Start of the Swift monitoring of the Galactic bulge central part

ATel #14438; Sergio Campana (INAF-OA Brera)
on 4 Mar 2021; 21:11 UT
Credential Certification: Sergio Campana (sergio.campana@brera.inaf.it)

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We initiated a monitoring campaign on the Galactic bulge center with the Swift/XRT and UVOT instruments. The campaign aims at monitoring the central 1 degree of our Galaxy (excluding the very central part), covered by 18 exposures lasting ~500s, every other week for the year 2021. We will monitor faint sources and try to discover new transients as done in the past by, e.g., Bahramian et al. (2020). During the first two runs (Feb 13 and Feb 27, summed to ~1ks), we revealed with Ximage detect (SNR>2):

 
Name       | XRT count rate    | RA(J2000)   |  DEC(J2000) | Err   | Possible ID & Distance       
CBS13794_1 |5.97E-02+/-9.4E-03 | 17 42 29.90 | -28 44 52.4 | 4.97" | HD160682 d=6.8"              
CBS13797_1 |2.08E+00+/-5.6E-02 | 17 46 04.38 | -29 30 42.7 | 3.57" | 1A1742-294 d=15.1"*          
CBS13798_1 |2.88E-02+/-6.2E-03 | 17 42 14.56 | -29 14 57.2 | 5.64" | 1RXS J174216.4-291454 d=8.6" 
CBS13800_1 |8.56E-03+/-3.8E-03 | 17 44 16.69 | -29 39 38.3 | 7.48" | 2XMM J174417.2-293943 d=9.3" 
CBS13802_1 |1.10E-02+/-3.8E-03 | 17 49 17.53 | -28 33 24.0 | 7.48" | NONE 
CBS13803_1 |1.12E-02+/-4.4E-03 | 17 47 25.34 | -28 09 37.2 | 7.09" | SSTGC 802032?? d=5.4"  
CBS13804_1 |7.86E-03+/-3.5E-03 | 17 43 31.44 | -28 27 55.0 | 8.27" | NONE  
CBS13804_2 |8.91E-03+/-4.1E-03 | 17 44 33.07 | -28 30 40.7 | 9.05" | 2MASS J17443315-2830321 d=8.6"  
CBS13807_1 |9.61E-03+/-4.0E-03 | 17 47 17.10 | -28 52 37.3 | 6.78" | CXOGCS J174717,7-285241 d=8.8"  
CBS13807_2 |1.18E-02+/-4.6E-03 | 17 46 34.03 | -28 50 59.1 | 5.76" | CXOGSS J174634.1-285104 d=6.9"  
CBS13807_3 |2.43E-01+/-1.9E-02 | 17 46 21.06 | -28 43 42.1 | 3.95" | AX J1746.3-2843 d=0.44"  
CBS13808_1 |7.52E-03+/-3.3E-03 | 17 43 51.57 | -28 46 33.1 | 6.93" | 3XMM J174351,2-284637 d=6.2" 
CBS13809_1 |1.89E-02+/-5.0E-03 | 17 47 02.28 | -29 11 09.3 | 6.21" | NONE  
CBS13809_2 |9.64E-03+/-3.9E-03 | 17 46 54.11 | -29 15 37.6 | 7.71" | XMM J174654.1-291542 d=5.0"  
CBS13810_1 |1.02E-02+/-3.8E-03 | 17 42 16.60 | -28 37 05.5 | 8.27" | SBM2001-23 d=6.0"  
CBS13810_2 |8.08E-03+/-3.7E-03 | 17 41 45.12 | -28 34 37.1 | 7.97" | NONE  
CBS13811_1 |1.31E-02+/-4.7E-03 | 17 48 05.24 | -29 26 19.7 | 5.53" | NONE  
CBS13811_2 |1.18E-02+/-4.5E-03 | 17 47 59.09 | -29 22 42.0 | 5.89" | NONE 

*just out of the field of view
Faint sources (at a level of ~0.01 c s^-1) are often detected in one exposure and not in the other. The limiting 0.3-10 keV unabsorbed luminosity reached in a single 500s exposure, assuming a power-law spectrum with photon index 2 and an absorbing column density of 5x10²² cm^-2, is ~2x10³⁴ erg s^-1 at 8 kpc. Source positions are uncertain given the short exposure, and possibly the quoted error, obtained with xrtcentroid, is slightly underestimated.
UVOT data are taken with the m2 filter (the purest UV filter) in event mode, to have the possibility to catch flares.