V5852 Sgr: An Unusual Nova Possibly Associated with the Sagittarius Stream
E. Aydi, P. Mróz, P. A. Whitelock, S. Mohamed, Ł. Wyrzykowski, A. Udalski, P. Vaisanen, T. Nagayama, M. Dominik, A. Scholz, H. Onozato, R. E. Williams, S. T. Hodgkin, S. Nishiyama, M. Yamagishi, A. M. S. Smith, T. Ryu, A. Iwamatsu, I. Kawamata
MMon. Not. R. Astron. Soc. , 1–11 (2015) Printed 5 November 2018 (MN L A TEX style file v2.2)
V5852 Sgr: An Unusual Nova Possibly Associated with theSagittarius Stream
E. Aydi , (cid:63) , P. Mr´oz , P. A. Whitelock , , S. Mohamed , (cid:32)L. Wyrzykowski ,A. Udalski ,P. Vaisanen , , T. Nagayama , M. Dominik , A. Scholz , H. Onozato , R. E. Williams ,S. T. Hodgkin , S. Nishiyama , M. Yamagishi , A. M. S. Smith , , T. Ryu , ,A. Iwamatsu , and I. Kawamata South African Astronomical Observatory, P.O. Box 9, 7935 Observatory, South Africa Astronomy Department, University of Cape Town, 7701 Rondebosch, South Africa Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland Southern African Large Telescope, P.O. Box 9, 7935 Observatory, South Africa Department of Physics and Astronomy, Graduate School of Science and Engineering, Kagoshima University, 1-21-35 Korimoto,Kagoshima 890-0065, Japan SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, KY169SS, UK. Astronomical Institute, Graduate School of Science, Tohoku University 6-3 Aramaki Aoba, Aoba ku, Sendai, Miyagi 980-8578, Japan Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA Institute of Astronomy, University of Cambridge, Madingley Rise, Cambridge, CB3 0HA, UK Miyagi University of Education, Aoba-ku, Sendai, Miyagi 980-0845, Japan Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan N. Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, 00-716, Warsaw, Poland Institute of Planetary Research, German Aerospace Center, Rutherfordstrasse 2, 12489 Berlin, Germany SOKENDAI, The Graduate University for Advanced Studies, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
Accepted: 08 June, 2016. Received ***; in original form 2015 December 30
ABSTRACT
We report spectroscopic and photometric follow-up of the peculiar nova V5852 Sgr(discovered as OGLE-2015-NOVA-01), which exhibits a combination of features fromdifferent nova classes. The photometry shows a flat-topped light curve with quasi-periodic oscillations, then a smooth decline followed by two fainter recoveries in bright-ness. Spectroscopy with the Southern African Large Telescope shows first a classicalnova with an Fe ii or Fe ii b spectral type. In the later spectrum, broad emissions fromhelium, nitrogen and oxygen are prominent and the iron has faded which could be anindication to the start of the nebular phase. The line widths suggest ejection velocitiesaround 1000 km s − . The nova is in the direction of the Galactic bulge and is heavilyreddened by an uncertain amount. The V magnitude 16 days after maximum enablesa distance to be estimated and this suggests that the nova may be in the extremetrailing stream of the Sagittarius dwarf spheroidal galaxy. If so it is the first nova tobe detected from that, or from any dwarf spheroidal galaxy. Given the uncertainty ofthe method and the unusual light curve we cannot rule out the possibility that it isin the bulge or even the Galactic disk behind the bulge. Key words: stars: binaries: close – novae, cataclysmic variables – white dwarfs.
Classical novae (CNe) form a subclass of the cataclysmicvariable stars. They are “close binary systems consisting (cid:63)
E-mail: [email protected] of a white dwarf (WD) and a Roche-lobe filling compan-ion” (Bode & Evans 2008). Matter accreted by the WDfrom the secondary accumulates and is compressed on theWD surface. When the critical temperature and density arereached a thermonuclear runaway (TNR) is triggered, caus-ing a nova eruption (Prialnik 1986). Typical ejecta veloci-ties and masses are ∼ − and between 10 − and c (cid:13) a r X i v : . [ a s t r o - ph . S R ] J un Aydi et al. − M (cid:12) , respectively (Gaposchkin 1957; Gallagher & Star-rfield 1978).Nova light curves are classified on the basis of their de-cline rate in various ways, but often in terms of time in days( t ) for the decline by 2 mag from maximum light (Payne-Gaposchkin 1964). More recently, Strope, Schaefer & Hen-den (2010) published an Atlas of light curves of 93 novaebased on visual estimates collected by AAVSO and they pro-posed a classification scheme based on the light curve shapeand rate of decline. They defined seven classes: S (smooth),P (plateau), D (dust dip), C (cusped secondary maximum),O (oscillations), F (flat-topped), and J (jitters). It is impor-tant to recognize that this classification is based on visuallight curves and given that the colours of novae typicallychange during their evolution we should not expect otherbands to track the visual curves with any degree of preci-sion. Nevertheless, this nomenclature has been used by oth-ers for I -band light curves of novae (e.g.Mr´oz et al. (2015)where they classified OGLE I -band light curves of 39 no-vae in the Galactic bulge using the same scheme) and wetherefore refer to it here.In addition to the photometric classification various ap-proaches have been taken to classify the spectral evolution ofnovae (McLaughlin 1944; Gaposchkin 1957; Williams 2012).Post-outburst spectra are divided into two spectroscopicclasses: the Fe ii type novae which show numerous narrowFe ii emission lines with P Cygni profiles and He/N type no-vae which show broad He, N and H emission lines. A thirdclass was also introduced, known as hybrid novae. Theseshow a transition between the two types (Fe ii and He/N) orsimultaneous emission lines of both types (Williams 1992).From a study of 22 Galactic novae, Della Valle & Livio(1998) established that novae showing He/N spectroscopicfeatures are faster, and have brighter maxima, compared tothose from the Fe ii class. The precise mechanism responsi-ble for the formation of the different spectral features afterthe explosion is poorly understood, but it is thought that theWD mass and the circumstellar environment play a crucialrole in this matter (Shafter et al. 2011).In this paper, we report the discovery of an unusualnova, V5852 Sgr, that may be a member of the trailingstream of the Sagittarius Dwarf Spheroidal Galaxy (hence-forth the Sagittarius stream). At about 20 kpc the Sagit-tarius Dwarf is our nearest neighbour and is being tidallydisrupted by its interaction with the Milky Way. It was dis-covered by Ibata, Gilmore & Irwin (1995) via the distinctivevelocities of its stars. It has since been shown to be stretchedinto a stream that loops around the Milky Way, with an es-timated total mass of about 5 × M (cid:12) (Majewski et al.2003), making it the largest dwarf spheroidal galaxy in theLocal Group.The nova shows combined features from different no-vae spectro-photometric classes and presents an interestingcase to study. In Section 2 we present the discovery and thephotometric (optical and IR) follow-up of the object. SALT(Southern African Large Telescope) spectroscopic opticaldata (between 4500 ˚A and 7800 ˚A) are detailed in Section 3.We present in Section 4. a discussion aiming to classify thenova, and establish its location and our conclusions. (a) Finding chart before the eruption.(b) Finding chart during the eruption (5 th of March 2015). Figure 1.
Finding charts for V5852 Sgr before and during theeruption in the I band. (North is up and East is to the left, imagesize 1 (cid:48) × (cid:48) ). V5852 Sgr was announced as a classical nova candidate andgiven the name OGLE-2015-NOVA-01, on 2015 March 5 byMr´oz & Udalski (2015) based on observations from the Opti-cal Gravitational Lensing Experiment (OGLE) survey. Thetransient was detected by the Early Warning System (EWS),which has been designed for the detection of microlensingevents. Fig. 1 presents charts of the nova before and dur-ing the eruption. From the time of the discovery, however,it was clear that the light curve (Fig. 2) does not resem-ble that of a microlensing event. The star is located in thedirection of the Galactic bulge at equatorial coordinates of( α, δ ) J2000 . = (17h:48m:12.78s, –32 ◦ :35’:13”.44) and Galac-tic coordinates of ( l, b ) = (357 ◦ .16, –2 ◦ .36). The source has been observed by the OGLE variability sur-vey since 2010 with a typical cadence of 1–2 d. This surveyuses the 1.3-m Warsaw Telescope located at Las Campanas c (cid:13) , 1–11 Observatory, Chile, operated by the Carnegie Institution forScience. Observations were taken through V - and I -bandfilters, closely resembling those of the standard Johnson-Cousins system. The photometry was carried out with theDifference Image Analysis (DIA) algorithm (Alard & Lupton1998; Won´ziak 2000). Details of reductions and calibrationscan be found in Udalski, Szyma´nski & Szyma´nski (2015).The light curve of the 2015 eruption data is shown in Fig. 2.In Tables 1 and 2 we present OGLE V - and I -band pho-tometry, respectively.Multicolor gri photometry was obtained using theLCOGT 1-m robotic telescope at the South African Astro-nomical Observatory (SAAO), Sutherland, South Africa on2015 March 19 and April 22-23. The data are presented inTable 3 and included in Fig. 2. The photometry was carriedout with DoPHOT (Schechter, Mateo & Saha 1993) andcalibrated with the aid of the AAVSO Photometric All-SkySurvey (APASS ) (Henden et al. 2009).The source was observed by the Swift satellite (Gehrelset al. 2004) on 2015 April 11, 15, and 20 with a total expo-sure time of 2.61 ks. No X-ray source was detected with theXRT (X-Ray Telescope; Burrows et al. 2005) at the posi-tion of the nova, which gives 3 σ upper limits of 0.022, 0.024,and 0.008 cts/s, respectively, in the 0.3-10 keV energy range.Upper limits were estimated with the sosta tool, which isa part of the XIMAGE package (version 4.5.1).We also analyzed images taken with the UVOT (Ultra-violet/Optical Telescope; Roming et al. 2005) on-board Swift and estimated the brightness of the source with the uvot-source tool, which performs aperture photometry. We de-tected a faint counterpart in the U -band (21 . ± .
58 mag),while the source was invisible in the
UV W (cid:48) × (cid:48) field of view, 0 (cid:48)(cid:48) .
45 pix-els and provides simultaneous
JHK S photometry. Expo-sures of 125 s were achieved by combining 25 dithered5 s exposures. Photometry on the 2MASS system wasperformed relative to the nearby stars 2MASS17481467–3235040 and 2MASS17481308–3235223. The rms of the in-strumental magnitudes of these reference stars was only σ J = 0 . , σ H = 0 . , σ K = 0 .
007 mag, demonstrat-ing that they do not vary.As can be seen in Fig. 1, the field is crowded, in par-ticular the nova is only 2 (cid:48)(cid:48) . J = 12 . H = 11 . K S = 10 . DAOPHOT in IRAF before the photometry was performed. The 2MASSmagnitudes of the reference stars were then used to derivethe zero points and an average taken. Photometry was doneusing apertures with radii of 5, 7 and 10 pixels. For mea-surements of
J, H <
14 and K S < . tures can differ by up to 0.2 mag. The measurements listedin Table 4 were made with a 7 pixel aperture. The eruption started between 2015 February 25 (HJD2457078.88) and 27 (HJD2457080.90), when the sourcebrightened by 2 mag from I = 19 . I = 17 .
6; by March3 (HJD 2457084.8) it had reached I = 14 .
4. This was fol-lowed by a short pre-maximum halt and then a nearly lin-ear rise by 1.5 mag in 14 days. The source peaked on 2015April 3 (HJD=2457115.87) at I = 12 . JHK S (Table 4) measurements were only ini-tiated when it was clear that the I light curve developmentwas unusual and, as can be seen in Fig. 2, the JHK S -curves,more or less follow the fall of the I curve. Fig. 3 shows thedevelopment of the infrared colours. The reddening vectorindicates that ( J − H ) is close to zero or slightly negativeand that it is remarkably constant, while ( H − K S ) getsslightly redder with time. A negative value for ( J − H ) hasbeen seen in other novae (Whitelock et al. 1984) and is at-tributed to very strong emission lines in the J band (theSAAO J filter used by Whitelock et al. is more sensitive tothe He I µ m line than is the IRSF J filter and thereforetheir J − H is very negative when this line is strong). Giventhat novae spectra are dominated by emission lines it is notpossible to properly transform photometry taken throughdifferent filters, so detailed comparisons of nova colours aredifficult.It is obvious that the overall development of the lightcurve (Fig. 2) is quite unlike any of the standard nova classesas can be seen by comparing it with the OGLE light curvesof bulge novae (Mr´oz et al. 2015). The decline at aroundHJD2457150 and the recovery around HJD2457230 couldpossibly have been the consequence of dust forming andgradually dispersing. However, Fig. 3 shows no indicationof the very red colours associated with significant quantitiesof dust. The colours of dust-forming novae are illustrated infig. 4 of Whitelock et al. (1984) for several other novae.We measured decline times in the I band over 2 and 3mags of t = 34 ± t = 45 ± I , and not V , it is notentirely clear how this should be interpreted.In the pre-eruption images we detected a possible veryfaint progenitor I pre = 20 .
77, which is invisible in the V -frames (as expected given the reddening discussed in sec-tion 2.4). We did not find any significant periodic variabilitybefore the eruption. c (cid:13)000
77, which is invisible in the V -frames (as expected given the reddening discussed in sec-tion 2.4). We did not find any significant periodic variabilitybefore the eruption. c (cid:13)000 , 1–11 Aydi et al.
HJD V ∆ V -2457000 (mag)110.84656 16.799 0.005131.79511 17.184 0.006138.84101 16.924 0.005163.78319 18.945 0.017 Table 1.
OGLE V -band photometry. The time-series photometryis available from the OGLE Internet Archive .HJD I ∆ I -2457000 (mag)66.86475 20.507 0.15370.89930 19.559 0.09274.85976 19.941 0.08975.83561 21.281 0.33576.90152 20.055 0.13578.87609 19.678 0.04380.89831 17.569 0.01083.80943 15.059 0.002 Table 2.
A sample of OGLE I photometry. The rest of the datacan be found on the electronic version and are available from theOGLE Internet Archive . In summary, our photometric data show a moderatelyfast outburst, followed by a flat-topped light curve with os-cillations and a smooth decline followed by two faint recov-eries in the brightness (between HJD 2457220 and 2457270and between 2457280 and 2457300). A comparison with thelight curve classes presented by Strope, Schaefer & Hen-den (2010) shows that the flat-topped light curve of V5852Sgr is similar to the F class light curves. In addition, thequasi-periodic oscillations during the peak in the intensityare similar to both J class and O class light curves. Thiscombination of features indicate the unusual light curve de-velopment of V5852 Sgr. Note that a flat-topped light curvewith oscillations is also seen for the 2011 light curve of therecurrent nova T Pyx. A detailed spectro-photometric studyby Surina et al. (2014) of the T Pyx 2011 outburst shows apre-maximum halt followed by a slow flat-topped outburstwith quasi-periodic oscillations of a period ∼ Given its position towards the bulge, we expect the novato be heavily reddened; determining exact values poses achallenge. At l = − o . b = − o . | l | < o , | b | < o ) for which Nishiyama et al. (2009) determinedan interstellar extinction law that is significantly differentfrom that generally assumed (Cardelli, Clayton & Mathis1989). Recently, Nataf et al. (2015) have shown that largevariations in the extinction ratios are common around theGalactic centre.The VVV extinction calculator, which is based on Gon-zalez et al. (2012), indicates E ( J − K S ) = 1 .
03 correspondingto A K = 0 . ± .
12 and A V = 6 .
44 mag on the Cardelli law ftp://ftp.astrouw.edu.pl/ogle/ogle4/NOVAE/BLG HJD Filter Exp. time Magnitude-2457000 (s)LCOGT100.60483 g
450 18.026 +/- 0.011100.61375 r
180 15.366 +/- 0.006100.61103 i
180 13.982 +/- 0.004134.54530 g
200 18.552 +/- 0.054134.54815 i
100 14.545 +/- 0.017134.55012 r
100 15.751 +/- 0.022134.57024 g
200 18.405 +/- 0.033134.60703 g
200 18.468 +/- 0.043134.60988 i
100 14.513 +/- 0.029134.61164 r
100 15.749 +/- 0.028134.64962 g
200 18.365 +/- 0.065134.65258 i
100 14.499 +/- 0.028134.65433 r
100 15.712 +/- 0.033135.49551 i
100 14.858 +/- 0.035135.52847 g
200 18.598 +/- 0.040135.53115 i
100 14.739 +/- 0.049135.53302 r
100 15.870 +/- 0.024136.41910 i
100 14.462 +/- 0.053136.42073 r
100 15.712 +/- 0.056
Swift
UV W > . a UV W > . a U a σ limit Table 3.
LCOGT and
Swift photometry of V5852 Sgr. or A K = 0 . ± .
12 and A V = 8 .
88 mag on the Nishiyamalaw. As an alternative approach we measured the centroidof the red giant clump ( V − I ) clump = 3 . ± .
03 on thecolor-magnitude diagram, using the OGLE data for starsin an area of 2 (cid:48) × (cid:48) around the nova. The intrinsic colorof the Galactic bulge red clump stars is 1.06 (e.g., Natafet al. 2013), so the color excess is E ( V − I ) = 2 . ± .
03 inthis direction. Using the extinction relations for the Galac-tic bulge from Nataf et al. (2013), we find A I = 3 . ± . A V = 6 . ± .
18 mag. Although this method es-timates the extinction foreground of the bulge red clumpstars, we can reasonably assume that most of the extinc-tion is in the foreground disc and therefore included. Thismethod should provide the best possible measure for theline-of-site to the nova and therefore in the following anal-ysis we assume: A V = 6 .
53 and A I = 3 .
60. It is importantto recognize that many of the conclusions of this paper arecritically dependent on this assumption.The distances to novae are often estimated from theirdecline rates, noting that fast novae are more luminousthan slow ones. However, the individual measurements haveconsiderable uncertainties and the validity of the so-called“maximum magnitude versus rate of decline” (MMRD) rela-tions was recently questioned by Kasliwal et al. (2011), Caoet al. (2012), and Shara et al. (2016). Furthermore, theserelations have not been defined in the I -band and no empir-ical relation between the rate of decline in I and V has beenderived (Munari private communication).A relationship sometimes used to derive distances isbased on the understanding that all novae have a similar V mag 15 days after maximum: M V = − ± .
44 (Downes& Duerbeck 2000). Coincidentally V5852 Sgr was measuredat V about 16 days after maximum (Table 1) with V = 17 . c (cid:13) , 1–11 Figure 2.
The photometric data from OGLE, LCOGT, and IRSF as a function of Heliocentric Julian Date (HJD), colour coded asindicated. The first two red bars indicate the dates when the eruption starts. The SALT spectra were obtained on April 22 and May 12. (making the reasonable assumption that maximum at V and I are close together in time), corresponding to V = 10 . m − M ) = 16 .
65 mag) in the range 17 to27 kpc. Because of the unusual light curve (section 2.3) andthe extension in the peak of brightness with quasi-periodicoscillations, this distance must be regarded as very uncer-tain.There are several possible explanations of the large dis-tance estimate. Most obviously it suggests the nova is be-hind the bulge, rather than in it. At about 22 kpc it wouldbe about the right distance to be associated with the Sagit-tarius stream (Ibata, Gilmore & Irwin 1995). Although itis a few degrees away from where the Sagittarius streamcrosses the Galactic plane, it is within the area where theOGLE group found RR Lyr variables that are members ofthe Sagittarius stream (see fig. 3 in Soszy´nski et al. (2014)).The nova is not coincident with the main density of OGLEvariables (with distances from Pietrukowicz et al. 2015), butit is plausibly within the volume occupied by the trailingstream from the Sagittarius dwarf which can be seen fromTorrealba et al. (2015) when their fig. 16 is interpolatedthrough the Galactic plane. Furthermore, the radial veloc-ity (see Section 3.3) although uncertain is consistent withmembership of the Sagittarius stream.We need to consider other possible explanations as al-ternatives to membership of the Sagittarius stream. First itcould be in the Galactic disk behind the bulge; at 22 kpc itis less than 1 kpc above the plane. This is discussed furtherin Section 3.3 in the context of the nova’s radial velocity.Secondly, given its peculiar light curve this nova may not follow the usual decay-rate luminosity relations and there-fore could be in the bulge. Thirdly, because of the peculiarityof the reddening law discussed above it is possible that theinterstellar extinction at V is higher than the A V = 6 . A V = 8 . The nova was observed on 2015 April 22 and May 12, usingthe Robert Stobie Spectrograph (RSS; Burgh et al. 2003;Kobulnicky et al. 2003), mounted on the Southern AfricanLarge Telescope (SALT) situated at the SAAO, Sutherland,South Africa. The observation on 2015 April 22 consists oftwo spectral ranges; [4500 ˚A – 5850 ˚A] and [5800 ˚A – 7000˚A]. The RSS long-slit mode was used with a 0 . (cid:48)(cid:48) slit at a res-olution of R ∼ c (cid:13) , 1–11 Aydi et al.
Table 4.
Near-infrared photometry from IRSF at
JHK S bands.HJD J ∆ J H ∆ H K S ∆ K S -2457000 (mag)134.52332 11.21 0.02 10.45 0.01 9.91 0.01137.49937 10.45 0.02 9.64 0.01 9.15 0.01138.46439 10.67 0.03 9.81 0.02 9.25 0.01139.55240 10.49 0.02 9.67 0.01 9.14 0.01140.40741 10.52 0.02 9.69 0.01 9.15 0.01143.61646 11.19 0.02 10.47 0.01 9.91 0.01145.55649 11.43 0.03 10.76 0.01 10.20 0.01146.50150 11.75 0.03 11.14 0.02 10.52 0.01147.43051 11.91 0.03 11.28 0.02 10.65 0.01149.66654 12.18 0.03 11.51 0.02 10.87 0.01152.49157 12.51 0.03 11.82 0.02 11.15 0.01155.45161 12.77 0.03 12.10 0.02 11.38 0.02158.57664 13.05 0.04 12.37 0.02 11.62 0.02161.62666 13.41 0.04 12.77 0.03 11.85 0.02162.64767 13.40 0.04 12.74 0.03 11.88 0.02163.60868 13.34 0.04 12.68 0.02 11.82 0.02165.41369 13.50 0.04 12.85 0.03 11.95 0.02167.57171 13.72 0.06 13.03 0.04 12.12 0.03168.65172 13.70 0.04 13.01 0.03 12.10 0.02171.65273 13.79 0.05 13.11 0.03 12.25 0.02193.51274 15.17 0.07 14.40 0.05 13.52 0.04195.61862 15.46 0.08 14.64 0.07 13.65 0.05196.61061 15.43 0.08 14.64 0.07 13.69 0.05201.53558 15.68 0.09 14.86 0.08 14.03 0.06203.59956 15.74 0.09 15.12 0.09 14.04 0.06204.60055 16.00 0.11 15.01 0.09 14.02 0.06205.31855 15.89 0.10 14.89 0.07 13.99 0.06206.37154 15.90 0.10 15.01 0.08 13.96 0.06207.34053 15.81 0.09 15.03 0.08 14.01 0.06209.26551 15.83 0.09 15.04 0.09 14.07 0.06210.41849 15.82 0.10 14.94 0.08 14.07 0.06214.23445 15.81 0.13 14.93 0.09 14.09 0.09217.27441 15.81 0.10 15.07 0.08 14.01 0.06218.19040 16.17 0.14 15.02 0.08 14.06 0.06240.39200 15.62 0.08 14.89 0.08 13.96 0.06241.47998 15.78 0.10 15.09 0.09 13.99 0.06242.31996 15.75 0.09 14.94 0.08 13.99 0.06243.40194 15.98 0.10 15.05 0.08 14.13 0.06245.38289 15.95 0.10 15.16 0.08 14.18 0.07246.43287 16.58 0.27 15.28 0.14 14.37 0.13249.44780 16.38 0.15 15.26 0.10 14.34 0.08250.45178 16.23 0.13 15.39 0.10 14.45 0.08259.29456 16.66 0.16 15.86 0.15 14.96 0.12260.31153 16.88 0.19 15.71 0.14 14.78 0.10263.28446 16.85 0.19 16.29 0.22 14.97 0.12 times of (4 ×
100 s) and (2 ×
150 s), respectively. The poorweather conditions and seeing resulted in a limited S/N.The May 12 observation was carried out, under goodseeing ( ∼ . (cid:48)(cid:48) ). The data cover three spectral ranges, [4500˚A – 5450 ˚A], [5400 ˚A – 6750 ˚A], and [6700 ˚A – 7850 ˚A].The RSS long-slit mode was used with the same narrowslit as above resulting in a resolution of R ∼ R ∼ ×
200 s), (4 × ×
100 s), respectively. The spectra are reducedand calibrated using the PySALT pipeline (Crawfordet al. 2010). The images are combined, the background issubtracted, and the spectra are extracted using the IRAF(Image Reduction and Analysis Facility) software (Tody
Figure 3.
Evolution of the
JHK S colours; asterisks, open cir-cles and closed circles represent observations before HJD 2457150,between HJD 2457150 and 2457200 and after HJD 2457200, re-spectively. The dotted vector shows the effect of correcting forreddening assuming E ( J − H ) = 0 .
69 and E ( H − K S ) = 0 .
34 (seesection 2.4)
In Figs. 4 to 8, we show the smoothed spectra, where the topspectrum represents the April 22 observation and the bot-tom spectrum represents the May 12 observation. The spec-tral lines were identified mostly using the list from Williams(2012). The only clear absorption features in either spec-trum are telluric. The April 22 spectra show several broadflat-topped Fe ii , H i , and N ii emission lines. However, in theMay 12 spectra, the Fe ii lines become weaker and almostdisappear, in contrast to the He and N lines that becomestronger. H β (Fig.4) and to some extent H α (Fig.7), showa double peek. These could be the consequence of bipolarejection or an optically thin shell of gas (see e.g. models ofNova Eri 2009 by Ribeiro et al. (2013)). Further in the red,between 6750 ˚A and 7850 ˚A, the May 12 spectrum showsstrong He, O and possibly C emission lines. The Balmer lines have asymmetric profiles, similar to othernovae in the transition stage. In the May spectrum H α andH β have double peaks with the red peak stronger than theblue one; [N ii ] 5755 ˚A, O i ii α , H β , and [N ii ] 5755 ˚A lines. Wefound that for H α the FWHM ∼ ±
200 km s − , for H β the FWHM ∼ ±
200 km s − , and for [N ii ] 5755 ˚A theFWHM ∼ ±
200 km s − .In order to derive the radial velocity of the nova, wemeasured the H α , H β , Fe ii ii ii c (cid:13) , 1–11 ˚A, and [N ii ] 5755 ˚A emission lines for the April 22 observa-tion. We also measured the H α , H β , N ii ii ] 5755˚A, C ii i ii ] 7319,7330 ˚A is a blend, so we use only N ii ii ] 5755 ˚A, and C ii ∼ ±
50 km s − .The determination of novae radial velocities is notstraight forward since many factors can affect it, includ-ing the WD orbital motion, optical depth effects, and mostimportantly the asymmetry of the ejecta (Williams 1994).In both observations, the lines are shifted towards the redwhich would not be expected from optically thick ejecta.It is not entirely clear why the radial velocity changesbetween the two epochs. Although both spectra appear tolack P Cygni absorptions it is possible that the first one wasnot optically thin and that absorption influences the blueside of the lines shifting them to the red, although the sizeof the shift makes this unlikely. It is also possible to speculatethat the eruption was very asymmetric and that what we seein the first spectrum is a redshifted clump of material, thatlater slows, although such an explanation seems contrived.The velocity of the Sagittarius stream has not been mea-sured in the direction of the nova, but the central part ofthe Sagittarius Dwarf Spheroidal galaxy has a mean veloc-ity of 140 km s − with a velocity dispersion of only 10 km s − (Ibata, Gilmore & Irwin 1995). The mean measurement fromthe more reliable lines in the second spectrum is consistentwith this value. On the other hand the Galactic bulge hasa high velocity dispersion and it is certainly not possible touse the velocity to rule out membership of the bulge.Williams (1994) discusses the radial velocities of Galac-tic novae that are mostly within the solar circle and in thedirection of the bulge. These have systematically large nega-tive velocities and, as Williams points out, this indicates thateither high internal absorption skews their emission lines tobluer velocities or most of the novae are moving out fromthe Galactic centre. It is much more difficult to skew thelines of a nova to the red, so if our nova is in the plane, be-hind the bulge, it would be moving away from the Galacticcentre. That possibility cannot be ruled out, but it seemsmore likely that it is in the bulge, or the Sagittarius stream. Table 5.
Heliocentric radial velocities of the emission lines forthe April 22 and May 12 observations. The uncertainty for all thevalues is ±
50 km s − .Line V rad (April 22) V rad (May 12)(km s − )H α
300 155H β
320 140Fe ii ii ii ii ] 5755 ˚A 150 45C ii i The spectra of V5852 Sgr leave no doubt that this is a CN.CNe are divided into two spectroscopic classes (Fe ii andHe/N). The two novae spectral classes have distinctly dif-ferent properties. He/N novae exhibit broad, high ionizationlines, rectangular profiles, few absorption features if any, anda rapidly decreasing visible luminosity. The line broadeningis due to high expansion velocities. The spectra are domi-nated by He, N, and H emission lines. The prominent Heand N transitions, the line widths and the rectangular lineprofiles, are all attributed to emission from high velocity gas(WD ejecta) (Williams 2012).Fe ii novae have spectra characterized by low excitationnarrow Fe ii , Na i lines, and CNO lines in the far red. Theyalso show prominent P Cygni absorption features character-istic of an optically thick expanding gas. The class of Fe ii can be divided into two sub-classes Fe ii n (narrow) and Fe ii b (broad) where respectively narrow or broad emission linesare present in the spectrum. Although no P Cygni absorp-tion features are seen in either spectra, the April 22 spec-trum presents features and lines that characterize the Fe ii spectroscopic class. A moderate FWHM of ∼ − and the flat-topped broad lines favors the possibility of aFe ii b (broad) spectroscopic class. The May 12 spectrumshows several N lines which might indicate a transition intoHe/N class. However the absence of some helium lines, (He i & CONCLUSIONS
V5852 Sgr was reported as a possible classical nova by theOGLE sky survey on 2015 March 5. Putting together thenova distance, luminosity, and radial velocity with theiruncertainties, leaves the membership of the nova open tothree possibilities. The first puts the nova in the Sagittar-ius stream and the second puts it in the plane behind thebulge moving away from the Galactic centre and the thirdputs it in the Galactic bulge (see Section 3.3). It is worthnoting that the mass of the Sagittarius dwarf is 10 – 10 M (cid:12) so the expected nova rate in that galaxy will only beone per 10 to 100 years. The nova rate in the bulge is ∼ c (cid:13)000
V5852 Sgr was reported as a possible classical nova by theOGLE sky survey on 2015 March 5. Putting together thenova distance, luminosity, and radial velocity with theiruncertainties, leaves the membership of the nova open tothree possibilities. The first puts the nova in the Sagittar-ius stream and the second puts it in the plane behind thebulge moving away from the Galactic centre and the thirdputs it in the Galactic bulge (see Section 3.3). It is worthnoting that the mass of the Sagittarius dwarf is 10 – 10 M (cid:12) so the expected nova rate in that galaxy will only beone per 10 to 100 years. The nova rate in the bulge is ∼ c (cid:13)000 , 1–11 Aydi et al.
Figure 4.
The flux plotted to arbitrary units between 4440 ˚A and 5450 ˚A. The April 22 spectrum (upper) shows several broad Fe ii emission lines and H β line. However, in the May 12 spectrum (lower), the Fe II lines become weaker and almost disappear in contrastto the He and N that start to appear. The H β line shows a double-peak. For clarity, the April 22 spectrum is vertically shifted. In theApril 22 spectrum, chip gaps are between: 4927.2 ˚A and 4953.4 ˚A and between: 5409.0 ˚A and 5433.9 ˚A. In the May 12 spectrum, thechip gaps are between: 4790.9 ˚A and 4810.0 ˚A and between: 5140.5 ˚A and 5158.4 ˚A. Figure 5.
As in Fig. 4, butbetween 5400 ˚A and 5850 ˚A. The April 22 spectrum (upper) shows broad and relatively weak N ii ii ] 5755 ˚A line. Inthe May 12 spectrum (lower), the [N ii ] line becomes stronger compared to the N II line. For clarity, the April 22 spectrum is verticallyshifted. characteristics of novae with this type of light curve. Themembership to the Sagittarius dwarf galaxy would make itthe first nova to be discovered in a dwarf spheroidal galaxy.The photometric follow up (OGLE, IRSF, LCOGT) revealeda peculiar nova light curve. The spectroscopic and photomet-ric data revealed combined features rarely observed in CNe.The nature of V5852 Sgr is best described as an unusualone. Based on our spectra and the classification criteria of Williams (1992) V5852 Sgr shows a Fe ii or Fe ii b spectro-scopic class in a transition to the nebular phase. Trackingthe spectroscopic and photometric evolution of the object isessential for a definitive classification. If the object is a clas-sical, moderately fast nova, the same spectroscopic featuresshould remain until the nebular phase develops. A detailedstudy of the elemental abundances is also essential. Hence,further observations are needed to definitively identify the c (cid:13) , 1–11 Figure 6.
As in Fig. 4, but between 5850 ˚A and 6500 ˚A. No strong emission lines are present in this spectral range. For clarity, the April22 spectrum is vertically shifted. In the April 22 spectrum, the chip gap is between: 6242.8 ˚A and 6267.1 ˚A. In the May 12 spectrum,the chip gaps are between: 5877.9 ˚A and 5902.7 ˚A and between: 6332.6 ˚A and 6355.9 ˚A.
Figure 7.
As in Fig. 4, but between (a) 6480 ˚A and 6640 ˚A, (b) 6620˚A and 6750 ˚A. The April 22 spectrum (upper) shows broad H α emission line (FWHM ∼ − ±
200 km s − ). In the May 12 spectrum (lower), the H α line shows a double-peak similar to theone of H β . Both [N ii ] 6548 ˚A and [N ii ] 6584 ˚A are expected to be present but merged with the broad H α . For clarity, the April 22spectrum is vertically shifted. In the April 22 spectrum, the chip gap is between: 6685.5 ˚A and 6708.1 ˚A. object and understand the physical mechanism responsiblefor this explosion. ACKNOWLEDGMENTS
A part of this work is based on observations made withthe Southern African Large Telescope (SALT), under theprogram 2014-2-DDT-006, and the IRSF, and makes use ofobservations from the LCOGT network. The IRSF projectis a collaboration between Nagoya University and the SouthAfrican Astronomical Observatory (SAAO) supportedby the Grants-in-Aid for Scientific Research on PriorityAreas (A) (No. 10147207 and No. 10147214) and Optical& Near-Infrared Astronomy Inter-University CooperationProgram, from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan and the NationalResearch Foundation (NRF) of South Africa. EA, PAW,SM, and PV gratefully acknowledge the receipt of researchgrants from the National Research Foundation (NRF) ofSouth Africa. We are grateful to Steve Crawford, MarissaKotze, and Brent Miszalski for assistance with the SALTobservations and We acknowledge helpful discussions withJoanna Miko(cid:32)lajewska, Krystian I(cid:32)lkiewicz, Katarzyna Drozdand Massimo Della Valle. We thank Ulisse Munari for hisinsight on the relative decay rates of novae in the I and V bands and Nikolai Samus for communicating the variablestar name of this nova in advance of publication.P.M. is supported by the ”Diamond Grant” No.DI2013/014743 funded by the Polish Ministry of Sci-ence and Higher Education. We thank the Swift
PI, NeilGehrels, for an allocation of ToO time. This research was c (cid:13) , 1–11 Aydi et al.
Figure 8.
As in Fig. 4, but between 6750 ˚A and 7850 ˚A. The May 12 spectrum shows strong C, He and O emission lines. In the May12 spectrum, the chip gaps are between: 7111.8 ˚A and 7134.4 ˚A and between: 7521.4 ˚A and 7542.1 ˚A. made possible through the use of the AAVSO PhotometricAll-Sky Survey (APASS), funded by the Robert MartinAyers Sciences Fund. The OGLE project has receivedfunding from the National Science Center, Poland, grantMAESTRO 2014/14/A/ST9/00121 to A.U.This work makes use of observations from the LCOGTnetwork, which includes three SUPAscopes owned by theUniversity of St Andrews. The Gaia transient follow-upprogram uses equal network time allocations from the Uni-versity of St Andrews and the South African AstronomicalObservatory (SAAO).
REFERENCES
Alard C., Lupton R. H., 1998, ApJ, 503, 325Bode M. F., Evans A., 2008, Classical NovaeBurgh E. B., Nordsieck K. H., Kobulnicky H. A., WilliamsT. B., O’Donoghue D., Smith M. P., Percival J. W., 2003,in Society of Photo-Optical Instrumentation Engineers(SPIE) Conference Series, Vol. 4841, Instrument Designand Performance for Optical/Infrared Ground-based Tele-scopes, Iye M., Moorwood A. F. M., eds., pp. 1463–1471Burrows D. N. et al., 2005, Space Sci. Rev., 120, 165Cao Y. et al., 2012, ApJ, 752, 133Cardelli J. A., Clayton G. C., Mathis J. S., 1989, ApJ, 345,245Crawford S. M. et al., 2010, in Society of Photo-OpticalInstrumentation Engineers (SPIE) Conference Series, Vol.7737, Society of Photo-Optical Instrumentation Engineers(SPIE) Conference Series, p. 25Della Valle M., Livio M., 1998, ApJ, 506, 818Della Valle M. D., 2002, in American Institute of PhysicsConference Series, Vol. 637, Classical Nova Explosions,Hernanz M., Jos´e J., eds., pp. 443–456Downes R. A., Duerbeck H. W., 2000, AJ, 120, 2007 Gallagher J. S., Starrfield S., 1978, ARA&A, 16, 171Gaposchkin C. H. P., 1957, The galactic novae.Gehrels N. et al., 2004, ApJ, 611, 1005Gonzalez O. A., Rejkuba M., Zoccali M., Valenti E., Min-niti D., Schultheis M., Tobar R., Chen B., 2012, A&A,543, A13Hanuschik R. W., 2003, A&A, 407, 1157Henden A. A., Welch D. L., Terrell D., Levine S. E., 2009, inAmerican Astronomical Society Meeting Abstracts, Vol.214, American Astronomical Society Meeting Abstracts c (cid:13) , 1–11 Nishiyama S., Tamura M., Hatano H., Kato D., Tanab´e T.,Sugitani K., Nagata T., 2009, ApJ, 696, 1407Payne-Gaposchkin C., 1964, The galactic novaePietrukowicz P. et al., 2015, ApJ, 811, 113Prialnik D., 1986, ApJ, 310, 222Ribeiro V. A. R. M., Bode M. F., Darnley M. J., BarnsleyR. M., Munari U., Harman D. J., 2013, MNRAS, 433,1991Roming P. W. A. et al., 2005, Space Sci. Rev., 120, 95Schechter P. L., Mateo M., Saha A., 1993, PASP, 105, 1342Shafter A. W. et al., 2011, ApJ, 734, 12Shara M. M., Doyle T., Lauer T. R., Zurek D., Neill J. D.,Madrid J. P., Welch D. L., Baltz E. A., 2016, ArXiv:1602.00758Soszy´nski I. et al., 2014, Acta Astron., 64, 177Strope R. J., Schaefer B. E., Henden A. A., 2010, AJ, 140,34Surina F., Hounsell R. A., Bode M. F., Darnley M. J.,Harman D. J., Walter F. M., 2014, AJ, 147, 107Tody D., 1986, in Society of Photo-Optical InstrumentationEngineers (SPIE) Conference Series, Vol. 627, Instrumen-tation in astronomy VI, Crawford D. L., ed., p. 733Torrealba G. et al., 2015, MNRAS, 446, 2251Udalski A., Szyma´nski M. K., Szyma´nski G., 2015, ActaAstron., 65, 1Warner B., 2008, in Classical Novae, Bode M. F., EvansA., eds., pp. 16–33Whitelock P. A., Carter B. S., Feast M. W., Glass I. S.,Laney D., Menzies J. W., Walsh J., Williams P. M., 1984,MNRAS, 211, 421Williams R. E., 1992, AJ, 104, 725Williams R. E., 1994, ApJ, 426, 279Williams R. E., 2012, AJ, 144, 98Won´ziak P. R., 2000, Acta Astron., 50, 421 c (cid:13)000