MAXI J1828-249: Swift UV counterpart and XRT spectral fit

ATel #5479; J. A. Kennea (PSU), M. Linares (IAC, Spain), H. A. Krimm (CRESST/GSFC/USRA), P. A. Evans (U Leicester),, P. Romano (INAF-IASF PA), V. Mangano (PSU), P. Curran (Curtin), K. Yamaoka (Nagoya U) and H. Negoro (Nihon U.)
on 17 Oct 2013; 04:20 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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At 19:52UT on October 16th, 2013, Swift took a 1ks long target-of-opportunity observation of MAXI J1828-249 (Nakahira et al, ATEL #5474), utilizing XRT in Windowed Timing (WT) mode to avoid pile-up, and with UVOT using the uvm2 filter. We detect an uncatalogued UV point source in the UVOT data at the following location: RA/Dec(J2000) = 277.241958, -25.029412, which is equivalent to:

RA(J2000) = 18h 28m 58.07
Dec(J2000) = -25d 01m 45.88

with an estimated uncertainty of 0.03 arc-seconds. This position lies 4.3 arc-seconds from center the XRT error circle reported by Kennea et al (ATEL #5478). The brightness of the new point source is uvm2 = 18.64 +/- 0.04 (stat) +/- 0.03 (sys), utilizing the AB system, this magnitude has not been corrected for interstellar reddening. We find no catalogued point source (searching using the VizieR tool - http://vizier.u-strasbg.fr/viz-bin/VizieR ), at this position and therefore suggest that this is likely the UV counterpart of MAXI J1828-249. We note that the appearance of a UV counterpart is similar to the behavior seen in Galactic Black Hole X-ray Binaries with modest extinction, e.g. MAXI J1659-152 (Kennea et al., 2011, ApJ, 736, 22; Kuulkers et al., 2013, A&A, 552, 32).

Analyzing the XRT WT mode data, we find that the source is very bright in X-rays, with a count rate of ~213 XRT count/s. Fitting the spectrum with an absorbed (XSPEC TBabs with abundance set to "wilm") disk blackbody (XSPEC diskbb) + power-law model, we find the following parameters: N_H = 0.44 +/- 0.04 x 10²² cm^-2, kT_in = 0.68 +/- 0.03 keV, photon index = 2.86 +/- 0.13, with a reduced chi^2 = 0.92 (343 dof). We note that an absorbed diskbb model is a poorer fit (reduced chi^2 = 2.26 for 345 dof), as is an absorbed power-law model (reduced chi^2 = 2.90 for 345 dof). The flux in the 0.5 - 10 keV band is 5.8 +/- 0.1 x 10^-9 erg s^-1 cm^-2, uncorrected for absorption.

Given the presence of a strong disk component required by the spectral fit, and the apparently soft power-law component, it appears as if MAXI J1828-249 may be evolving towards the soft state. We note however that given the limited energy range of XRT, measurement of the power-law photon index may be inaccurate when performing fits that contain multiple continuum components. Fixing the photon index at the INTEGRAL measured value of 1.7 (Filippova et al., ATEL #5476), gives a poorer, but still statistically acceptable fit (reduced chi^2 = 1.08 for 344 dof).

Observations of this new transient with Swift are on-going.