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Register a new userXMM-Newton Observations of the Be/X-ray transient A0538-66 in quiescence
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P. Kretschmar
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We present XMM-Newton observations of the recurrent Be/X-ray transient A0538-66, situated in the Large Magellanic Cloud, in the quiescent state. Despite a very low luminosity state of (5-8)E33 ergs/s in the range 0.3-10 keV, the source is clearly detected up to ~8 keV. and can be fitted using either a power law with photon index alpha=1.9+-0.3 or a bremsstrahlung spectrum with kT=3.9+3.9-1.7 keV. The spectral analysis confirms that the off-state spectrum is hard without requiring any soft component, contrary to the majority of neutron stars observed in quiescence up to now.
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We report the result of an XMM-Newton observation of the black-hole X-ray transient XTE J1650-500 in quiescence. The source was not detected and we set upper limits on the 0.5-10 keV luminosity of 0.9e31-1.0e31 erg/s (for a newly derived distance of 2.6 kpc). These limits are in line with the quiescent luminosities of black-hole X-ray binaries with similar orbital periods (~7-8 hr)
A0538-66 is a Be/X-ray binary (Be/XRB) hosting a 69 ms pulsar. It emitted bright X-ray outbursts with peak luminosity up to $sim 10^{39}$ erg/s during the first years after its discovery in 1977. Since then, it was always seen in quiescence or during outbursts with $L_x lesssim 4 times 10^{37}$ erg/s. In 2018 we carried out XMM-Newton observations of A0538-66 during three consecutive orbits when the pulsar was close to periastron. In the first two observations we discovered a remarkable variability, with flares of typical durations between $sim$2-50 s and peak luminosities up to $sim 4times 10^{38}$ erg/s (0.2-10 keV). Between the flares the luminosity was $sim 2times 10^{35}$ erg/s. The flares were absent in the third observation, during which A0538-66 had a steady luminosity of $2times 10^{34}$ erg/s. In all observations, the X-ray spectra consist of a softer component, well described by an absorbed power law with photon index $Gamma_1approx 2-4$ and $N_Happrox 10^{21}$ cm$^{-2}$, plus a harder power-law component ($Gamma_2approx 0-0.5$) dominating above $sim$2 keV. The softer component shows larger flux variations than the harder one, and a moderate hardening correlated with the luminosity. The fast flaring activity seen in these observations was never observed before in A0538-66, nor, to our best knowledge, in other Be/XRBs. We explore the possibility that during our observations the source was accreting in a regime of nearly spherically symmetric inflow. In this case, an atmosphere can form around the neutron star magnetosphere and the observed variability can be explained by transitions between the accretion and supersonic propeller regimes.
In 2018, XMM-Newton observed the awakening in X-rays of the Be/X-ray binary (Be/XRB) A0538-66. It showed bright and fast flares close to periastron with properties that had never been observed in other Be/XRBs before. We report the results from the observations of A0538-66 collected during the first all-sky survey of eROSITA, an X-ray telescope (0.2-10 keV) on board the Spektrum-Roentgen-Gamma (SRG) satellite. eROSITA caught two flares within one orbital cycle at orbital phases $phi = 0.29$ and $phi = 0.93$ (where $phi=0$ corresponds to periastron), with peak luminosities of $sim 2-4 times 10^{36}$ erg/s (0.2-10 keV) and durations of $42 leq Delta t_{rm fl} leq 5.7times 10^4$ s. The flare observed at $phi approx 0.29$ shows that the neutron star can accrete considerably far from periastron, although it is expected to be outside of the circumstellar disk, thus providing important new information about the plasma environment surrounding the binary system. We also report the results from the photometric monitoring of A0538-66 carried out with the REM, OGLE, and MACHO telescopes from January 1993 until March 2020. We found that the two sharp peaks that characterize the orbital modulation in the optical occur asymmetrically in the orbit, relative to the position of the donor star.
IGR J18483-0311 was discovered with INTEGRAL in 2003 and later classified as a supergiant fast X-ray transient. It was observed in outburst many times, but its quiescent state is still poorly known. Here we present the results of XMM-Newton, Swift, and Chandra observations of IGRJ18483-0311. These data improved the X-ray position of the source, and provided new information on the timing and spectral properties of IGR J18483-0311 in quiescence. We report the detection of pulsations in the quiescent X-ray emission of this source, and give for the first time a measurement of the spin-period derivative of this source. In IGRJ18483-0311 the measured spin-period derivative of -(1.3+-0.3)x10^(-9) s/s likely results from light travel time effects in the binary. We compare the most recent observational results of IGRJ18483-0311 and SAXJ1818.6-1703, the two supergiant fast X-ray transients for which a similar orbital period has been measured.
On 2003 September 17 INTEGRAL discovered a bright transient source 3 degrees from the Galactic Center, IGR J17544-2619. The field containing the transient was observed by XMM-Newton on 2003 March 17 and September 11 and 17. A bright source, at a position consistent with the INTEGRAL location, was detected by the European Photon Imaging Camera (EPIC) during both September observations with mean 0.5-10 keV unabsorbed luminosities of 1.1x10^35 and 5.7x10^35 erg s-1 for an (assumed) distance of 8 kpc. The source was not detected in 2003 March, with a 0.5-10 keV luminosity of < 3.8x10^32 erg s-1. The September 11 and 17 EPIC spectra can be represented by a power-law model with photon indices of 2.25+/-0.15 and 1.42+/-0.17, respectively. Thus, the 0.5-10 keV spectrum hardens with increasing intensity. The low-energy absorption during both September observations is comparable to the interstellar value. The X-ray lightcurves for both September observations show energy dependent flaring which may be modeled by changes in either low-energy absorption or power-law index.
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