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INTEGRAL observation of MAXI J0911-655

ATel #8986; E. Bozzo, C. Ferrigno (ISDC, Switzerland), A. Papitto (INAF-OAR, Italy), A. Sanna, L. Burderi (Univ. of Cagliari, Italy), N. Rea (Univ. of Amsterdam, Netherlands; CSIC-IEEC, Spain), T. di Salvo (Univ. of Palermo, Italy)
on 26 Apr 2016; 05:59 UT
Credential Certification: E. Bozzo (enrico.bozzo@unige.ch)

Subjects: X-ray, Binary, Transient

Referred to by ATel #: 9738, 10425

The recently discovered transient MAXI J0911-655 (Atels #8872, #8884, #8914, #8971) was observed during a dedicated INTEGRAL campaign performed from 2016 April 23 at 23:43 to April 25 at 06:51 UTC.

The source was detected by IBIS/ISGRI at a significance of 16 sigma in the 20-40 keV energy range and 11 sigma in the 40-80 keV energy range (effective exposure time 46 ks). The estimated fluxes from the mosaics were of 9.3+/-0.6 mCrab (i.e. ~7.0E-11 erg/cm^2/s) and 10.7+/-0.9 mCrab (i.e. ~7.4E-11 erg/cm^2/s), respectively.

The source was also detected by the two JEM-X instruments at a significance of 11 sigma in the 3-10 keV energy range and about 4 sigma in the 10-20 keV energy range (effective exposure time 58 ks). The corresponding fluxes estimated from the JEM-X mosaics were 6.8+/-0.6 mCrab (i.e. ~9.6E-11 erg/cm^2/s) and 5.5+/-1.4 mCrab (i.e. ~5.1E-11 erg/cm^2/s), respectively. These values might be significantly different once the JEM-X off-line energy calibration will be carried out.

By using the INTEGRAL near real time data, we performed a preliminarily broad-band spectral fits of the ISGRI and JEM-X data together. The spectrum of MAXI J0911-655 could be well fit (reduced chi^2/d.o.f. = 0.7/36) by using a simple absorbed power-law with photon index of 2.4+/-0.2 (we fixed the absorption column density to 4E21 cm^(-2), see Atel #8884). The flux measured from the spectral fit is 3.0E-10 erg/cm^2/s in the 3-100 keV energy range.

A more detailed spectral analysis will be performed on the consolidated INTEGRAL data including refined calibrations especially for the JEM-X units.

No thermonuclear bursts have been detected in the JEM-X lightcurve, and thus the nature of the accreting compact object in MAXI J0911-655 remains debated.