Swift Bulge Survey: X-ray activity of bursters KS 1741-293, IGR J17445-2747, SAX J1750.8-2900, and symbiotic X-ray binary XMMU J174445.5-295044

ATel #12843; C. O. Heinke (Alberta), A. Bahramian (Curtin), R. Wijnands (Amsterdam), T. J. Maccarone (Texas Tech), N. Degenaar (Amsterdam), J. J.M. in 't Zand (SRON), J. A. Kennea (Penn State), E. Kuulkers (ESA), L. Rivera-Sandoval (Texas Tech), A. W. Shaw, G. R. Sivakoff (Alberta), J. Strader (MSU), A. J. Tetarenko (East Asian Observatory)
on 5 Jun 2019; 18:29 UT
Credential Certification: Craig Heinke (cheinke@virginia.edu)

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The Swift Bulge Survey performs rapid shallow mapping of 16 square degrees of the Galactic Bulge with short (120-s in 2019) observations (see Atels #10265,#10273,#10305,#10355,#10419,#10422,#10428,#12751). After our first 2019 observations on April 4, 18 and May 8-9, we conducted further Swift Bulge Survey observations on May 16, May 23, and May 30.

In our May 30 observations, the transient X-ray burster KS 1741-293 (e.g. Atel #9387; De Cesare et al. 2007, MNRAS, 380, 615) was detected with the Swift/XRT at 0.2 cts/s (0.5-10 keV). We note that the Swift/XRT position of KS 1741-293 is consistent with the Chandra source CXOGC J174451.6-292042, in agreement with the identification by Degenaar, Wijnands and Muno (2008, Atel #1531), and inconsistent with the position suggested by Marti et al. (2007, A&A, 462, 1065). Using Degenaar et al's spectral parameters (N_H=1.7e23, photon index 1.6), we use PIMMS to estimate a flux F_X(0.5-10 keV, unabsorbed)=1.1e-10 erg/cm^2/s, and (for an 8 kpc distance) L_X(0.5-10)=8e35 erg/s. We estimate an upper limit, combining the three preceding Swift survey observations, of L_X<6e34 erg/s, so we conclude that we have detected a new outburst of KS 1741-293.

The transient X-ray burster IGR J17445-2747 (Atels #10256,#10265,#10272,#10273,#10305,#10395) was barely detected (F_X(0.5-10 keV)=1e-11 erg/cm^2/s) on May 9-10 (Atel #12751). We detect 0-1 photons within 30" (vs. 5 on May 9-10) in follow-up Swift/XRT observations on May 17, May 22, May 23, May 25, May 28, or June 1, nor in our Survey observations on May 23 and May 30. The follow-up observations give individual upper limits of typically F_X(unabs, 0.5-10 keV)<1e-12 erg/cm^2/s. For an assumed 8 kpc distance, we infer that we have likely observed a very faint X-ray outburst from IGR J17445-2747, with a peak observed L_X of 8e34 erg/s (0.5-10 keV), though it may have reached a brighter peak L_X. The outburst lasted less than one month.

The transient X-ray burster SAX J1750.8-2900 (e.g. Atels #12048, #1431) has been detected throughout our 2019 survey observations, showing strong variability. Atel #12751 reported an increase from 3.2e-11 to 7e-10 erg/cm^2/s from April 4 to May 9; we then see a sharp drop on May 16 back to 3e-11 erg/cm^2/s, then a slow rise again up to 3.7e-10 erg/cm^2/s (or L_X(0.5-10 keV)=3e36 erg/s, at 8 kpc) by May 30.

The likely symbiotic X-ray binary XMMU J174445.5-295044 (Bahramian et al. 2014, MNRAS, 441, 640) was detected at roughly 0.04 ct/s in the April 18, May 9, May 23, and May 30 observations, only undetected on April 4. For a 3.1 kpc distance (Bahramian et al. 2014), Gamma=1.18, and N_H about 1e23 cm^-2 (Heinke et al. 2009, ApJ, 701, 1627), this corresponds to F_X (2-10 keV, unabs)=1e-11 erg/cm^2/s, L_X(2-10 keV)=1e34 erg/s.

We thank the Swift team for their support of these observations.