Swift reports the discovery of the galactic transient Swift J1753.7-2544

ATel #4769; H. A. Krimm (CRESST/GSFC/USRA), S. D. Barthelmy (GSFC), W. Baumgartner (CRESST/GSFC/UMBC), J. Cummings (CRESST/GSFC/UMBC), N. Gehrels (GSFC), C. B. Markwardt (GSFC), D. Palmer (LANL), T. Sakamoto (AGU), M. Stamatikos (OSU/GSFC), J. Tueller (GSFC), T. Ukwatta (MSU)
on 29 Jan 2013; 15:53 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

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The hard X-ray transient monitor of the Swift Burst Alert Telescope (BAT) has triggered on and discovered a new galactic transient source (Cummings et al, GCN Circ. 14151).

The source, to which we give the name Swift J1753.7-2544, was first detected by the BAT on 2013 January 24 (MJD 56316) and has been mostly brightening ever since.

The BAT rates in the 15-50 keV band have been

Jan 24: 0.0050 +/- 0.001 ct/s/cm^2 (23 mCrab)
Jan 25: 0.0074 +/- 0.001 ct/s/cm^2 (33 mCrab)
Jan 26: 0.014 +/- 0.002 ct/s/cm^2 (64 mCrab)
Jan 27: 0.0090 +/- 0.001 ct/s/cm^2 (41 mCrab)
Jan 28: 0.017 +/- 0.002 ct/s/cm^2 (80 mCrab)
Jan 29: 0.017 +/- 0.002 ct/s/cm^2 (80 mCrab)

The best BAT position is:

RA(J2000) = 268.429, Dec(J2000) = -25.742
RA = 17h 53m 43s, Dec = -25d 44' 31"
Glon= 3.664, Glat = +0.099

The estimated error (statistical plus systematic) is 3 arc minutes (90% confidence) This position is derived from the BAT transient monitor.

At 06:16:29 UT on February 28, 2013 (MJD 56320), Swift J1753.7-2544 triggered BAT in a 1280-second image trigger. On-board software identified it as a gamma-ray burst, but the Swift team suggested that it is more likely a galactic transient (Cummings et al, GCN Circ. 14151).

Using the BAT event data set from T+165 to T+963, we report further analysis of Swift J1753.7-2544 The BAT ground-calculated position from the event data is RA, Dec = 268.429, -25.745 deg which is within 0.2 arcminutes of the BAT monitor position. The partial coding was 92%. The mask-weighted light curve shows no significant structure and an average count rate in the 15-100 keV band of roughly 0.03 ct/s/cm^2.

The time-averaged spectrum from T+165 to T+963 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.07 +- 0.25. The fluence in the 15-150 keV band is 2.1 +- 0.3 x 10^-6 erg/cm2. (90% confidence level.

Sakamoto et al (GCN Circ. 14153) reported no detection in MAXI (4-10 keV) during short observations near the time of the BAT trigger.

A Swift target of opportunity observation has been requested. However, due to a Sun constraint, there will be no Swift XRT or UVOT observations before Feb. 4, 2013.

BAT hard X-ray transient monitor light curve for Swift J1753.7-2544