The obscured pulsar IGR J18179-1621 entered a bright state

ATel #13625; P. Esposito (IUSS-PV), G. L. Israel (INAF-OAR), N. Rea, A. Borghese, F. Coti Zelati (ICE-CSIC, IEEC), on behalf of a larger collaboration
on 10 Apr 2020; 09:43 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Paolo Esposito (paoloesp@iasf-milano.inaf.it)

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XMM-Newton observed serendipitously the position of IGR J18179-1621 (Bozzo et al. 2012, A&A, 545, A83; Li et al. 2012, MNRAS, 426, L16; Nowak et al. 2012, ApJ, 757,143) on 2020 March 15 and again on 2020 April 4. The XMM-Newton monitoring campaign is aimed at observing the outburst evolution of the new magnetar Swift J1818-1607 (Esposito et al. 2020, astro-ph/2004.04083).

IGR J18179-1621 was discovered in 2012 with INTEGRAL during an outburst that lasted few weeks. A Swift/XRT monitoring led to the discovery of strong pulsations at 11.82 s (Halpern 2012, ATel #3949) and showed a flux decreasing from 2.9e-10 erg/cm^2/s on 2012 February 29 to 8e-12 erg/cm^2/s in about 19 days (Bozzo et al. 2012). The source was suggested to be an accreting high-mass X-ray binary.

In the first XMM-Newton observation (22.1 ks exposure time; 2020 March 15), IGR J18179-1621 fell in an off-axis portion of the EPIC MOS2 CCD camera. Using the SAS tool EUPPER and the exposure map, we set a 3 sigma upper limit on its count rate of 0.0041 counts/s. Using the WebPIMMS and adopting the spectrum below, we find an upper limit on the observed flux of approximately 3e-13 erg/cm^2/s in the 2-10 keV band.

In the second observation (27.7 ks exposure time; 2020 April 4), the source was clearly detected in the EPIC pn and MOS2 cameras. Its spectrum can be modelled as a hard power law modified for the interstellar absorption with photon index -0.6 +/- 0.2 and absorption (20 +/- 3)e22 cm^-2. The observed 2-10 keV flux was (2.1 +/- 0.1)e-11 erg/cm^2/s. A strong periodic flux modulation is detected at 11.8268(1) s. The high time resolution XMM-Newton data show a double peak profile with two slightly asymmetric peaks. All uncertainties are at 1 sigma confidence level.

The brightening of a factor larger than 50 in about 20 days may indicate either that the source is entering a new outburst, or that it is on the decay path from an outburst that peaked after 2020 March 15 and has a time scale of weeks. Multi-band observations of the source are encouraged.