P. Kahabka

A ROSAT PSPC catalogue of X-ray sources in the SMC region ?

We present a catalogue of 517 discrete X-ray sources in a 66 eld covering the Small Magellanic Cloud (SMC). The catalogue was derived from the pointed ROSAT PSPC observations performed between October 1991 and May 1994 and is complementary to the Large Magellanic Cloud (LMC) catalogue published by Haberl & Pietsch (1999). We followed the same identication scheme and used, among other information, X-ray hardness ratios and spatial extent to classify unknown sources as candi- dates for active galactic nuclei (AGN), foreground stars, supernova remnants (SNRs), supersoft sources (SSSs) and X-ray binaries. For 158 sources a likely source type is given, from which 46 sources are suggested as background AGN (including candidates resulting from a compari- son of X-ray and radio images). Nearly all of the X-ray binaries known in the SMC were detected in ROSAT PSPC observations; most of them with luminosities below 10 36 erg s 1 suggesting that the fraction of high luminos- ity X-ray binary systems in the Magellanic Clouds (MCs) is not signicantly larger than in our galaxy. Seventeen X-ray sources are associated with SNRs found in earlier work and we suggest here two additional extended sources as SNR candidates. Three very soft sources are newly clas- sied as SSSs from which one is identied with the sym- biotic star LIN 358 in the SMC.

A ROSAT PSPC X-ray survey of the Small Magellanic Cloud

Wepresenttheresultsofasystematicsearchfor point-like and moderately extended soft (0:1 2: 4k eV) X-ray sources in a raster of nine pointings covering a eld of 8:95 deg 2 and performed with the ROSAT PSPC between October 1991 and October 1993 in the direc- tion of the Small Magellanic Cloud (SMC). We detect 248 objects which we include in the rst version of our SMC catalogue of soft X-ray sources. We set up seven source classes dened by selections in the count rate, hardness ratio and source extent. We nd ve 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 (SNRs), 19 candidate foreground stars and 53 candidatebackgroundactivegalacticnuclei(andquasars). We give a likely classicationfor60% of the catalogued sources. The total count rate of the detected point-like andmoderatelyextendedsourcesinourcatalogueis6.9 0.3 s 1 , comparable to the background subtracted total rate from the integratedeld of6:10:1s 1 .

Exosat Observations of the 35-Day Cycle of Her X-1; Evidence for Neutron Star Precession

We have observed Her X-1 with the EXOSAT satellite throughout a 35-day cycle at about four day sampling intervals. These measurements include for the first time, an accurate measurement of the 1.24 sec pulse profiles seen during the short-on state at the middle of the 35-day cycle. The data show a large change in the pulse profiles between the main and short-on states, suggesting a precession of the magnetic dipole axis of the rotating neutron star. We propose that the clock mechanism resides in the free precession of the neutron star and examine further consequences of this precession with respect to the inner structure of the accretion disk.

The super-soft source XMMU J052016.0-692505 in the LMC: a likely white dwarf Be/X-ray binary

Aims: We report the discovery of the super-soft X-ray source XMMU J052016.0-692505 in the LMC with XMM-Newton. Methods: We analyse the EPIC spectra of XMMU J052016.0-692505 and study the likely optical counterpart LMCV2135. Results: Using an absorbed blackbody spectrum we derive a bolometric luminosity of ⪆ 1034 erg s-1 for the X-ray source at LMC distance. Assuming that the bolometric luminosity does not exceed the Eddington luminosity of a ~ 1.0 Mȯ star we derive a blackbody temperature of (25-70) eV and an LMC absorbing column density of < 6× 1021 cm-2. The likely optical counterpart of XMMU J052016.0-692505 is the LMC variable star LMCV2135 which is a MACHO and OGLE variable. The infrared and optical colors and magnitudes of this star are consistent with a hot star of likely spectral type B. The long-term MACHO light curve shows variability with a timescale of ˜ 500 and ˜ 1000 days. The optical spectra obtained at the 1.9-meter telescope of the South African Astronomical Observatory show strong Hα and Hβ emission lines (with EWH_α˜ 34 A) which indicate a B0-3e star. The radial velocities of the Hα and Hβ emission lines show a variation from ˜ 400-450 km s-1 to ˜ 5-20 km s-1 which is consistent with the systemic velocity of the LMC and an intrinsic variation most likely due to the rotation of the Be disk. We discuss LMCV2135/XMMU J052016.0-692505 as a Be/white dwarf binary system in the LMC. The super-soft X-ray spectrum of the source could be due to a stable nuclear burning white dwarf with a mass of ˜ 0.9-1.0 Mȯ.

X-Ray Energy Spectra of the Supersoft X-Ray Sources CAL 87 and RX J0925.7–4758 Observed with ASCA

We report observation results of the supersoft X-ray sources CAL 87 and RX J0925.7-4758 with the X-ray CCD cameras (Solid-State Imaging Spectrometers [SISs]) on board ASCA. Because of the superior energy resolution of the SIS (ΔE/E ~ 10% at 1 keV) relative to previous instruments, we could study detailed X-ray spectral structures of these sources for the first time. We have applied theoretical spectral models to CAL 87 and constrained the white dwarf mass and intrinsic luminosity as 0.8-1.2 M☉ and 4 × 1037-1.2 × 1038 ergs s-1, respectively. However, we have found the observed luminosity is an order of magnitude smaller than the theoretical estimate, which indicates that the white dwarf is permanently blocked by the accretion disk, and we are observing a scattering emission by a fully ionized accretion disk corona (ADC) whose column density is ~1.5 × 1023 cm-2. Through simulation we have shown that the orbital eclipse can be explained by the ADC model, such that a part of the extended X-ray emission from the ADC is blocked by the companion star filling its Roche lobe. We have found that very high surface gravity and temperature, ~1010 cm s-2 and ~100 eV, respectively, as well as a strong absorption edge at ~1.02 keV, are required to explain the X-ray energy spectrum of RX J0925.7-4758. These values are only possible for an extremely heavy white dwarf near the Chandrasekhar limit. Although the supersoft source luminosity should be ~1038 ergs s-1 at the Chandrasekhar limit, the observed luminosity of RX J0925.7-4758 is nearly 2 orders of magnitude smaller, even assuming an extreme distance of ~10 kpc. To explain the luminosity discrepancy, we propose a model in which very thick matter that was previously ejected from the system, as a form of jets, intervenes the line of sight and reduces the luminosity significantly because of Thomson scattering.

Supersoft sources in XMM-Newton small magellanic cloud fields - a symbiotic and a close binary or cooling neutron star

We report the detection and study of two faint ROSAT supersoft X-ray sources in the SMC field withXMM-Newton, RX J0059.1−7505 and RX J0059.4−7118. Due to the larger eff ective area ofXMM-Newton we can constrain the X-ray spectra of both sources. RX J0059.1−7505 is optically identified with the symbiotic LIN 358 in the SMC. A ∼20 eV blackbody component dominates the observed spectrum. The soft blackbody component is consistent with steady nuclear burning in a shell although the spectrum is more complex than a simple blackbody continuum. RX J0059.4−7118 is not optically identified and we derive with the Optical Monitor (OM )a V magnitude >19.3 assuming an M0 spectral type. The X-ray spectrum is fitted with a blackbody component with a temperature of ∼90 eV and an additional spectrally hard component which can be reproduced with a powerlaw. The luminosity of RX J0059.4−7118 would be ∼4 × 10 34 erg s −1 at the distance of the SMC. This is too large for a Cataclysmic Variable (CV). The spectral appearance is not in agreement with a supersoft source in the SMC. Thus we suggest that RX J0059.4−7118 is a Galactic source. As the optical magnitude derived from the OM data may be too faint for a normal Galactic CV we examined the possibility that RX J0059.4−7118 is a polar CV in the Galaxy, an isolated cooling neutron star (INS) at distance ∼(1−2) kpc, a pulsar with a brown dwarf companion, or a Galactic quiescent low-mass X-ray binary (qLMXB). We favor the hypothesis of a Galactic CV because of variability in the EPIC-pn data with a timescale of ∼1 h. A third supersoft ROSAT source, RX J0050.5−7455, is not detected with XMM-Newton.

Faint super-soft X-ray sources in XMM-Newton large magellanic cloud fields

Aims. We report the discovery of three faint, super-soft X-ray sources in LMC fields observed with XMM-Newton. Methods. We analyse the three new sources together with RX J0537.7-7034 and RX J0507.1-6743, both known since ROSAT. Results. We identify XMMU J050803.1-684017 with the LMC planetary nebula LHA 120-N 102 = LMC SMP 29. The EPIC-pn spectrum of XMMU J050803.1-684017 can be fitted with a blackbody spectrum with an effective temperature of ~(26-51) eV and a bolometric luminosity of (0.1-30)×10³⁶ erg s⁻¹, depending on the assumed absorption. The source is consistent with the nucleus of a planetary nebula. The EPIC-pn spectrum of XMMU J052530.5-671501 is characterized by a blackbody temperature of ~(38-120) eV, a best-fit bolometric luminosity of ~3×10³⁷ erg s⁻¹ and absorbing LMC column of ~8⁺²(-8) 10²¹ cm⁻². This most likely highly-absorbed super-soft source resembles the LMC super-soft source CAL 87. The EPIC-pn spectrum of XMMU J052215.0-701658 yields a blackbody temperature of ~(24-83) eV, but there are only about 40 source counts and the spectral parameters of the source are not well constrained. We observe RX J0537.7-7034 with EPIC-pn to be about a factor of 15 fainter compared to the ROSAT observation 11 years earlier. RX J0507.1-6743 is observed as an absorbed super-soft source by XMM-Newton and the improved X-ray position coincides with a MACHO star which is consistent with a symbiotic star.

ROSAT survey view of the SMC

We analysed a 8 × 8 degree field centered at the body of the Small Magellanic Cloud observed during ROSAT all-sky-survey from October to November 1990. We detected more than 40 sources, 15 could be identified with Einstein detections and further 9 with Simbad catalogue entries. We detected a new supersoft source in the SMC cluster NGC 269 which could be identified with the symbiotic star SMC3. In the body of the SMC we found a clear enhancement in flux at energies above 0.5 keV, which is at least partially explained by point sources detected during deep pointed observations.

Discovery of an X-ray binary in the outer SMC wing

The discovery of an X-ray binary RX J0209.6−7427 in the outer wing of the SMC is reported from two archival ROSAT PSPC observations. The data show variability in the X-ray light curve with a timescale of ∼40 days that probably is related to the binary orbital period of the system. A V = 14 mag star in the X-ray error circle of the source is the likely optical counterpart. The optical spectrum of the star shows Hα emission with an equivalent width of −10.8 ± 0.2 A. The spectral type of the star is constrained as B0-B1.5IV-Ve and consistent with that of a Be star. RX J0209.6−7427 is therefore a Be-type X-ray binary. The mean X-ray luminosity of the source is derived to be ∼1.0 × 10 38 erg s −1 (for a distance of 60 kpc), which is comparable to luminosities derived for Be-type X-ray binaries in the body of the SMC. This makes RX J0209.6−7427 the first candidate Be-type X-ray binary discovered in the outer SMC wing that directly extends into the Magellanic Bridge. This finding is consistent with the existence of a young (∼10-30 Myr) stellar population in the outer region of the SMC wing. Star formation due to an encounter of the SMC with the LMC and the Magellanic system with the Galaxy may explain this likely high-mass X-ray binary in an area far from the main body of the SMC.

Hubble Space Telescope Discovery of an Optical Counterpart to the Supersoft X-Ray Source in the Globular Cluster M3*

We report the detection with the Hubble Space Telescope of an optical counterpart to the transient supersoft X-ray source 1E 1339.8+2837 in the globular cluster M3. The counterpart is found near the faint end of the subgiant branch in the V versus V - I color-magnitude diagram but is extremely bright in the U band. Variability is detected over a range of timescales, suggesting the presence of an accretion disk and perhaps also ellipsoidal variations of the subgiant secondary. The optical colors of the binary are similar to those of cataclysmic variables recently discovered in 47 Tuc and NGC 6397. We suggest that magnetically channeled accretion may explain the relatively low X-ray luminosity of this sources supersoft state.