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MASTER-SAAO Follow Up observations of the X-ray Transient MAXI J0051-736/SMC X-2

ATel #8093; V. Lipunov, E. Gorbovskoy (Lomonosov MSU), D. Buckley (SAAO), E. O. Gress, P. Balanutsa, V. Kornilov, N. Tiurina, A. Kuznetsov, I. Gorbunov, E. Popova, D. Vlasenko (M. V. Lomonosov Moscow State University, SAI), O. Gress, N. Budnev, K. Ivanov (Irkutsk State University, Applied Physics Institute), A. Tlatov, V. Senik D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), Yu. Sergienko, A. Gabovich, V. Yurkov (Blagoveschensk Educational State University), V. Krushinskiy (Ural Federal University), R. Rebolo, M. Serra-Ricart, N. Lodieu, G. Israelian (The Instituto de Astrofisica de Canarias)
on 24 Sep 2015; 17:43 UT
Credential Certification: Nataly Tyurina (tiurina@sai.msu.ru)

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

Referred to by ATel #: 8207

MASTER II robotic telescope (MASTER-Net: http://observ.pereplet.ru ) located in SAAO was pointed to the new X-ray Transient MAXI J0051-736 (ATel #8088) 179 sec after notice time and 15011 sec after trigger time at 2015-09-23 23:42:16 UT. On our first (180s exposure) set we haven`t found optical transient within MAXI error-box.

The 5-sigma upper limit has been about 19.5 mag

The automatic reduction give photometry of the optical counterpart of the SMC X-2.

 date       |  mag  | err  | n_imgs 
 2014-12-17 | 14.53 | 0.04 |     18 
 2015-01-30 | 14.67 | 0.01 |     20 
 2015-07-31 | 14.74 | 0.02 |      3 
 2015-09-23 | 14.63 | 0.03 |      4 
 2015-09-24 | 14.66 | 0.06 |     52 

SMC X-2 is an accretor pulsar (Kennea et al., ATel #8091) (spin period = 2.37 s (Yokogawa et al., PASP, 2001, 23, 227). Really this is mixed neutron star stage: AP - Accretor-Propeller (Lipunov,V.M., Astrophysics of Neutron Stars,322 pages, Springer-Verlag, Berlin, Heidelberg, New York. Astronomy and Astrophysics Library, 1992 )

There are two optical stars Oe (Northern) and Be (South) on the line of sight with 2 arcsec separation. So it is not known which star belong to X-ray pulsar system. We have 2 arcsec per pixel and our magnitude is the sum of optical stars fluxes.

The optical flux for unfiltered MASTER instrumental magnitude m=14.6 is about 9.2 10-12erg/s/cm2. So we can conclude that optical flux is not changed more than 5 10-13erg/s/cm2 during last 280 days. From other hand the Swift X-ray flux detected 24 september (at the same time as our optical observations) was about 5.4 10-10 ergs/s/sm2.

OGLE optical monitoring show that Be-star practically not changed (less than 10 percent), but Oe star representes high amplitude variation (delta m ~ 1 magnitude) (Schmidtke et al.,2006, Astron.J.132,page 971.). Because short accretor period binary system must be extremely close for such Be+NS binaries less than 10-20 days. So its X-ray behaviour connected with optical star instability, but not with orbital period (like for A0535+26). Because Oe star is not changed we propose that Be star is the real optical counterpart of the SMC X-2 X-ray pulsar.

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