Swift J091211.2-274429: a new X-ray transient

ATel #16702; Phil Evans (U. Leicester), Sam Oates (U. Lancaster)
on 11 Jul 2024; 13:16 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Phil Evans (pae9@star.le.ac.uk)

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Swift-XRT has detected and observed a a new X-ray transient, Swift J091211.2-274429. The transient was first detected in a 1.5-ks observation beginning on 2024-07-01 10:07:49 UT. The best XRT position of the transient is RA, Dec = 138.0464, -27.7411 degrees, which is equivalent to:

RA (J2000): 09h 12m 11.12s
Dec(J2000): -27d 44m 28.1s

with an uncertainty of 3.5" (radius, 90% confidence).

An optical object is also detected in the UVOT, at a position of

RA (J2000): 09h 12m 11.21s
Dec(J2000): -27d 44m 27.8s

with an estimated uncertainty of 0.9 arcsec (radius, 90% confidence), consistent with the XRT position and a known catalogued optical source.

No catalogued X-ray source is reported at this position; a 3-sigma upper limit from previous Swift-XRT observations is 3.99e-3 ct/sec (0.3-10 keV), whereas the count-rate at discovery was is 1.4 (+/- 0.4)e-2 ct/sec. The transient was originally classified within the LSXPS system (Evans et al. 2023) as needing follow up, since the detection flux was slightly less than 3 sigma above the upper limit. Daily monitoring of the object with Swift-XRT reveals an initial steady flux level, of around 0.014 ct/sec, then after ~2 days a sudden drop by a factor of ~2, before the count-rate returned to its original level ~5 d later.

The X-ray spectrum can be modelled with an absorbed power-law, with a photon index of 1.6 (+0.6, -0.4). The absorption column is not well constrained, with the lower-limit going to 0; the 90% upper limit is 3.0e23 cm^-2. Using this spectrum the peak 0.3-10 keV flux of Swift J091211.2-274429 is found to be 3.44 (+/- 2.5) e-13 erg cm^-2 s^-1.

The optical counterpart is catalogued, with USNO-B1 magntiudes of B2=21.31, R2=20.81; the source is also in the WISE All-Sky Data Release (Cutri et al., 2012), with magnitudes: W1=16.253, W2=15.826, W3>12.634, W4=9.145 and in PanSTARRS PS1. The WISE colours do not correlate with the stellar or galactic object types shown in Fig. 12 of Wright et al. (2010).

In UVOT the object is detected in the uvw1, uvm2 and uvw2 filters only. The magnitude does not currently appear to be varying, but is brighter than archival UVOT values. Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011) for the archival and most recent summed observations are given below.

 
Filter   Tmid(MJD)  Exp(s)     Mag(AB)/3sigUL 
 
   v    58600        297      >19.9 
   v    60492        39       >18.7 
   b    58600        297      >20.7 
   b    60492        39       >19.4 
   u    58600        297      >21.3 
   u    60497        1260     >22.3 
  w1    58600        463      >21.8 
  w1    60497        2180     21.7+\-0.2 
  m2    58600        888      >22.2 
  m2    60497        5203     21.8+\-0.1 
  w2    58600        1193     22.7+\-0.4 
  w2    60497        4186     21.8+\-0.1 

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.016 in the direction of the object (Schlegel et al. 1998).

The nature of this transient is therefore currently unknown. Swift observations continue, and follow up is recommended. The automated XRT analysis of this transient is available at: https://www.swift.ac.uk/LSXPS/transients/5682

References:
Breeveld et al. 2011, AIP Conf. Proc. 1358, 373
Cutri R.M., et al., VizieR On-line Data Catalog: II/311
Evans P.A., et al., 2023, MNRAS, 518, 174
Schlegel et al. 1998, ApJ, 500, 525
Wright E.L., et al., 2010, AJ, 140, 1868

Swift J091211.2-274429 - Swift-XRT