Seyfert 1 galaxies (NLS1). About one third of the new Seyfert's have nearby companion galaxies displaying either emission or absorption lines at the same redshift. Among them are a couple of systems showing direct morphological evidence for interaction. The large fraction of interacting galaxies among our sample suggests a scenario where interaction is the main trigger of AGN activity.

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arXiv1998

The ROSAT Deep Survey IV. A distant lensing cluster of galaxies with a bright arc

An unusual double-lobed extended X-ray source (RX J105343+5735) is detected in the ROSAT ultra-deep HRI image of the Lockman Hole. The angular size of the source is 1.7 X 0.7 arcmin^2 and its X-ray flux is 2 X 10^-14 erg cm^-2 s^-1. R-band imaging from the Keck telescope revealed a marginal excess of galaxies brighter than R=24.5, but Keck LRIS spectroscopy of 24 objects around the X-ray centroid did not yield a significant number of concordant redshifts. The brightest galaxy close to the centre of the eastern emission peak appears to be a gravitationally lensed arc at z=2.570, suggesting that the X-ray object is associated with the lens, most likely a cluster of galaxies. Based on a comparison of lensing surface mass density, X-ray luminosity, morphology and galaxy magnitudes with clusters of known distance, we argue that RX J105343+5735 is a cluster at a redshift around 1. Future X-ray, ground-based optical/NIR and high resolution HST observations of the system will be able to clarify the nature of the object.

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arXiv1997

The Neutron Star in the Supernova Remnant PKS 1209-52

We re-analyzed soft X-ray data collected with the ROSAT and ASCA observatories on a candidate neutron star (NS) near the center of the supernova remnant PKS 1209-52. We fitted the observed spectra with NS atmosphere models. The hydrogen atmosphere fits yield more realistic parameters of the NS and the intervening hydrogen column than the traditional blackbody fit. In particular, for a NS of mass 1.4 M_\odot and radius 10 km, we obtained the NS surface temperature T_{eff}=(1.4-1.9)*10^6 K and distance d=1.6-3.3 kpc versus T=(4.2-4.6)*10^6 K and (implausible) d=11-13 kpc for the blackbody fit, at a 90% confidence level. Our fits suggest that the surface magnetic field is either very weak, B < 10^{10} G, or it exceeds 2*10^{12} G. The hydrogen column density inferred from the atmosphere fits, n_H=(0.7-2.2)*10^{21} cm^{-2}, agrees fairly well with independent estimates obtained from UV observations of nearby stars, radio data, and X-ray spectrum of the shell of the supernova remnant, whereas the blackbody and power-law fits give considerably lower and greater values, n_H=(0.2-0.4)*10^{21} and (5.2-7.0)*10^{21} cm^{-2}, respectively. The inferred NS surface temperature is consistent with standard NS cooling models.

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arXiv1997

RX J2115.7-5840: a short-period, asynchronous polar

We report phase-resolved optical polarimetric, photometric and spectroscopic observations of RX J2115.7-5840 (= EUVE J2115-58.6, Craig 1996) which confirms the system to be a magnetic cataclysmic binary of the polar (AM Herculis) subclass. The optical light curve is sometimes flat and occasionally displays a pronounced bright phase, reminiscent of the self-eclipse of a small accretion spot by the revolving white dwarf, as seen in self-eclipsing polars. Our period search reveals ambiguous results only which can be interpret assuming that the white dwarf is not synchronously rotating with the binary orbit. We find circularly polarized cyclotron radiation with V/I ranging from 0% to -15% on one occasion, from -8% to +15% on another occasion. Compared with other polars, the self-occulted accretion region of RXJ2115 had an extreme red cyclotron spectrum. In addition, the system has an extreme hard X-ray colour during the ROSAT all-sky survey observation. Both properties suggest a low value of the magnetic field strength, and our best estimate gives B = 11+-2 MG. Due to the absence of significant M-star features in our low-resolution spectra we estimate the minimum distance to RXJ2115 to be d > 250pc (for an M5+ secondary star).

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arXiv1996

Search for correlations between COBE DMR and ROSAT PSPC All-Sky survey data

Results from a cross-correlation analysis between the COBE DMR 4 year, and ROSAT PSPC All-Sky Survey data are presented. Statistical comparisons between microwave and X-ray maps can probe interesting astrophysical environments and processes, such as e.g. the Sunyaev-Zel'dovich effect in clusters of galaxies or the Integrated Sachs-Wolfe effect. In order to test the diffuse, extragalactic X-ray background as probed by ROSAT, against the COBE DMR large-scale CMB structure, our analysis was performed in most detail in a ROSAT selected region of the sky (40 deg < b, 70 < l < 250 deg) in an X-ray energy range with minimal Galactic structure and residual X-ray contamination. Comparing to other regions of the sky and neighbouring energies and frequencies, we find indication for a positive Galactic correlation on large scales. Removing a quadrupole term on a sky map with a Galactic cutout or related gradients in the selected regions leaves no correlations above a 1-sigma level on smaller angular scales. We conclude that there is no significant extragalactic correlation on scales for which the combined data are sensitive (7 deg - 40 deg) and that Galactic correlation is significant only on angular scales of the order of the quadrupole. In the context of removing large angular scale gradients we give results on best fit X-ray dipoles from various ROSAT data and discuss these with respect to the difficulty of finding a cosmological dipole. The lowest correlation upper limits we can place are about 15 % of COBE CMB fluctuations and about 10 % of the ROSAT extragalactic XRB fluctuations. We discuss these results with respect to possible correlation mechanisms.

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arXiv1996

Multiwavelength observations of isolated neutron stars as a tool to probe the properties of their surfaces

We show that an analysis of multiwavelength observations of isolated neutron stars based on neutron star atmosphere models can be used not only to evaluate the neutron star effective temperature, but also to determine chemical composition of its surface. To demonstrate how this new method can be applied to a specific object, we chose the old isolated neutron star candidate RXJ1856.5-3754, whose soft X-ray radiation has been studied recently by Walter et al. (1996). We fitted the soft X-ray spectrum of this object with neutron star atmosphere models of different chemical compositions and used these fits to calculate the source spectrum over a broad wavelength range. We showed, in particular, that the optical/UV flux expected from this object depends drastically on the composition of its surface. In particular, the neutron star covered with hydrogen would be 5-6 magnitudes brighter than the neutron star with an iron surface. The object should also be observable with $EUVE$; the EUV flux is expected to be almost twice higher for the iron surface than for the hydrogen one. Thus, multiwavelength observations of this object would enable one to examine, for the first time, chemical composition of the neutron star surface. The method proposed can be applied to other nearby isolated neutron stars.

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arXiv1995

ROSAT observations of the radio and gamma-ray pulsar PSR 1706-44

We report on the detection of PSR 1706-44 in two ROSAT-PSPC observations. The recorded source counts are unpulsed with a $2σ$ pulsed fraction upper limit of 18%. Spectral analysis did not distinguish between black-body and power law models; however, we argue that the lack of pulsations and the similarity in the pulsar's spin parameters to those of the Vela pulsar favour a power law model $dN/dE\propto E^{-2.4\pm 0.6}$ and indicate synchrotron emission from a pulsar-powered nebula as the origin of the detected X-radiation. The X-ray flux derived for the power law model is f_x=3.2^{+6.5}_{-1.8} x E-12 erg/ s/ cm^2 within the 0.1-2.4 keV energy range. An upper limit for the neutron star's surface temperature is put at Log T_s^\infty \sim 6.03 K for a neutron star with a medium stiff equation of state (FP-model with M=1.4 Mo, R=10.85 km). Slightly different values for $T_s^\infty$ are computed for the various neutron star models available in the literature, reflecting the difference in the equation of state. No soft X-ray emission is detected from the supernova remnant G343.1-2.3, proposed to be associated with PSR 1706-44.

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