X-ray and radio observations of the re-brightening event in MAXI J1659-152

ATel #3358; J. C.A. Miller-Jones (ICRAR - Curtin), O. K. Madej (SRON), P. G. Jonker (SRON), J. Homan (MIT), E. M. Ratti (SRON) & M. A.P. Torres (SRON)
on 18 May 2011; 08:01 UT
Credential Certification: James C.A. Miller-Jones (jmiller@science.uva.nl)

Tweet

Recent Swift observations of the black hole X-ray transient MAXI J1659-152 during the decay from its 2010 outburst showed evidence for a marked X-ray re-brightening (ATel #3339). On behalf of a larger collaboration, we report on quasi-simultaneous radio and X-ray observations of the source during the decay and subsequent re-brightening, using the EVLA and Chandra, respectively. These observations are part of a longer-term monitoring campaign tailored to follow the X-ray and radio decay of the source.

On 2011 April 18, Yang & Wijnands (ATel #3298) reported an unabsorbed X-ray flux of 2.2e-13 erg cm^-2 s^-1 (0.5-10 keV). We made 4.9 GHz EVLA observations on 2011 April 19 and 21, which failed to detect the source to a combined 3-sigma upper limit of 20 microJy beam^-1.

Following the reported X-ray re-brightening (ATel #3339), we observed MAXI J1659-152 with Chandra for 18 ks on 2011 May 12 (start time 07:36 UT). The source was slightly piled-up at the 10% level. Model fitting the spectrum with a simple absorbed power law including the pile-up component, we derived a power law index of 1.65+/-0.03, a column density of 3.1+/-0.1e21 cm^-2, and an unabsorbed flux of 10.6+/-0.6e-12 erg cm^-2 s^-1 (0.5-10 keV). This is significantly higher than the flux reported from the Swift observations of 2011 May 6 (ATel #3339), indicating that the re-brightening has continued.

We also made a quasi-simultaneous radio observation with the EVLA on 2011 May 13 (05:18-09:28 UT), for comparison with the Chandra X-ray data. We detected a point source of flux density 81+/-8 microJy beam^-1 at 4.9 GHz. The EVLA was in BnA configuration during our observations, with a resolution of 1.4 x 0.9 arcsec^2. Our measured EVLA position was within 0.11 arcsec of that determined by the EVN on 2010 September 30 (ATel #2906). If the radio emission were due to particle acceleration where the jets launched during the 2010 outburst impacted on the surrounding interstellar medium, we could use the measured offset and time elapsed since the state transition reported on 2010 October 12 (ATel #2927) to estimate the average jet speed, for which we derive a value of 0.01c (d/3 kpc). This is significantly slower than seen in other systems where downstream X-ray and radio re-brightening has been observed, such as XTE J1550-564 (Corbel et al. 2002, Science, 298, 196) and H1743-322 (Corbel et al. 2005, ApJ, 632, 504). We therefore find it most probable that the observed radio emission arises from compact core jets launched from the inner regions of the accretion flow.

We note that the distance of 7+/-3 kpc derived by Kuulkers et al. (2011, arXiv:1102.2102) is likely to be an overestimate. The absolute disc magnitude in outburst assumed from the orbital period relation does not take into account the inclination angle of the disc, which the presence of X-ray dips in this source shows to be high. This implies a smaller projected disc area and hence a fainter absolute magnitude in outburst, explaining the relatively small difference between the measured quiescent and outburst magnitudes. Instead, we can assume that the quiescent r-band magnitude of 22.4 (ATel #2976) is generated purely by the companion (likely an M2V-M5V dwarf, with absolute magnitude R=8.4-10.5). With the extinction A_R=0.9 derived from the column density, we then estimate a likely distance range of 1.6-4.2 kpc. Higher distances would require the disc to make a substantial contribution to the quiescent optical emission.

For our estimated range of distances, our measured X-ray flux corresponds to luminosities in the range 3e33-2e34 erg/s. Our quasi-simultaneous radio and X-ray detections then suggest that MAXI J1659-152 is underluminous in the radio band by a factor of several relative to the empirically-determined radio/X-ray correlation.