Swift XRT/UVOT monitoring of MAXI J1659-152 during its low luminosity phase

ATel #3249; Y. J. Yang, R. Wijnands, D. M. Russell (University of Amsterdam)
on 30 Mar 2011; 16:07 UT
Credential Certification: Rudy Wijnands (rudy@space.mit.edu)

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We report on the continuous Swift follow-up observations of the black hole candidate MAXI J1659-152 (ATel #2873, ATel #2877, GCN #11296) currently in its low luminosity, hard state (ATel #3201). Two more observations were performed after our previous report (ATel #3201). The observations were taken on 2011-03-14 20:01:45 UT for 2.4 ks, and on 2011-03-27 00:33:03 UT for 2.1 ks, and were all carried out with Photon Counting (PC) mode.

Analysis shows that the source flux is slowly but steadily decreasing. The observations could all be adequately fitted with an absorbed power-law model. We obtained a NH=2.9(+0.9, -0.8)e+21 cm^-2 with the power-law photon index 2.0+/-0.2 for the observation taken on March 14, and a NH=2.8(+1.0, -0.8)e+21 cm^-2 with the photon index 1.9+/-0.3 for the observation taken on March 27. The unabsorbed fluxes measured for these two observations are 1.4e-11 and 1.1e-11 erg cm^-2 s^-1 for 0.3-10 keV, and 6.9e-12 and 5.5e-12 erg cm^-2 s^-1 for 2-10 keV respectively. The UV/optical counterpart was also detected with UV/optical telescope on board Swift. Current magnitudes for all six filters are v > 19.2; b = 20.0 +/- 0.4; u = 18.9 +/- 0.2: uvw1 = 19.3 +/- 0.3; uvm2 > 20.2 and uvw2 = 20.0 +/- 0.3. We note that the proposed quiescent magnitude of the source is ~22.4 in r band (ATel #2976).

In addition, we investigate if there is any spectral variation during the low luminosity phase. We analyzed all four Swift PC mode observations (on 02-06, 02-28, 03-14 and 03-27) taken during the decaying phase of the source. All four observations are adequately fitted with the absorbed power-law model. In order to best constrain any possible spectral change, we first obtained the average best fitted column density NH=3.0(+0.4, -0.3)e+21 cm^-2 from the combined spectrum of all four observations. By fixing the column density to 3e+21 cm^-2, we then obtained the corresponding power-law photon indices 1.8+/-0.1, 1.7+/-0.3, 2.0+/-0.2 and 1.8+/-0.2 receptively. This demonstrates that the source could have had the same spectral shape during all four observations, although minor spectral variability cannot be excluded. To further investigate the luminosity and the spectral behavior of MAXI J1659-152 at this low-luminosity, hard state, more Swift monitoring observations are planned. We encourage continued monitoring of this source also at other wavelengths.

We thank the Swift team for their scheduling of these observations. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.