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Fermi Reveals New Light on Novae in Gamma rays

Novae are now firmly established as a high-energy (>100 MeV) gamma-ray source class by the Fermi Large Area Telescope (LAT). In symbiotic binary systems such as V407 Cyg 2010, there is a firm theoretical framework for the production of shock-accelerated particles in the nova ejecta from interactions with the dense wind of the red giant companion. Yet, the high-energy gamma-ray emission detected in classical novae involving less evolved stellar companions cannot be explained in the same way and could instead be produced in internal shocks in the ejecta. We summarize the Fermi-LAT gamma-ray observations of novae, highlighting the main properties that will guide further studies. Additionally, we report on the soft gamma-ray (~0.1 MeV) continuum detection of the oxygen-neon type classical nova V382 Vel 1999 with the OSSE detector aboard the Compton Gamma Ray Observatory in light of its Fermi-era analog, V959 Mon 2012.


Fermi LAT Gamma-ray Novae Discoveries
Novae were not among the wide array of high-energy γ-ray (>100 MeV) source types considered prior to the launch of the Fermi Large Area Telescope (LAT) [3]. That changed when the symbiotic-like recurrent nova V407 Cyg 2010 explosion was discovered optically and identified with an independently detected Fermi-LAT transient source [28,1,6]. In retrospect, the idea that such symbiotic systems can be high-energy γ-ray emitters could have been anticipated as a product of shock-accelerated particles in the nova ejecta through interactions with the dense wind of the red giant companion as considered for RS Oph [39]. However, the later LAT detections of three classical novae (CNe) were truly unexpected [2] (see Figure 1) because they were produced in compact binary systems composed of a white dwarf and main sequence companion. The detected CNe also had different optical/ultraviolet properties, with V959 Mon identified as an explosion from an oxygen-neon (ONe) nova, and V1324 Sco and V339 Del appear to be carbon-oxygen (CO) novae.
Despite the compositional differences, the γ-ray light curves for the ensemble four systems appear similar, being brighter initially with most of the emission observed within ∼ 2 weeks, and total detected durations of 17 − 27 days ( Figure 2) [2]. Because V959 Mon was discovered first by Fermi-LAT as a γ-ray transient in the Galactic plane [8], then identified as a nova ∼ 2 months later following its optical discovery [18,9,7], a comparison of the optical and γ-ray light curves is not possible. For the other three systems (Figure 3; see [1] for V407 Cyg), the γ-ray peaks lagged the optical peaks (by up to ∼ 6 days in the case of V339 Del), with the first significant LAT detection occurring before (V1324 Sco), after (V339 Del 1 ) and on the same day (V407 Cyg) of the observed optical peak. V1324 Sco was unusual in that its optical activity began about 1-month prior to its first LAT detection on 15 June, having displayed an increase of 6 mag between June 1.77 − 3.33 after an initial discovery of an enhanced optical flux in mid-May [40,27].
The observed >100 MeV spectra of the four novae are also similar, with broad spectral peaks that cutoff at energies ∼ 1 − 4 GeV and emission observed up to ∼ 6 − 10 GeV (the highest energy extension was observed in V1324 Sco). Spectral fits with hadronic and leptonic models were indistinguishable (e.g., Figure 4 [left]). The available distance estimates to the novae of < ∼ 4 − 5 kpc indicate they are nearby, and the resultant range of >100 MeV luminosities and total emitted energies spanned a small range of ∼ (3 − 4) × 10 35 ergs s −1 and ∼ (6 − 7) × 10 41 ergs, respectively, with ∼ 2× larger values for V1324 Sco. The V339 Del distance of 4.2 kpc in [2] was confirmed [32] and the 2.7 kpc distance for V407 Cyg is unchanged. For V959 Mon (distance of 3.6 kpc; [36]), adopting smaller inferred distances of ∼ 1.5 to 2.2 kpc [26,24] would translate to a γ-ray luminosity ∼ 5× to 3× smaller. As cautioned for V1324 Sco (see SOM of [2]), the adopted 4.5 kpc distance [20] could be underestimated due to dust and has a likely range of 4 − 8 kpc, consistent with independent estimates [27,17] so its luminosity value could be even higher. To what extent the γ-ray parameters (luminosities, spectra, and durations) of the LAT-detected novae are similar is not yet understood, but this can assuredly be addressed with future observations ( § 4).

Compton OSSE Continuum Detection of V382 Vel 1999
The Fermi-LAT >100 MeV classical novae detections motivated us to revisit previous obser- vations of other novae from the Compton Gamma Ray Observatory (CGRO). In the case of the bright classical nova V382 Velorum 1999 (V382 Vel), that peaked at ∼2.5 mag about one-day after it was discovered May 22.4 UT [41], Oriented Scintillation Spectrometer Experiment (OSSE) [22] observations were obtained over 26 days beginning May 27. The primary objective was to search for nuclear line emission, which was not detected ( [23]; see [19] for the flux limits for the lines at 0.478 MeV from 7 Be and 1.275 MeV from 22 Na). Instead, [23] found a 7.0σ continuum signal over the 0.05 − 0.3 MeV energy range in the first 12 days of the OSSE observations (CGRO Viewing Period, VP 817.5), that could not at the time be unambiguously associated with the nova because of OSSE's large field of view (3.8 • × 11.4 • ). There was no significant detection in the subsequent 14-day observation interval (VP 819.5) that began June 8.
With the benefit of hindsight, these Compton-OSSE continuum observations of V382 Vel are not surprising considering it is an analogue of the Fermi-LAT detected ONe-type nova V959 Mon [36,9]. Specifically, the OSSE soft γ-ray detection within the first ∼ 2 weeks after its optical peak  and its null-detection in the weeks that followed, is similar to the >100 MeV light curves of the Fermi-LAT detected novae (Figure 2), strongly suggesting the positive detection is not a result of source confusion.
The OSSE 0.05 − 0.3 MeV spectrum from VP 817.5 (encompassing the interval 4 − 16 days after optical peak) that we can now associate with V382 Vel (no significant detection was obtained from 0.3 − 10 MeV) is compared to the average Fermi-LAT >100 MeV spectrum of V959 Mon 2012 up to 22-days after its first significant 1-day LAT detection (Figure 4 [right]). Because these novae are similarly distant (1.7 − 2.5 kpc for V382 Vel, [16,35]; see § 1 for V959 Mon), this can be considered an early time composite spectrum of an ONe-type nova. Interestingly, a single BeppoSAX observation of V382 Vel obtained 15 days after optical maximum resulted in a 0.1 − 10 keV detection and 15−60 keV upper limit (2σ ) < 6.7×10 −12 erg cm −2 s −1 attributable to a kT ≈ 6 keV plasma [29]. It is difficult to gauge how the BeppoSAX data connect to the OSSE spectrum because it probed a single epoch coinciding with the end of the Compton VP (perhaps suggesting fading of the ∼0.1 MeV emission), but such early X-ray observations can similarly probe internal shocked emission in the nova ejecta (see [25]).  Figure 2, with optical light curves (bottom panels) over the same time interval. For V1324 Sco, the optical data are those compiled by [27] predominantly from the AAVSO. The V339 Del figure is taken directly from [2] where the optical data are from the AAVSO in 0.25 day bins and includes the additional unfiltered discovery magnitude (black square) [21]; note the small second peak ∼2 days after the first.

New Fermi LAT Gamma-ray Novae Detections
Since the first four γ-ray novae discoveries ( § 1), significant LAT detections of two more followed, V1369 Cen 2013 [10,11] and Nova Sgr 2015 No. 2 [13,14], as a result of Fermi target-ofopportunity (ToO) observations prompted by their optical discoveries [33,34]. Both reach nakedeye brightness optically, but in contrast to the previous γ-ray detected novae, several subsequent optical peaks were observed up to ∼1 − 2 months after their first optical peaks. The first significant 1-day γ-ray detections began ∼ 2 days after the first optical peaks, similar to that reported in V339 Del. The distances to these two novae are not yet known, but they are fainter in γ-rays on average than the previous LAT-detected novae, and are characterized by more sporadic lowsignificance 1-day detections extended over moderately longer durations (up to ∼1 − 2 months), broadly mimicking the multi-peaked optical activity ( Figure 5).

Future Perspectives
The γ-ray detections of V407 Cyg 2010 and the two CNe in 2012 resulted naturally from Fermi-LAT's all-sky γ-ray monitoring. They paved the way for the secure detections of fainter γray fluxes from three naked-eye novae enabled by Fermi ToO observations following their optical discoveries by the amateur community. The detection rate of γ-ray novae is ∼1 yr −1 over the first ∼7 yrs of the Fermi mission, consistent with the rate of Galactic novae at < ∼ 4 − 5 kpc distances, suggesting all novae are potential γ-ray sources [2]. The more recent detections may be revealing a wider diversity in γ-ray properties, being fainter and seemingly characterized by longer total durations. Further serendipitous and targeted γ-ray detections through the continued Fermi mission, as well as studies of non-LAT detected novae (e.g., [7]) could reveal the true range of γ-ray prop-  . Vertical bars indicate 1σ uncertainties for data points with significances > 2σ , and arrows indicate 2σ limits and the best-fit hadronic (from π 0 -decay) and leptonic (from inverse Compton and lower-level bremsstrahlung emission) model curves are overlaid. See [2] for details and for the plots for the other two LAT detected classical novae (V1324 Sco, V339 Del) and for the symbiotic nova V407 Cyg (see [1] as well). erties. Coordinated γ-ray and multi-wavelength observations can test scenarios for how and where the putative shocks are generated (e.g., internal shocks or strong turbulence driven in the ejecta [2], or in wind-wind interactions [15]) and point to the underlying γ-ray emission mechanism.
Fermi-LAT observations of the recurrent nova V745 Sco 2014 [37] indicated low significance, 2 − 3σ γ-ray emission on February 6 and 7, coincident with its optical outburst detected on February 6.694 [12], suggesting a connection between these events. Though not definitive, the short γ-ray duration would imply a'fast' analogue of V407 Cyg [4], with the (∼ 3×) larger distance of V745 Sco helping to explain the 5× fainter γ-ray peak flux observed. Such systems demonstrate the important role of the companions in the γ-ray production that has yet been fully explored in CNe. Of the known recurrent novae, one can anticipate future outbursts [31] with the immediate ones to watch being perhaps V3890 Sgr (expected in 2015 with some great uncertainty) and U Sco (2020 ± 2 years, again during the Fermi-era after its 2010 eruption).
In light of the Fermi-LAT discovery that classical novae can be bright continuum emitters at >100 MeV, the possibility that these spectra extend down to lower, ∼ 0.1 MeV energies is supported by the revisited Compton/OSSE observations of V382 Vel 1999. This should motivate early observations with INTEGRAL to search for both soft γ-ray continuum, in addition to nuclear lines, in future Fermi-novae. Current facilities that can probe the hard X-ray band like Suzaku, NuSTAR, and the upcoming ASTRO-H mission [38] will be invaluable, as well as future MeV missions (e.g., AstroMeV; http://astromev.in2p3.fr). Extrapolating their LAT spectra to higher energies suggest novae could be detected by CTA (see e.g., [5] for V407 Cyg), and can give crucial hints as to the emission mechanism. Simultaneity of such observations will be key, so perhaps then the most im- portant recurrent novae to anticipate for the Fermi mission in the CTA-era are RS Oph (2021? Its last explosion in 2006 did not have γ-ray coverage) and T CrB (2026?), a very nearby ∼ 0.8 kpc system that could produce a remarkably bright MeV-GeV-TeV γ-ray source, and a transient neutrino signal expected in the hadronic scenario [30] potentially detectable at even higher energies.