Swift detects a bright flare from IGR J16418-4532

ATel #14924; B. Sbarufatti (INAF-OAB/PSU), P. Romano (INAF-OABrera), S. D. Barthelmy (GSFC), H. A. Krimm (CRESST/GSFC/USRA), P. A. Evans (U Leicester), J. A. Kennea (PSU), K. L. Page (U Leicester), M. H. Siegel (PSU)
on 19 Sep 2021; 00:11 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Boris Sbarufatti (bxs60@psu.edu)

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The Swift Burst Alert Telescope (BAT) triggered on the high mass X-ray binary IGR J16418-4532 on 2021 September 18 at07:58:20 UT (image trigger 1073821). Swift immediately slewed to the target.

Using the BAT data set from T-239 to T+963 sec from the recent telemetry downlinks, we report that the time-averaged spectrum from T+0.00 to T+64.00 sec is best fit by a simple power-law model with a photon index of 2.95 +/- 0.43. The fluence in the 15-150 keV band is (4.0 +/- 0.9)E-07 erg cm^-2. All quoted errors are at the 90% c.l.

The XRT began observing the field at 08:00:34 UT, 134 s after the BAT trigger. The XRT light curve shows the descending part of a flare that exceeded 30 counts s^-1 and decays down to 0.1 counts s^-1 in about 17 ks. The mean XRT/WT spectrum (first orbit of data, exposure 799 s) can be fit by an absorbed power-law model with a photon index of 1.72 (+0.11, -0.11) and an absorbing column density of NH=(7.7+0.5-0.5)E22 cm^-2. The average 0.3-10 keV observed (unabsorbed) flux is 1.4E-9 (3.5E-9) erg cm^-2 s^-1, making this one of the brightest states observed by XRT. The mean XRT/PC spectrum (orbits second to fourth, 3.9 ks exposure) can be fit by an absorbed power-law model with a photon index of 0.98 (+0.28, -0.27) and an absorbing column density of NH=(5.5+1.2 -1.0)E22 cm^-2. The average 0.3-10 keV observed (unabsorbed) flux is 8.0E-11 (1.2E-10) erg cm^-2 s^-1.

Previously, Swift observed bright flares from this source on 2011 February 17 (Romano et al., 2012, MNRAS, 419, 2695), 2012 June 3 (Romano et al., 2012, ATel #4148), 2013 April 2 (Romano et al., 2012, ATel #4939), 2013 September 19 (Krimm et al., 2013, ATel #5398), and 2015 April 27 (Romano et al., 2015, ATel #7454).

The historical light curve from the BAT hard X-ray transient monitor (Krimm et al, 2013, ApJS, 209, 14; 15-50 keV) can be found at http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/IGRJ16418-4532 .