Swift detection of a rapidly brightening soft X-ray spectrum from Nova SMC 2016

ATel #9733; Kim Page, Julian Osborne (U. Leicester), Paul Kuin(MSSL/UCL), Steve Shore (U. Pisa), Steven Williams (U. Lancaster), M. J. Darnley (LJMU)
on 8 Nov 2016; 13:38 UT
Credential Certification: Paul Kuin (npkuin@gmail.com)

Tweet

Nova SMC 2016 (MASTER OT J010603.18-744715.8) was discovered by MASTER-OAFA on 2016 Oct 14 (ATel #9621); prediscovery observations were reported by OGLE-IV in ATel #9622 and from the database of the MASTER Very Wide Field Cameras (ATel #9631), with a peak unfiltered magnitude of 8.5 +/- 0.1 mag on 2016 Oct 09 at 19:24:02.85 UT. The rise to peak was also captured by a robotic camera in Sao Jose dos Campos, Brazil (ATel #9684), with the outburst seen to start on 2016 Oct 09 at 05.30 +/- 0.1 UT.

Swift observations started a day after the discovery announcement, on 2016 Oct 15. Three observations were taken on Oct 15, 16 and 17, before decreasing the cadence to approximately once every three days until Oct 27, at which point the nova entered an observing constraint. During this time, no X-ray source was detected, to a 3-sigma upper limit of 1.6E-3 ct/s in 8.4 ks.

In ATel #9688, Darnley & Williams reported that the nova had entered the nebular phase on 2016 Oct 29, an indication that X-ray emission might soon be expected. On 2016 Nov 07, after the source position became observable to Swift again, a new X-ray source was clearly detected at the nova position, at a count rate of 0.405 +/- 0.020 ct/s. This first X-ray detection occurred 28.3 days after peak optical magnitude. The source spectrum is soft, with the counts all detected below 1 keV. A second snapshot of data, collected later the same day (28.9 days after peak) shows the X-ray count rate to have increased significantly to 3.35 +/- 0.05 ct/s.

The spectrum from the first detection can be parametrised with a blackbody of kT = 23 +- 10 eV and an absorbing column of (3.8 +0.9/-0.6)E21 cm^-2. The second spectrum has a hotter soft component with BB kT = 39 +/- 6 eV, with less absorption: N_H = (6.2 +2.6/-4.2)E20 cm^-2. An optically thin APEC component is required in addition, in each case to account for the emission above ~0.6 keV. The 0.3-10 keV observed flux from the soft component is 1.2E-11 erg/cm^2/s for the first observation, and 4.0E-11 erg/cm^2/s for the second. Alternatively, using a model atmosphere grid instead of the BB component provides a temperature of 61 +2/-8 eV for the supersoft emission, with N_H < 2.2E21 cm^-2 for the earliest spectrum; for this model, the 0.3-10 keV observed flux is 1.31E-11 erg/cm^2/s. The second spectrum is fitted with an atmosphere model of kT = 58 +/- 2 eV, with N_H < 0.8E20 cm^-2; the 0.3-10 keV flux is 3.71E-11 erg/cm^2/s

The UVOT obtained photometry followed by a 901s long UV grism exposure on November 7th while the XRT was observing. The UV spectrum shows emission lines with the following fluxes:

 
ID     wavelength  flux           notes 
       (Angstrom)  (erg/cm2/s) 
N III]     1750     6.5e-11 
Al III     1860     9.4e-12 
C III]     1909     4.1e-11 
N II       2143     7.9e-12 
C II       2324     4.6e-12 
Mg II      2800     1.7e-11 
[Ne V]     3346     1.2e-11 
[Ne V]     3426     1.5e-12 
[Ne III]   3868     2.2e-12 (poss. blend) 
[Ne III]   3967     3.8e-13 
Hdelta     4101     2.8e-12 
Hgamma     4340     5.7e-12 
N III      4636     4.2e-12 (but likely blended) 
Hbeta      4861     2.8e-12  
[O III]    5007      ---   very complex blend (+2nd order line) 
Halpha     6563     5.6e-12 
He II      5411     2.1e-12  

The Balmer line profiles have barely resolved peaks at indicative of velocities of 1000 km/s and a HWZI of about 3000 km/s.

The magnitude in uvw2 appears variable. In the 10s image taken starting 2016-11-07T01:23 UT uvw2 = 11.38 +/- 0.05 (stat) +/- 0.03 (sys), while in the next 32s image taken starting 2016-11-07T01:41 UT uvw2 = 10.78 +/- 0.19 (stat) +/- 0.10 (sys). By 2016-11-07T18:53 UT we find uvw2 = 10.99 +/- 0.02 (stat) +/- 0.03 (sys). The magnitude system is Vega.