Rapid Decline in Radio Flux Density of Nova Sco 2015 Followed By Rise at High Frequencies

ATel #7194; J. Linford (Michigan State), T. Nelson (Minnesota), L. Chomiuk (Michigan State), J. Sokoloski (Columbia), K. Mukai (UMBC/NASA GSFC), T. Finzell (Michigan State), J. Weston (Columbia), M. Rupen (NRC-HIA) and A. Mioduszewski (NRAO)
on 9 Mar 2015; 19:29 UT
Credential Certification: Justin Linford (jlinford@msu.edu)

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We are monitoring Nova Sco 2015 (PNV J17032620-3504140) at radio wavelengths with the Karl G. Jansky Very Large Array (VLA). We have observations from three epochs: 2015 Feb 14.5, 2015 Feb 18.5-19.5, and 2015 Feb 24.6-Mar 01.5. All VLA observations were made in the 8-bit mode and in B configuration (maximum baseline of 11.1 km). The L-band (1-2 GHz) observations were made with a total bandwidth of 1 GHz. All other observing frequencies had a total bandwidth of 2 GHz. All bands were split between a lower and upper sideband. The results from 2015 February 14.5 previously reported in ATEL #7085 are also included here to fully illustrate the radio behavior.

 
Frequency (GHz) | Date (UT)    | Flux Density (mJy)  
----------------|--------------|-------------------  
1.26            | 2015-02-19.5 | 1.57 +/- 0.08       
                | 2015-03-01.5 | <0.31                
----------------|--------------|-------------------  
1.74            | 2015-02-19.5 | 1.21 +/- 0.07       
                | 2015-03-01.5 | <0.42               
----------------|--------------|-------------------  
4.55            | 2015-02-14.5 | 4.13 +/- 0.02       
                | 2015-02-19.5 | 0.65 +/- 0.02       
                | 2015-03-01.5 | 0.22 +/- 0.02       
----------------|--------------|-------------------  
7.38            | 2015-02-14.5 | 2.79 +/- 0.01       
                | 2015-02-19.5 | 0.44 +/- 0.01       
                | 2015-03-01.5 | 0.19 +/- 0.02       
----------------|--------------|-------------------  
13.5            | 2015-02-18.5 | 0.42 +/- 0.02       
                | 2015-02-24.6 | 0.39 +/- 0.02       
----------------|--------------|-------------------  
17.4            | 2015-02-18.5 | 0.34 +/- 0.02       
                | 2015-02-24.6 | 0.46 +/- 0.02       
----------------|--------------|-------------------  
28.2            | 2015-02-14.5 | 0.82 +/- 0.06       
                | 2015-02-18.5 | 0.30 +/- 0.05       
                | 2015-02-24.6 | 0.79 +/- 0.05       
----------------|--------------|-------------------  
36.5            | 2015-02-14.5 | 0.68 +/- 0.08       
                | 2015-02-18.5 | 0.38 +/- 0.07       
                | 2015-02-24.6 | 0.85 +/- 0.08    

As reported in ATEL #7085, the initial spectrum rose toward lower frequencies indicating the presence of synchrotron radiation, confirming the "embedded" nature of the nova as proposed in ATEL #7060. The flux density decreased significantly between the first two epochs, and then began to rise at high frequencies between the second and third epochs. The current spectrum rises toward higher frequencies and appears to be relatively flat between 4.55 and 7.38 GHz. This indicates the presence of both synchrotron and thermal free-free emission. The low frequency flux density appears to still be in decline, with the most recent 1.26 and 1.74 GHz observations resulting in non-detections.

To the best of our knowledge, this is the first time such an obvious transition from synchrotron-dominated to thermal-dominated emission has been observed in a nova. We will continue to monitor this nova with both radio and X-ray instruments, and we strongly encourage others to monitor it at as many wavelengths as possible.