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Swift observations of 1RXS J180408.9-342058

ATel #7039; H. A. Krimm (CRESST/GSFC/USRA), J. A. Kennea (PSU), M. H. Siegel (PSU) and B. Sbarufatti (INAF-OAB/PSU)
on 10 Feb 2015; 02:07 UT
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

Subjects: Optical, Ultra-Violet, X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 7096, 7100, 7255, 7352

We report on recent observations of the galactic transient source 1RXS J180408.9-342058. This source was recently reported to be in outburst starting on 22 January 2015 in the Swift/BAT transient monitor (Krimm et al., ATel #6997) and confirmed by MAXI (Negoro et al., ATel #7008). It twice triggered the BAT on Swift: on 31 Jan. 2015 (Barthelmy et al., GCN #17367) and on 6 February 2015 (Barthelmy et al., GCN #17416). The source was in Sun constraint at the time of the 1st trigger, but the 2nd trigger led to an observation with the Swift narrow-field instruments. This was followed by a pre-planned observation of the source.

The BAT transient monitor shows that the source steadily increased in brightness in the 15-50 keV band since first detection, but has plateaued since 5 Feb. 2015, at an average rate of (0.025 +/- 0.0014) ct/s/cm^2, or ~100 mCrab. The light curves from both triggers show variations on the time scale of tens of seconds, but no significant structure. Both triggers are best fit by a simple power-law model. For the 31 Jan. trigger, the power law index of the time-averaged spectrum from T+231 to T+666 sec is 2.72 +/- 0.68 and for the 6 Feb. trigger, from T+0.0 to T+912.0, it is 2.04 +- 0.21. The fluence in the 15-150 keV band for the first (second) trigger is 3.9 +/- 1.4 x 10^-7 (2.1 +/- 0.3 x 10^-6) erg/cm2. The quoted errors are at the 90% confidence level.

The XRT data covers two epochs. The 1st includes 3.8 ksec of Windowed Timing mode data starting at 6 Feb 2015, 20:45:14.928 UTC and the 2nd includes 980 sec of Photon Counting mode data starting at 7 Feb 2015 00:04:20.777 UTC. The first epoch shows a mean count rate of 21 s-1 (0.3-10 keV) and an absorbed power law fit with nH = 3.52 (+/- 0.17) X 10^21 cm-2, photon index = 0.965 +/- 0.018. The unabsorbed flux is 1.502 (+/- 0.013) X 10^9 erg cm-2 s-1. The second epoch shows a mean count rate of 18.5 s-1 and an absorbed power law fit with nH = 5.7 (+/- 1.3) X 10^21 cm-2, photon index = 1.10 +/- 0.13. The unabsorbed flux is 1.55 (+/- 0.09) X 10^9 erg cm-2 s-1. The flux is a factor of ~160 times the quiescent rate observed in 2012 (Chevenez et al., ATel #4050). The nH values are slightly in excess of the Galactic value in that direction (2.01 X 10^21 cm-2, Kalberla et al. 2005).

The first epoch shows a clear Type-I X-ray burst peaking at 21:24:16 UTC on 6 Feb and lasting ~100 seconds. This behavior, which is similar to what was seen in 2012 confirms the neutron star transient nature of the source.

The best fit XRT position is
RA(J2000) = 18h 04m 08.40s = 271.03501 (deg)
Dec(J2000) = -34d 20' 51.6'' = -34.34766 (deg),
with an error radius 1.8'' (90% confidence).

The preliminary UVOT position of the optical counterpart is:
RA (J2000) = 18h 04' 08.38'' = 271.03493 (deg.)
Dec (J2000) = -34d 20' 51.3'' = -34.34757 (deg.)
with an estimated uncertainty of 0.49'' (radius, 90% confidence). Both of these positions are consistent with the source position from the 2012 observations (Chevenez et al., ATel #4050).

The UVOT data shows no change in the optical source over the time span of the observations. Preliminary detections using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are:

Filter T_start(s) T_stop(s) Exp(s) Mag

white 5755 5955 197 17.58 +/- 0.04
v 1320 2032 97 17.68 +/- 0.17
b 1939 2303 38 17.89 +/- 0.17
u 1914 1934 19 17.04 +/- 0.17
w1 1888 2255 39 17.28 +/- 0.18
m2 6370 6570 197 17.62 +/- 0.11
w2 5961 6161 197 17.86 +/- 0.10

The magnitudes are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.48 in the direction of the burst (Schlegel et al. 1998). This is the first time this source has been detected optically. The estimated limiting magnitude for earlier observations in April and May 2012 is V=20.

Swift/BAT transient monitor light curve for 1RXS J180408.9-342058