arXiv subjects

W. Pietsch

Publications and source records attributed to W. Pietsch.

125 records · Page 7Linked to original sources

arXiv2000

Probing the gas content of the dwarf galaxy NGC 3109 with background X-ray sources

We established the catalog of X-ray point sources in the field of the Magellanic-type spiral galaxy NGC 3109 (DDO 236) from two ROSAT PSPC observations. Of the 91 X-ray sources 26 are contained within the HI extent of NGC 3109 as derived by Jobin & Carignan (1990) with the VLA. For 10 of these we can determine accurate hardness ratios delta HR2<=0.2. We find 3 candidate AGN, 2 candidate X-ray binaries and one source which may belong to either class. We also find 2 candidate foreground stars. In a field of 8'x8' observed with the NTT in the I-band and centered on the nucleus of NGC 3109 we determine candidate optical counterparts in the X-ray error circle of 7 ROSAT PSPC sources. We apply a spectral fit to the ROSAT spectrum of the X-ray brightest absorbed candidate AGN behind NGC 3109, RX J1003.2-2607. Assuming a galactic foreground hydrogen column density of 4.3 10^20 cm^-2 we derive from the X-ray spectral fit, assuming reduced metallicities (~0.2 solar), a hydrogen column density due to NGC 3109 of 11(+7,-5) 10^20 cm^-2. This value is slightly larger than the hydrogen column density derived from the 21-cm observations of ~8 10^20 cm^-2. We estimate that the molecular mass fraction of the gas is not larger than ~60%.

astro-ph↗

arXiv2000

X-ray observations of the starburst galaxy NGC 253:II. Extended emission from hot gas in the nuclear area, disk, and halo

Spatial and spectral analysis of deep ROSAT HRI and PSPC observations of the near edge-on starburst galaxy NGC 253 reveal diffuse soft X-ray emission, which contributes 80% to its total X-ray luminosity (L$_{\rm X} = 5 10^{39}$ \ergsec, corrected for foreground absorption). The nuclear area, disk, and halo contribution to the luminosity is about equal. The starburst nucleus itself is highly absorbed and not visible in the ROSAT band. We describe in detail spectra and morphology of the emission from the nuclear area, disk and halo and compare our results to observations at other wavelengths and from other galaxies. (abridged)

astro-ph↗

arXiv2000

New NLS1 galaxies detected among RASS galaxies

Six Narrow-Line Seyfert 1 galaxies have been discovered in optical follow-up spectroscopy of nuclei of galaxies selected for their high X-ray flux in the ROSAT All Sky Survey. We give X-ray and optical parameters of the sources and show optical spectra.

astro-ph↗

arXiv1999

RX J0420.0-5022: An isolated neutron star candidate with evidence for 22.7 s X-ray pulsations

We report the discovery of a new isolated neutron star candidate, RX J0420.0-5022, showing evidence (4 sigma) for 22.7 s X-ray pulsations in ROSAT data. NTT observations of the field around the soft X-ray source do not reveal any likely optical counterpart brighter than B = 25.25 implying an X-ray to optical flux ratio of >10^3.3 and ruling out other possible kinds of X-ray emitters. The X-ray spectrum of RX J0420.0-5022 can be described with blackbody emission with temperature kT of about 57 eV and four ROSAT detections are consistent with no flux variations on time scales of years. The X-ray pulsations, if confirmed, make RX J0420.0-5022 the second long-period isolated neutron star candidate after RX J0720.4-3125. As for this latter source similar conclusions about the magnetic field strength of the neutron star can be drawn depending on evolutionary scenarios.

astro-ph↗

arXiv1999

Peculiar properties of 2 unusually X-ray bright early type galaxies

Two X-ray bright early type galaxies, selected for their unusually high X-ray to optical flux ratio, have been observed at high spatial resolution with the ROSAT HRI. Both sources are clearly extended (more than 250 kpc in radius), thus excluding a nuclear origin of the high X-ray emission. A small group of galaxies observed around IC 1262 could be the most natural explanation for the high X-ray luminosity observed. NGC6159 could be similarly explained, although there are to date no spectroscopic confirmations of a group of galaxies associated with it. The X-ray properties of these two sources are discussed in the context of the properties of groups of galaxies. A peculiar very bright feature, in the shape of an arc, is detected at the center of the IC 1262 image. This could be the signature of a recent merger, or of peculiar high velocities in the group.

astro-ph↗

arXiv1998

A ROSAT PSPC X-Ray Survey of the Small Magellanic Cloud

We present the results of a systematic search for point-like and moderately extended soft (0.1-2.4 keV) X-ray sources in a raster of nine pointings covering a field of 8.95 deg^2 and performed with the ROSAT PSPC between October 1991 and October 1993 in the direction of the Small Magellanic Cloud. We detect 248 objects which we include in the first version of our SMC catalogue of soft X-ray sources. We set up seven source classes defined by selections in the count rate, hardness ratio and source extent. We find five high luminosity super-soft sources (1E 0035.4-7230, 1E 0056.8-7146, RX J0048.4-7332, RX J0058.6-7146 and RX J0103-7254), one low-luminosity super-soft source RX J0059.6-7138 correlating with the planetary nebula L357, 51 candidate hard X-ray binaries including eight bright hard X-ray binary candidates, 19 supernova remnants, 19 candidate foreground stars and 53 candidate background active galactic nuclei (and quasars). We give a likely classification for ~60% of the catalogued sources. The total count rate of the detected point-like and moderately extended sources in our catalogue is 6.9+/-0.3 s^-1, comparable to the background subtracted total rate from the integrated field of ~6.1+/-0.1 s^-1.

astro-ph↗

arXiv1998

X-ray observations of the starburst galaxy NGC 253: I. Point sources in the bulge, disk and halo

We report the results of a deep spatial, spectral, and timing analysis of ROSAT HRI and PSPC observations of the edge-on starburst galaxy NGC 253. In this first paper, point-like X-ray sources detected within the galaxy and in the field are discussed. In total, 73 X-ray sources have been collected in the NGC 253 field, 32 of which are associated with the disk of the galaxy. Though 27 of these disk sources are detected with the HRI (some being resolvable with the PSPC), the remaining 5 PSPC-only detected sources are likely not to be real point sources, being instead due to fluctuations within the X-ray structure of the disk. The source close to the center of the galaxy is extended (L_x = 1e39 erg/s in the ROSAT 0.1-2.4 keV band), and is most likely associated with the nuclear starburst activity. The remaining sources have luminosities ranging from 7e36 erg/s to 3e38 erg/s, yielding an integrated point source luminosity of 1e39 erg/s. The halo of NGC 253 is filled with diffuse, filamentary X-ray emission. Seven sources are located (or projected) in this diffuse emission region. Time variability arguments, together with optical identifications, are put forward to explain 4 sources as background objects, the other 3 sources likely being spurious detections caused by local enhancements in the diffuse emission of the halo of NGC 253. The diffuse X-ray emission components of NGC 253 will be discussed in a separate paper.

astro-ph↗

arXiv1998

NGC 3079: X-ray emission from the nuclear super-bubble and halo

We report the results of the spatial and spectral analysis of the ROSAT HRI and PSPC observations of the edge-on spiral galaxy NGC 3079. We detected several sources in the field of NGC 3079 with both PSPC and HRI, and complex emission from the inner 5' around NGC 3079. We have identified possible counterparts for several of the sources outside NGC 3079 by comparison with optical plates and catalogues. The X-ray emission from NGC 3079 has a Lx = 3E40 erg/sec and can be resolved into the following three components: (i) Extended emission in the innermost region, with Lx = 1E40 erg/sec, coincident with the super-bubble seen in optical images. The active nucleus may contribute to the emission as a point source. (ii) Emission from the disk of the galaxy, with Lx = 7E39 erg/sec, that can be partly resolved by the HRI in 3 point-like sources with luminosities of \~6E38erg/sec each. (iii) Very soft X-shaped emission from the halo, with Lx = 6E39 erg/sec, extending to a diameter of 27 kpc. The X-ray luminosity of NGC 3079 is higher by a factor of 10 compared to other galaxies of similar optical luminosity and we argue that this may be caused by the presence of an AGN rather than by starburst activity. The influence of the AGN on the companion galaxies in the NGC 3079 group are discussed.

astro-ph↗

arXiv1998

Properties of new X-ray selected AGN

We present the results of a program to identify so far unknown active nuclei (AGN) in galaxies. Candidate galactic nuclei have been selected for optical spectroscopy from a cross-correlation of the ROSAT all sky survey (RASS) bright source catalog with optical galaxy catalogs. In this paper we present results from the second optical observing run.

astro-ph↗

arXiv1998

ROSAT observations of the Sculptor galaxy NGC7793

We report here the results of spectral imaging observations with the ROSAT PSPC of the field surrounding the very nearby (D=3.38 Mpc) Sculptor galaxy, NGC7793. Many point sources are detected within the field, several of them showing evidence for variability. Seven sources are seen to lie within the optical confines of the system, the brightest (Lx ~ 9x10e38 erg/s), lying to the southern edge of NGC7793. This source, also observed to be variable, is seen now, not to be associated with a z=0.071 redshift QSO, as was previously thought. A number of the other sources within the NGC7793 disc are likely to be due to X-ray binaries, supernova remnants or superbubbles within the galaxy itself. Other sources may be associated with background AGN. In addition to the point source emission, rather uniform unresolved emission is detected in and around NGC7793 extending to a radius of perhaps 4 kpc. This emission is likely to be contaminated to some degree by unresolved point sources, as its temperature (kT~1 keV) is rather hotter than is seen for the diffuse gas components in other nearby spiral galaxies. Comparing the X-ray properties of NGC7793 with those of the remaining Sculptor group members, suggests that it may, in terms of activity, lie somewhere between its quiescent and starburst neighbours.

astro-ph↗

arXiv1998

ROSAT observations of the IC4329A galaxy group

We report here the results of high-resolution and spectral imaging X-ray observations, with both the ROSAT HRI and PSPC, of the field surrounding the nearby Seyfert galaxy IC4329A. Many point sources are detected, the brightest being associated with IC4329A itself, having an extremely bright Lx of 6x10e43erg/s, and spectral properties compatible with a single power-law model, with a spectral break at 0.7keV. Two other bright sources are detected associated with the companion galaxy IC4329, and a likely quasar 14' to the south-west. We have also established, through optical observations, that three further X-ray point sources, intriguingly positioned with respect to IC4329A, are in fact nothing to do with the system. In addition to point source emission, residual, unresolved emission is detected surrounding the IC4329A/IC4329 pair, extending for some 200kpc. This emission appears markedly two-component, comprising of a spectrally hard and smooth component, circularly-distributed about the central galaxy pair, and a spectrally soft, more clumpy component, positioned almost entirely to the south-east of IC4329A. The hard component of the residual emission itself appears two-component, one component being due to the `wings' of the intensely bright IC4329A source, the other, apparently due to hot (~1.5keV) gas, likely associated with the galaxy group of which IC4329A and IC4329 are members. The soft component of the residual emission may be a larger version of the superwinds seen around some ultraluminous far-infrared galaxies, or may even represent a `stripped wake' of intragroup gas. Evidence for shocked gas due to the central IC4329A/IC4329 interaction is also found between the two central galaxies.

astro-ph↗

arXiv1998

New active galactic nuclei detected in ROSAT All Sky Survey galaxies; Part I: Verification of selection strategy

We present the first results of a program to identify so far unknown active nuclei (AGN) in galaxies. Candidate galactic nuclei have been selected for optical spectroscopy from a cross-correlation of the ROSAT All Sky Survey (RASS) bright source catalog with optical galaxy catalogs. A high X-ray flux has been used as pointer to galaxies with a high probability to contain active nuclei. Only galaxies have been accepted for the program for which no activity was noted in NED. For many of the galaxies no radial velocity was reported before. The optical spectra demonstrate that the galaxies cover a redshift range of 0.014 to 0.13 and that most of them host active nuclei. For 75 percent of the 33 candidates the X-ray emission is caused by the AGN. In addition several of the remaining candidates host Seyfert 2/LINER nuclei that, however, most certainly cannot explain the X-ray emission alone. Three BL Lac objects have been detected serendipitously in galaxy fields that have been followed up by short ROSAT HRI observations to confirm the X-ray galaxy identification with improved position accuracy and point response function. The sources show X-ray to radio flux ratios typical for X-ray selected BL Lac objects. The results presented in the paper prove the selection strategy as very successful to detect previously unknown AGN of all Seyfert 1 types in nearby galaxies encouraging the extension of this program. The detection of new nearby AGN will be used to initiate a detailed investigation of their multi-wavelength properties and a comparison with the more distant AGN population.

astro-ph↗

arXiv1997

Identification of selected sources from the ROSAT Galactic Plane Survey - I

We report on optical searches in the error circles of 93 ROSAT survey sources located at low galactic latitudes (|b| < 20 deg). These sources were extracted from the ROSAT Galactic Plane Survey using various selection criteria on hardness ratio, X-ray and optical brightness and integrated galactic absorption in the direction of the source. We find optical identifications in 76 cases, among which are 25 new AGN, 6 new CVs and a new Be/X-ray binary. In order to illustrate the relevance of the source selections applied here, we cross-correlated the ROSAT all-sky survey bright source list with SIMBAD. Different classes of X-ray emitters populate distinct regions of a multi dimensional parameter space involving flux ratios, galactic latitude and Nh. This relatively good segregation offers the possibility to build source samples with enhanced probability of identification with a given class. Complete optical identification of such subsamples could eventually be used to compute meaningful probabilities of identification for all sources using as basis a restricted set of multi-wavelength information.

astro-ph↗

arXiv1997

Properties of the X-ray pulsar GX 301-2 in hard X-rays

In 1993-1994 a series of observations of the X-ray pulsar GX 301-2 by HEXE onboard Mir-Kvant was made. A period of pulsations was measured (it varied between 675 and 678 s) and pulse profiles in different energy bands were produced. The measured luminosity in the 20-100 keV energy range changed substantially between 8x10^34 and 7x10^35 d^2 erg/s (d is the distance to the source in kpc). The obtained spectrum is quite satisfactory described by the canonical model for X-ray pulsars with gamma=1.3, E_c~23 keV, E_f~9 keV. It changed weakly between the observations, but was softest at brightness maximum. Significant variations of the spectral hardness over the pulse phase were detected, but the accumulated data are insufficient to quantify variations in spectral parameters. No significant traces of cyclotron lines were found. An interpretation of the pulse profiles as superposition of emissions from two flat polar caps (with inclusion of gravitational lensing) leads to an estimate of the angle between the magnetic axis and axis of rotation of 40-70 deg and an angle between the direction to the observer and the rotation axis of 75-85 deg.

astro-ph↗

arXiv1997

ROSAT PSPC X-ray observations of the nearby spiral galaxy M83

The nearly face-on SBc galaxy M83 (NGC 5236) was observed for 25 ksec with the ROSAT PSPC. We detected 13 point-like sources in this galaxy, 10 of which were previously unknown. We measured extended X-ray radiation from almost the whole optically visible galaxy. Comparing the diffuse soft and hard X-ray emission components, we observed a different asymmetric distribution and a slower radial decrease of the intensity profile of the soft X-ray emission. Both these results support the existence of a huge spherical gas halo of 10-15 kpc radius. On the other hand, the radial scale lengths of the hard X-ray radiation, that of the thermal radio emission and the profile of the optical surface brightness are similar, favouring the idea that all these emission processes are connected to star formation in the galaxy's disk. M83 is the first face-on galaxy where the diffuse X-ray emission spectrum can be characterized by a two-temperature thermal plasma: a soft X-ray emitting warm `halo component' and an internally absorbed hot `disk component' which is dominating the emission in the hard (0.5-2.0 keV) ROSAT energy range. The combination of X-ray and radio polarization observations allows an estimate of the plasma parameter beta = U_therm/U_magn which is found to be 0.2+/-0.1. This result supports the hypothesis that magnetic fields play an important role for the evolution and structure of galactic gas haloes. The high energy input rate in the active star-forming disk of M83 seems to be responsible for the outflow of hot gas and the halo formation.

astro-ph↗

arXiv1997

X-Ray Binary Systems in the Small Magellanic Cloud

We present the result of a systematic search for spectrally hard and soft X-ray binary systems in the Small Magellanic Cloud (SMC). This search has been applied to ROSAT PSPC data (0.1-2.4 keV) collected during 9 pointed observations towards this galaxy covering a time span of 2 years from October 91 till October 93. Selection criteria have been defined in order to confine the sample of candidates. Finally 7 spectrally hard and 4 spectrally soft sources were selected from the list as candidates for binaries in the SMC. The sample is luminosity limited (>3.10**35 erg/s). SMC X-1 has been observed during a full binary orbit starting with a low-state covering an X-ray eclipse and emerging into a bright long-duration flare with two short-duration flares separated by 10 hours. The Be type transient SMC X-2 has been redetected with ROSAT. Variability has been found in the sources RX J0051.8-7231 and RX J0052.1-731 already discovered with Einstein. RX J0101.0-7206 has been discovered at the north-eastern boundary of the giant SMC HII region N66 during an X-ray outburst and half a year later during a quiescent phase. A variable source, RX J0049.1-7250, located north-east of the SMC supernova remnant N19 and which may either be an X-ray binary or an AGN turns out to be strongly absorbed. It may be located behind the SMC. If it is an X-ray binary then it radiates at the Eddington limit in the X-ray bright state. Another variable and hard X-ray source RX J0032.9-7348 has been discovered at the south-eastern border of the body of the SMC. A high mass X-ray binary nature is favored for this source. We searched for CAL87 like systems in the SMC catalog and found none. A new candidate supersoft source RX J0103.8-7254 has been detected. We cannot exclude that it is a foreground object.

astro-ph↗

arXiv1996

The ROSAT Galactic Plane Survey: analysis of a low latitude sample area in Cygnus

We present the analysis of the point source content of a low galactic latitude region selected from the ROSAT all-sky survey. The test field is centered at l = 90deg, b = 0deg and has an area of 64.5 deg2. A total of 128 soft X-ray sources are detected above a maximum likelihood of 8. Catalogue searches and optical follow-up observations show that in this direction of the galactic plane, 85% of the sources brighter than 0.03 PSPC cts/s are identified with active coronae. F-K type stars represent 67% (+/-13%) of the stellar identifications and M type stars account for 19% (+/- 6%). These results together with those of similar optical campaigns demonstrate that the soft X-ray population of the Milky Way is largely dominated by active stars. The modelling of this population suggests that most of the stars detected by ROSAT in this direction are younger than 1 Gyr. The small number of unidentified sources at low X-ray flux put rather strong constraints on the hypothetical X-ray emission from old neutron stars accreting from the interstellar medium. Our observations clearly rule out models which assume no dynamical heating for this population and a total number of Nns = 10^9 neutron stars in the Galaxy. If accretion on polar caps is the dominant mode then our upper limit may imply Nns ~ 10^8.

astro-ph↗