Swift Bulge Survey detections of the X-ray transients SAX J1750.8-2900 and IGR J17445-2747

ATel #12751; 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 13 May 2019; 17:02 UT
Credential Certification: Craig Heinke (cheinke@virginia.edu)

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We have begun a second installment of our Swift Bulge Survey (Atels #10265,#10273,#10305,#10355,#10419,#10422,#10428), a rapid shallow mapping of ~16 square degrees of the Galactic Bulge. Each tile is observed for 120 seconds with the XRT and UVOT aboard the Neil Gehrels Swift satellite, enabling X-ray sensitivity down to 4-10e34 erg/s, depending on extinction, for sources near the Galactic Center. A two-week cadence (when Swift planning allows) enables the detection of very faint X-ray transients as far as the Galactic Bulge. Our first three sets of observations in 2019 were taken on April 4, April 18, and May 8-9.

We report the detection of SAX J1750.8-2900 in each epoch, with 0.5-10 keV unabsorbed fluxes rising from 3.2e-11 (April 4), to 2e-10 (April 18), to 7e-10 ergs/cm^2/s (May 9, 2019), assuming an absorbed power-law with N_H=3e22 and photon index 2 (e.g. Atel #1490). SAX J1750.8-2900 was last seen in Sept. 2018 by INTEGRAL (Atel #12048), and there have been no further observations until our April 4 observation. Its previous outbursts have tended to last years (see Parikh & Wijnands 2017, MNRAS, 472, 2742), but the rapid rise we observe suggests that this is the beginning of an outburst, not a continuation of the 2018 outburst.

In our May 8 observation set, we detect a new outburst of IGR J17445-2747 (Atel #10256,#10265,#10272,#10273,#10305,#10395), a known X-ray burster (Mereminsky et al. 2017, Astr. Lett., 43, 656) with a likely giant star companion (Shaw et al. in prep). We detect 5 photons in a 122 s exposure, suggesting an X-ray flux F_X(0.5-10 keV, unabsorbed)~1e-11 ergs/cm^2/s for N_H~6e22, photon index=2, and thus, for an 8 kpc distance, L_X~7e34 ergs/s. In our two April 2019 observations, there are no photons within 2' of IGR J17445-2747, suggesting a 1-sigma upper limit of F_X<1.8e-12 ergs/cm^2/s, or L_X<1.3e34 erg/s. This source had not been detected since an outburst recorded by INTEGRAL and our prior Swift Bulge Survey observations, which was observed until May 13, 2017.

Our Swift/XRT survey will continue biweekly through Sept. 2019. We thank the Swift team for their support of these observations.