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Swift J1658.2-4242: Swift BAT and XRT refined analysis

ATel #11310; A. Y. Lien (GSFC/UMBC), J. A. Kennea (PSU), S. D. Barthelmy (GSFC), A. P. Beardmore (U. Leicester), P. A. Evans (U Leicester), H. A. Krimm (NSF/USRA), D. Malesani (DARK/NBI), C. B. Markwardt (GSFC), D. M. Palmer (LANL), K. L. Page (U Leicester), T. Sakamoto (AGU), M. Stamatikos (OSU), T. N. Ukwatta (LANL)
on 16 Feb 2018; 19:10 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Amy Lien (amy.y.lien@nasa.gov)

Subjects: X-ray, Gamma Ray, Transient

Referred to by ATel #: 11311, 11318, 11321, 11322, 11334, 11336, 11358, 11375

Using the data set from the recent telemetry downlink, we report further analysis of the new Galactic transient, Swift J1658.2-4242 (trigger #810300) (Barthelmy et al., GCN Circ. 22416).

Using the BAT data from T+210 to T+963 sec, the BAT ground-calculated position is RA, Dec = 254.537, -42.712 deg which is
RA(J2000) = 16h 58m 08.8s
Dec(J2000) = -42d 42' 42.1"
with an uncertainty of 3.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 78%.

The BAT mask-weighted light curve shows a continuous weak emission that starts before and ends after the event data range from T+210 s to T+963 s. Data from the BAT transient monitor (Krimm et al. 2013) show the source starts flaring around Feb. 14, 2018, and has reached a brightness of ~ 70 mCrab (15-50 keV). This is generally consistent with the detections reported by INTEGRAL (Grebenev et al., ATEL 11306).

The BAT time-averaged spectrum from T+210.00 to T+963.02 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.98 +- 0.28. The fluence in the 15-150 keV band is 2.2 +- 0.4 x 10-6 erg/cm2. All the quoted errors are at the 90% confidence level.

Swift/XRT began observation of the transient at 06:54 UT and observed the target in a mixture of Windowed Timing and Photon Counting mode for a total of 3.1 ks. In XRT data we localize the transient, utilizing UVOT to reduce the systematics on astrometry, to the following coordinates: RA/Dec(J2000) = 254.55266, -42.69845, which is equivalent to:

RA(J2000) = 16h 58m 12.64s,
Dec(J2000) = -42d 41m 54.4s,

with an estimated uncertainty of 1.6 arc-seconds radius (90% confidence). We note that this position lies 0.4 arc-seconds from the IR source VVV J165812.62-424154.78, from the VISTA Variables DR2 catalog (Minniti et al., 2017), which has magnitudes H=16.2 and K=15.252. In addition G343.2514+00.0537, a source in the GLIMPSE catalog, is also consistent with the XRT localization.

Spectral fits to the PC and WT data are consistent, although as WT data are unaffected by pile-up we report that fit here. The spectrum is well fit by a highly absorbed power-law model, with NH = 1.9 +/- 0.5 1023 cm-2, and photon index = 1.7 +/- 0.5. The average flux, corrected for absorption, is ~1.6 x 10-9 erg/s/cm2 (0.3 - 10 keV). The light-curve shows some evidence of fading during the observed period, although planned later observations of the new transient will be required to determine any fading.

We find no evidence of pulsations or QPOs in the XRT data, although as the data are mainly taken in PC mode and suffer from pile-up, this does not place strong constraints on the presence of any periodicities in the data.

The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/810300/BA/

The results of the BAT transient monitor are available at https://swift.gsfc.nasa.gov/results/transients/weak/SwiftJ1658.2-4242/