A NuSTAR Observation of the fast recurrent nova V745 Scorpii

Marina Orio, Vikram Rana, Jeno Sokoloski, Fiona Harrison

V745 Scorpii �  Fastest Galactic Recurrent Nova, with times for a 2 and

3 mag decay t2=2 days and t3=4 days �  Symbiotic binary (like RS Oph, T CrBor) �  Last outburst observed on February 6, 2014 �  Detected with Fermi on outburst day at 3 and 4 σ level

with flux of order of 10-10 erg cm-2 s-1 (Cheung et al. 2014)

�  Observed with NuSTAR on February 16 in a net 22.5 ksec exposure (with interruptions) in the 3-79 keV range. It is only second nova observed in this range with NuSTAR, and the first nova that was actually detected

�  Constantly monitored with Swift, luminous central SSS had already appeared on February 10 with unabsorbed flux of the order of 10-8 erg cm-2 s-1 below 1 keV

The optical light curve

Questions that can be addressed with hard X-rays observations

�  Do the hard X-rays and gamma rays originate in a common region?

�  Are the hard X-rays a later phase in the cooling of shocked material?

�  Can we detect down-scattered Comptonized gamma ray radiation, and can thus the X-rays give insight on gamma-ray emission “below” the Fermi detection threshold?

�  In a symbiotic nova, are the X-rays instead not always due to the nova wind impact on the the red giant wind?

�  Or are they perhaps associated with jet emission (bipolar outflows)?

�  Is the hard X-ray region depleted of iron and can be associated with the “He/N spectrum” as a separate outflow (from the WD) while at later phases an iron rich spectrum must be associated with outflows from the secondary? (Williams 2013).

NuSTAR detection

FPMA FPMB 3-79 keV 3.354e-01 +/- 3.945e-03 3.158e-01 +/- 3.872e-03

3-20 keV 3.349e-01 +/- 3.895e-03 3.154e-01 +/- 3.812e-03 3-10 keV 3.241e-01 +/- 3.824e-03 3.046e-01 +/- 3.736e-03

10-20 keV 9.471e-03 +/- 6.956e-04 9.793e-03 +/- 7.256e-04

(See Rana et al. 2014, also see Harrison 2013 for a description of telescopes and detectors)

At the same time, the Swift XRT count rate in the 0.3-10 keV range was 10.16 +/- 0.10 (obs id. 00033136033) due to the luminous central supersoft X-ray source (the WD itself)

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Spectrum of V745 Sco. The nova was observed with the FPMA(black) and FPMB telescopes (red) and fitted with an XSPEC VAPEC model of ionized plasma in collisional ionization equilibrium with kT=2.64 keV, Fe/Fe¤=0.5, F(3-10 keV)=1.6 x 10-11 erg cm-2 s-1. For an emitting region as large as the whole nova shell on day 10, the electron density would be ne=8.3 x 106 cm－3.

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Here we show the Swift XRT and the NuSTAR spectra together. and a fit in which we also added a WD atmosphere by Rauch with T=1 million K and a second,much cooler component of collisionally ionized plasma with kT=180 eV. (the fit to the Swift spectrum will still be improved).

Hard X-rays as probe of gamma ray emission �  Do all novae in outburst emit gamma rays like the

five detected with Fermi, and is the Fermi LAT detection threshold simply too high?

�  Do the emit for longer than just the short burst of emission observed with Fermi?

�  The optical depth for Compton scattering in a nova in the initial stage is only of the order of 10-5 , and there should be enough electrons to downgrade gamma rays to X-rays

�  We propose that hard x-rays can be used as a probe of gamma ray emission.

Initial conclusions �  We did not detect any “very hard” X-rays on day 10 after the

outburst (above 20 keV) �  The lack of emission at energy larger than 20 keV and the

fact that the spectrum cannot be fitted with a power law argue against any downgraded Comptonized gamma ray emission, with a remaining gamma-ray flux limit on the day 10 of the order of 10-11 erg cm-2 s-1

�  An initial observation of plasma at kT=8 keV with Swift on the day of the outburst proves that the ejecta must have significantly cooled in only 10 days

�  In this nova Fe appears to be depleted, but the detection of a prominent iron feature due to the triplet around 6.7 keV highlights the power of NuSTAR for iron abundance diagnostics

References

�  Cheung C. C., Jean P., Shore S. N. 2014, The Astronomer’s Telegram, 5879

�  Harrison, F. A., Craig, W W., Christensen, F. E., et al. 2013, ApJ, 770, 103

�  Rana V., Orio M., Sokoloski J., Harrison F. 2014, The Astronomer’s Telegram, 5913

�  Williams R., 2013, AJ, 146, 55