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Chandra HRC confirms that M15 X-2 is the currently flaring source in M15

ATel #3393; G. R. Sivakoff (U. Alberta), C. O. Heinke (Alberta), J. C.A. Miller-Jones (ICRAR - Curtin), D. Altamirano (Amsterdam), E. Kuulkers (ESA/ESAC, Spain), , M. Morii (Tokyo Tech)
on 1 Jun 2011; 14:19 UT
Credential Certification: Gregory R Sivakoff (grs8g@virginia.edu)

Subjects: Radio, X-ray, Binary, Globular Cluster, Neutron Star, Transient, Variables

Referred to by ATel #: 5327, 5396, 5490, 15586

Radio observations of the ongoing X-ray flare in the globular cluster M15 (ATel #3356, #3363), suggested that the ultracompact X-ray binary M15 X-2 was the flaring source (ATel #3378). This is surprising as ultracompact X-ray binaries such as M15 X-2 (22 minute period, Dieball et al. 2005, ApJ, 634, L105) are generally not expected to show factors of >10 flux variations over timescales of days to weeks. We report here that M15 X-2 is one of an increasing number of ultracompact X-ray binaries that actually show strong X-ray variability that is likely accretion related. These systems challenge our understanding of accretion in ultracompact X-ray binaries (e.g., Simon 2005, A&A 436, 263; Maccarone et al. 2010, MNRAS, 406, 2087).

To verify the origin of the ongoing X-ray flare, we undertook a Chandra X-ray Observatory HRC-I observation of M15 on 2011 May 30 UT 16:35 - 17:00 UT. These observations clearly show that M15 X-2 has varied dramatically (~ factor of 9) compared to archival HRC-I observations. We summarize the data below, converting full band HRC-I count rates (in 1.5" radius circles) to unabsorbed luminosities using PIMMS, assuming a simple Gamma=1.7 power-law absorbed by an N_H=1.2E21 cm^-2 column density at a distance of 10.3 kpc. We estimate there is a 20% systematic error in the conversion. Since AC 211 undergoes eclipses, we use the ephemeris given in Ioannou et al. (2003 A&A 399, 211) to determine the orbital phase at the time of the observation.

 
                    M15 X-2                              AC 211                  
          ----------------------------  ---------------------------------------- 
   MJD     Count Rate   L_X 0.5-10keV   Count Rate  L_X 0.5-10keV  Orbital Phase 
              cnt/s       1e36 erg/s      cnt/s     1E36 erg/s 
--------  ------------  -------------   ----------  -------------  ------------- 
52103.35   7.08+/-0.03        2.6       1.7+/-0.01       0.7           0.80 
52124.73   6.68+/-0.03        2.5       1.8+/-0.01       0.7           0.79 
52143.27   7.69+/-0.03        2.9       5.6+/-0.02       2.3           0.79 
54348.64   6.56+/-0.06        2.1       2.3+/-0.03       0.8           0.83 
55711.69  52.20+/-0.19       18.3       5.7+/-0.06       2.4           0.70 
 

We also report that when we compare EVLA observations of M15 on 2011 May 26, UT 11:27-12:27 (MJD~55707.50, AC211 orbital phase of 0.64) to our earlier reported (ATel #3378) observations on 2011 May 22, UT 12:11-13:11 (MJD~55703.53, AC211 orbital phase of 0.07), we detect significant variations in flux density for both M15 X-2 and the accretion disc corona X-ray binary, AC211.

For M15 X-2, we measured flux densities of 57 +/- 11 microJy/beam at 5 GHz and 55 +/- 7 microJy/beam at 7 GHz, yielding a spectral index of -0.09 +/- 0.58. These flux densities are approximately a factor of 0.4 times that measured 4 days earlier, which could be due to either correlated X-ray/radio variability or a transition to a softer accretion state with a reduced radio jet.

For AC 211, we measured flux densities of 782 +/- 14 microJy/beam at 5 GHz and 756 +/- 8 microJy/beam at 7 GHz, yielding a spectral index of -0.09 +/- 0.06. These flux densities are an approximately a factor of 2.7 times that measured 4 days earlier. It is possible that AC 211 was eclipsed in the first epoch, which would imply that the radio emission, which likely comes from radio jets as suggested by the flat spectrum, would have to be very compact to be eclipsed by the companion star at phase zero.

We encourage continued multi-wavelength observations of this flaring source.

We also thank the Chandra X-ray Observatory and EVLA for their rapid response.