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Changes in the timing properties of MAXI J0556-332

ATel #3112; T. Belloni, S. Motta, T. Munoz-Darias, H. Stiele (INAF - Brera Observatory, Italy)
on 18 Jan 2011; 05:24 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Tomaso Belloni (belloni@merate.mi.astro.it)

Subjects: X-ray, Binary, Transient

Referred to by ATel #: 3116, 3119, 3327, 3349, 3650, 4524

A new longer RXTE observation of MAXI J0556-332 (ATel #3102, #3103, #3104, #3106, #3110) was performed on 2011 January 17 from 11:57UT to 19:31UT, for a total of 5 RXTE orbits. The flux does not show eclipse-like drops like in the previous two observations. Folding at the maximum period of 28 hrs allowed by aligning the previous two eclipse-like events shows that the new data are consistent with the absence of an eclipse. All sub-multiples of this period can be excluded, with the exception of P=9.33 hrs, although in that case the data would cover precisely the eclipse-free phase range. The energy spectrum is similar to that observed in the other two observations. However, the hardness ratio (6-10 keV / 3-6 keV) shows an increase from 0.21-0.23 to 0.30. A Power Density Spectrum shows a clear quasi-periodic oscillation at an average frequency of 1.2 Hz. Significant variability of the QPO frequency is observed throughout the observation, resulting in a broad peak. A broad-band noise component is also present, with a characteristic frequency of ~0.4 Hz. The total 0.1-10 Hz fractional rms variability (3-30 keV) is 24%, that of the QPO peak is 14%. The observed properties do not yet allow to classify the source. The hardness values and their changes, together with the variations in power spectral density, are consistent with a black-hole binary in transition from the soft to the hard-intermediate state. However, the energy spectrum is similar to those of neutron-star sources. More observations at all bands are urged.