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Draft version January14, 2016 PreprinttypesetusingLATEXstyleemulateapjv.01/23/15 RAPIDLY RISING TRANSIENTS FROM SUBARU HYPER SUPRIME-CAM TRANSIENT SURVEY Masaomi Tanaka1,2, Nozomu Tominaga3,2, Tomoki Morokuma4,2, Naoki Yasuda2, Hisanori Furusawa1, Petr V. Baklanov5,6,7, Sergei I. Blinnikov5,2,8, Takashi J. Moriya9, Mamoru Doi4,2, Ji-an Jiang4, Takahiro Kato10, Yuki Kikuchi4, Hanindyo Kuncarayakti11,12, Tohru Nagao13, Ken’ichi Nomoto2,14, and Yuki Taniguchi4 Draft version January 14, 2016 6 ABSTRACT 1 We present rapidly rising transients discovered by a high-cadence transient survey with Subaru tele- 0 2 scope andHyper Suprime-Cam. We discoveredfive transientsat z =0.384−0.821showingthe rising rate faster than 1 mag per 1 day in the restframe near-ultraviolet wavelengths. The fast rising rate n and brightness are the most similar to SN 2010aq and PS1-13arp, for which the ultraviolet emission a within a few days after the shock breakout was detected. The lower limit of the event rate of rapidly J rising transients is ∼ 9% of core-collapse supernova rates, assuming a duration of rapid rise to be 1 1 day. We showthatthe lightcurvesofthe threefaintobjects agreewith the coolingenvelope emission 1 from the explosion of red supergiants. The other two luminous objects are, however, brighter and faster than the cooling envelope emission. We interpret these two objects to be the shock breakout E] from dense wind with the mass loss rate of ∼ 10−3 M⊙ yr−1, as also proposed for PS1-13arp. This mass loss rate is higher than that typically observed for red supergiants. The event rate of these H luminous objects is >1%of core-collapsesupernovarate, andthus, ourstudy implies thatmore than ∼ . h ∼1% of massive stars can experience an intensive mass loss at a few years before the explosion. p Keywords: supernovae: general - o r t 1. INTRODUCTION such short-timescale transients. For supernovae (SNe), s shock breakout emission should have timescale of ∼ 1 a The transient sky has been intensively explored by hr for the case of red supergiant progenitors (e.g., Falk [ various surveys in the last decade. Especially, op- 1978; Klein & Chevalier 1978; Matzner & McKee 1999). tical surveys using wide-field cameras, such as Palo- 1 The subsequent cooling emission lasts for a few days marTransientFactory(PTF,Law et al.2009;Rau et al. v (e.g., Waxman et al. 2007; Chevalier & Fransson 2008; 2009), Catalina Real-Time Transient Survey (CRTS, 1 Nakar & Sari 2010). For the case of blue supergiants Drake et al. 2009), and Pan-STARRS1 (PS1, e.g., 6 or Wolf-Rayet stars, these timescale are even shorter. Kaiser et al. 2010), have significantly contributed to 2 Otherpossible short-timescaletransientsinclude, forex- 3 building our knowledge on the transient phenomena in ample, the disk outflow from black hole forming SNe (< 0 the Universe. a few days, Kashiyama & Quataert 2015) and accretion . Oneoftheimportantdiscoveryspacesfortransientsur- 1 veys is phenomena with a short timescale, i.e., < 1 day. inducedcollapseofwhitedwarfs(∼1day,Metzger et al. 0 ∼ 2009). Inadditiontothese,theremightalsobeunknown There are, in fact, several theoretical expectations for 6 kindoftransientswitha shortdurationsinceourknowl- 1 1NationalAstronomicalObservatoryofJapan,Mitaka,Tokyo edge on the short-timescale transients is still limited. v: 1812-K85a8v8li,IJnasptaitnu;[email protected] icaTtoedexhpiglohr-ecathdeenscheorstu-trivmeyesschaalevetrsatnasriteendt.skFyo,rsoexmaemdpelde-, i X verse(WPI),TheUniversityofTokyo,Kashiwa,Chiba277-8583, Kiso Supernova Survey (KISS, Morokuma et al. 2014; Japan r 3Department ofPhysics,FacultyofScienceandEngineering, Tanaka et al. 2014, using 1.05m Schmidt telescope and a KonanUniversity,Kobe,Hyogo658-8501, Japan ∼4 deg2 wide field camera, Sako et al. 2012) and High- 4Institute of Astronomy, Graduate School of Science, The cadence TransientSurvey (HiTS, Forster et al. 2014, us- UniversityofTokyo,Mitaka,Tokyo181-0015,Japan 5Institute forTheoretical andExperimental Physics(ITEP), ing4mBlancotelescopeand∼3deg2DarkEnergyCam- BolshayaCheremushkinskaya 25,117218Moscow,Russia era,Flaugher et al.2015)adopt∼1hrcadenceaimingat 6NovosibirskStateUniversity,Novosibirsk630090, Russia thedetectionofSNshockbreakout. Therearealsosome 7National Research Nuclear University MEPhI, 115409 ambitious surveys to explore even shorter timescales Moscow,Russia 8All-Russia Research Institute of Automatics (VNIIA), (e.g., Becker et al. 2004; Rau et al. 2008; Berger et al. 127055 Moscow,Russia 2013), although no extragalactic transients with < 30 9ArgelanderInstituteforAstronomy,UniversityofBonnAuf min timescale have been detected. ∼ demHügel71,D-53121Bonn,Germany 10Department of Physics, The University of Tokyo, Tokyo Recently, we have started a high-cadence transient 113-0033, Japan survey with the 8.2m Subaru telescope and 1.77 deg2 11Millennium Institute of Astrophysics, Casilla 36-D, Santi- Hyper Suprime-Cam (HSC, Miyazaki et al. 2006, 2012), ago,Chile 12DepartamentodeAstronomía,UniversidaddeChile,Casilla as a part of Subaru HSC Survey Optimized for Opti- cal Transients (SHOOT). SHOOT also adopts ∼ 1 hr 36-D,Santiago, Chile 13Research Center for Space and Cosmic Evolution, Ehime cadence focusing on the detection of SN shock break- University, Matsuyama790-8577, Japan out (Tominaga et al. 2015a). In this paper, we present 14HamamatsuProfessor 2 Tanaka, M., et al. Table 1 Table 2 Logofobservations Classificationofdetected sources UT Epoch Instrument mode seeinga Classification (arcsec) Numberofsources 2014-07-02 Day1 HSC imaging(g,r) 0.5 Total 2014-07-03 Day2 HSC imaging(g,r) 0.6 412 2014-08-05 Day35 FOCAS imaging(g,r) 0.9 Fakea Astronomicalobjects spectroscopy 215 197 2014-08-06 Day36 FOCAS imaging(g,r) 0.9 Star/quasarb Non-star spectroscopy 166 31 2015-05-24 Day327 HSCb imaging(g,r) 1.0 Center Offset 2015-06-22 Day356 FOCAS spectroscopy 0.5 16(8c) 15(1c) 2015-08-19 Day414 HSCb imaging(r) 1.4 Note. — a Non-astronomical sources Note. — a Fullwidthathalfmaximum. b Usedasreference such as bad image subtraction, bad refer- images. ence, or cosmic ray events. b Point sources (including moving objects with a negligible rapidly rising transients discovered in SHOOT. Here we motion). c Number of declining objects in define rapidly rising transients as objects that rise more thesamples. than 1 mag within restframe 1 day, i.e., the rising rate convolution kernel is derived by minimizing the differ- |∆m/∆t| > 1 mag day−1. We describe our observations ence between convoluted PSFs of two images. Although andsampleselectioninSection2. Then,wecomparethe our 7 survey fields are selected based on the availability obtained light curves with previously known SNe and of the past imaging data, most of the survey fields lack transients in Section 3. Rising rates of various types imaging data that are deep and wide enough to be used of transients are summarized in Section 4. Based on asreferencesforournewHSCimages. Thus,weusedthe these comparison, we discuss the nature of these tran- data taken at the first visit of Day 1 as reference images sients in Section 5. Finally we give conclusions in Sec- for sample selection. tion 6. Throughout the paper, we assume the following The data reduction described above was performed cosmological parameters: ΩM = 0.273, ΩΛ = 0.726, and in realtime using the on-site data analysis system H0 =70.5kms−1 Mpc−1 (Komatsu et al.2009). Allthe (Furusawa et al. 2011) and a dedicated transient system magnitudes are given in AB magnitude. (Tominaga et al. 2015a). By using these systems, transient candidates were typically selected within the same 2. OBSERVATIONSANDSAMPLESELECTION night (Tominaga et al. 2014a,b, 2015b,c). 2.1. HSC observations Toobtainthefinalreferenceimages,wealsoperformed We performed a high-cadence transient survey with HSCimagingobservationson2015May24UT(Day327, Subaru/HSC for two continuous nights, 2014July 2 and for g- and r-band) and 2015 Aug 19 UT (Day 414, for 3 UT (hereafter Day 1 and 2, respectively). The log r-band). All the photometric values given in this paper of our observations is given in Table 1. Seven field-of- are derived by aperture photometry with 7 pixel radius views (≃ 12 deg2) were repeatedly visited with about 1 (1.18 arcsec) in the difference images using these final hrcadence. Oursurveywascarriedoutmostlyinoptical reference images. g-band,targetingthedetectionoftheveryearlyphaseof 2.2. Sample selection SNe (Tominaga et al. 2015a). Within one night, we had 3 or 4 visits in g-band (here one “visit” consists of five We adopted the following selection processes to select 2-minexposures). We alsotook 1visitdata inr-band in candidates for rapidly rising transients. As mentioned each night to obtain g−r color. above, we used the first images taken on Day 1 as refer- The HSC data were reduced using the HSC pipeline ence images for the selection process. Therefore, source (version 3.6.1) developed based on the LSST pipeline detection in the subtracted images is sensitive only to (Ivezic et al. 2008; Axelrod et al. 2010). After standard objects showing variability within 2 nights. reduction for each frame, 5 exposure images were co- Detected sources in the subtracted images containnot added. For astrometry and photometric calibration, onlyrealastronomicalsourcesbutalsofakesourcessuch we used the Sloan Digital Sky Survey DR8 catalog as spikes around bright stars, and artifacts due to mis- (Aihara et al. 2011). For stacked images for 1 visit (i.e., subtraction or mis-alignment (e.g., Bailey et al. 2007; 10 min exposure), a typical limiting magnitude is about Bloom et al. 2012; Brink et al. 2013). Thus, we selected 26 mag (5 sigma limiting magnitude for point sources) objects detected in the subtracted images at least twice in both g- and r-bands. with>5σ significance. Afterthisselection,1407sources We performed image subtraction using the HSC remain. We first performed initial visual screening, re- pipeline. The pipeline adopts the algorithm developed sulting in 430 sources with SHOOT14XX names (412 by Alard & Lupton (1998) and Alard (2000), which are independent sources because of 18 duplication in over- used for the ISIS package15 and the HOTPANTS pack- lappedregionsin the reducedimages). Then, we further age16. The algorithm uses a space-varying convolution performed detailed classification. Results of the classifi- kernel to match the PSFs of two images. The optimal cations are summarized in Table 2. Among 412 independent sources, 215 sources are still 15 http://www2.iap.fr/users/alard/package.html fakesofthesubtractedimageswhiletheother197sources 16http://www.astro.washington.edu/users/becker/v2.0/hotpants.ahtrmellikely to be astronomical sources. The astronomical Rapidly Rising Transients from Subaru/HSC Transient Survey 3 SHOOT14gp SHOOT14or SHOOT14ha SHOOT14jr SHOOT14ef Discovery Discovery Discovery Discovery Discovery Reference Reference Reference Reference Reference Difference Difference Difference Difference Difference Figure 1. Images of rapidly rising transients (g- and r-band two-color composite images). From top to bottom, each panel shows the discoveryimagestakenonDay2,imagestakenonDay1(usedasreferencesforthesampleselection),anddifferenceimages(Day2−Day 1). Eachpanelhas8′′×8′′ size. Northisupandeastisleft. Thecolorscaleforthediscoveryandreferenceimagesaresettobethesame. Table 3 Rapidlyrisingtransients fromSubaru/HSC transientsurvey Object R.A. Decl. Redshift |∆m/∆t|a (J2000.0) (J2000.0) (magday−1) SHOOT14gp 23:20:20.80 +28:25:00.54 0.635 >3.10 SHOOT14or 15:26:24.18 +47:47:07.34 0.821 3.12+−10..1710 SHOOT14ha 23:21:44.91 +28:54:49.80 0.548 >1.19 SHOOT14jr 16:33:49.99 +34:28:05.36 0.384 1.61+−00..3392 SHOOT14ef 21:31:08.77 +09:32:54.10 0.560 >1.31 Note. — a Measureding-banddata. Errors represent 1σ. Forthe objects thatarenotdetected inthedifference imagesonDay1(Day1 −Day327), 3σ lowerlimitsaregiven. sources are dominated by stellar-shape sources, such as the magnitude difference since the final reference images starsorquasars(166sources). Theremaining31sources werenotavailableandtruemagnitudesoftheobjectson are associated with extended sources (galaxies). Among Day1 were notknownat the time ofspectroscopy(2014 these sources, 16 sources are located at the center of Aug). Therefore, even after the selection processes, our galaxies. Sincetheymaybeactivegalacticnucleiortidal initialsamplescouldincludenotonlyrapidlyrisingtran- disruptionevents,we avoidedthese objectsfor follow-up sients but also normal SNe around the peak brightness observations. Since 8 out of 16 objects show declining if the flux difference within 2 nights is large enough. In flux, it is likely that the majorityof these 16 sources are fact,byourfollow-upspectroscopicobservations(Section activegalactic nuclei. Remaining 15sourceshavean off- 2.3),3outof8objectswereidentifiedasnormalSNe (at set from the center of the galaxies, and selected as SN z=0.13,0.25, and 0.40). In addition, after obtaining the candidates. final reference images on Day 327, we confirmed that The final SN candidates consist of 14 brightening ob- these three objects are already bright on Day 1. The jects. From this final sample, we performed follow-up rising rates for these three objects are |∆m/∆t|<1 mag observations of most reliable 12 objects. Among these day−1,whichisalsoconsistentwithnormalSNe. There- 12objects,we measuredredshifts for 8 objectswhile the fore, we omit these three objects from our samples. other4objects(andtheirhostgalaxies)weretoofaintto Figure 1 shows images of 5 rapidly rising transients, takespectra. Theremaining2objectswerenotobserved. namedasSHOOT14gp,14or,14ha,14jr,and14ef(Table Note that the sample selection for spectroscopy was 3). Photometry of these 5 objects is shown in Table 4. madebasedonthefluxdifferencewithin2nights,noton 4 Tanaka, M., et al. Table 4 14gp [O II] Hβ [O III] Photometry ofrapidlyrisingtransients 2 (z=0.635) MJD Filter Magnitudea Instrument SHOOT14gp 56840.542 g >25.53 HSC 0 56840.577 g >25.57 HSC 6 1(z4=o0r.821) 56841.513 g 23.74+−00..0098 HSC 4 55556666888844440111....555568440278 gggg 222333>...77720215+−+−+−.0000005......8000000887776 HHHHSSSSCCCCbb -1-2-1s cm A) 602 1(z4=h0a.548) 56874.475 g >25.45 FOCAS g 4 55556666888847440410....244486577369 grrr SHO222O654>...T753241114+−+−+−4.o0000009r......9661431535093 FOHHHCSSSCCCAS -18Flux f (10 erλ 4602 1(z4=jr0.384) 56840.332 g 26.88+−00..7454 HSC 2 56841.283 g 25.11+−00..1132 HSC 56841.326 g 25.01+−00..1120 HSC 1 00 14ef 56841.487 g 24.99+−00..1110 HSC 8 (z=0.560) 56840.310 g 26.85+−00..6440 HSCb 6 5566884713..336155 gg 25>.0245+−.006..90087 FHOSCCAbS 24 56840.431 r >25.61 HSC 0 5566887431..247162 rr 2255..7295+−+−0000....52329580 FOHCSCAS 3000 Res 4t0 w0a0velen 5g0t0h0 (A) 6000 3700 3750 4900 5000 SHOOT14ha Figure 2. Spectra of host galaxies of SHOOT14gp, 14or, 14ha, 56840.542 g >25.77 HSC 14jr, and 14ef (from top to bottom). The wavelengths of strong 56840.577 g >25.79 HSC emissionlines([Oii]λ3727,Hβ,and[Oiii]λ4959,5007)aremarked 56841.513 g 25.29+−00..3235 HSC withthedashedlines. Therightpanelsshowthedataaroundthese 56841.547 g 25.27+−00..2282 HSC lines. 56841.582 g 24.95+−00..1196 HSC 56840.560 g >25.87 HSCb 2.3. Follow-up observations 56841.548 g 25.11+−00..2107 HSCb We performed imaging and spectroscopic observations 56874.601 g >25.42 FOCAS 56840.479 r >25.48 HSC of 5 objects (Table 3) using the Faint Object Camera 56840.479 r >25.48 HSC and Spectrograph (FOCAS, Kashikawa et al. 2002) of 56841.456 r 25.26+−00..3225 HSC the Subaru telescope. Observations of the four objects 56873.501 r >25.03 FOCAS (SHOOT14gp,14or,14ha,and 14jr)were carriedout on 56874.589 r >25.07 FOCAS 2014Aug5and6UT(Day35and36,respectively)while SHOOT14jr 555555666666888888444444111000......532532039249083629 gggggg 222222444555......476598575065+−+−+−+−+−+−000000000000............011233010222820673819175 HHHHHHSSSSSSCCCCCC bo3sPdub5aeAFbt6nsNeeot,dcrrrTo.tavneacSTtdlthtyhipieoooeafnnnocFolskrytwOahotiigCfhneterheSAr.oH-hStSbbhoOHajesienOmtOcfidtgTaOnswag1aTwlih4aln1eeixgrlr4fyeeegfw)dtpen.haeroaeerttaeynnd,cdwoeenw1etie4rem2ceo0trpane1geowd5retefbsJdoroueruetmntsmheieencaidt2ngre2ggdHiim-(niODnaaanTlaggldyye-- 56840.389 g 25.76+−00..2271 HSCb r-bands. Atypicallimitingmagnitudesare≃25.0−25.5 56841.377 g 24.61+−00..0098 HSCb mag (Table 4). 56840.442 r 25.84+−00..7423 HSC Forspectroscopy,weusedmulti-objectmodewith0′.′8- 56841.422 r 24.87+−00..2139 HSC widthslitandlong-slitmodewith1′.′0-widthslit(onlyfor 56873.262 r >25.36 FOCAS SHOOT14ef). With the 300B (300 lines mm−1) grism SHOOT14ef 56840.554 g >26.30 HSC and the SY47 order-sort filter, our configuration gives 56840.591 g >26.41 HSC a wavelength coverage of 4700 - 9000 Å and a spectral 56840.610 g >26.19 HSC resolution of R = λ/∆λ ∼ 600. The data were reduced 56841.525 g 25.57+−00..2128 HSC with the IRAF packages in a standard manner. 56841.559 g 25.72+−00..2139 HSC Thetransientcomponentsarenotdetectedinourspec- 56841.596 g 25.70+−00..2271 HSC traasexpectedfromtheresultsofimagingobservations. 56841.615 g 25.74+−00..3203 HSC Figure 2 shows the spectra of the host galaxies for these 56840.585 g >26.50 HSCb five objects. The [O ii] λ3727 emission line is detected 56841.574 g 25.67+−00..2107 HSCb from all the host galaxies, which indicates that they 56840.467 r >26.08 HSC 56841.445 r >26.06 HSC are all star forming galaxies. The redshifts range from z =0.384 (SHOOT14jr) to z =0.821 (SHOOT14or). Note. — a All the photometry are derived inthesubtractedimagesusingthefinalreference images. Errors represent 1σ. For the cases of non-detection,3σupperlimitsaregiven. Magni- tudes are corrected only for Galactic extinction. b Photometry inthe1-nightstackimages. Rapidly Rising Transients from Subaru/HSC Transient Survey 5 -19.0 14gp 14or 14ha 14jr 14ef (z=0.635) (z=0.821) (z=0.547) (z=0.384) (z=0.560) -18.5 g (rest 2620 A) g (rest 3080 A) g (rest 3450 A) g (rest 3060 A) r (rest 3420 A) r (rest 4020 A) r (rest 4500 A) r (rest 3990 A) -18.0 e d u nit -17.5 g a m -17.0 e ut ol -16.5 s g (rest 2920 A) b A r (rest 3810 A) -16.0 -15.5 -15.0 -1.0 -0.5 0.0 -1.0 -0.5 0.0 -1.0 -0.5 0.0 -1.0 -0.5 0.0 -1.0 -0.5 0.0 Restframe days Rest frame days Rest frame days Rest frame days Rest frame days Figure 3. Light curves of the five rapidly rising transients on Days 1 and 2. The g- and r-band photometry is shown in blue and red points. Trianglesshow3σupperlimit. Fortheg-banddata,photometryfor1visit(5×2-minexposures)isshowninpalebluecolorwhile photometry inthe1-nightstackeddataisshowninbluecolor. 3. LIGHT CURVES galaxies. 3.1. Overview SHOOT14orand14jraredetectedintheimagesofDay 1−Day327. We measuretherisingratesfromDay1to Figure3showslightcurvesofoursamplesonDay1and Day2usingtheg-band1-daystackedimages: |∆m/∆t|= Day 2. Hereafter, the epochs of stacked g-band data on 3.12+1.11 and 1.61+0.39 mag day−1 for SHOOT14or and Day 2 are taken to be t=0 unless otherwise mentioned. −0.70 −0.32 14jr, respectively (errors represent 1σ, Table 3). Note The photometry is performed in the subtracted images that the rising rate is measured in the restframe, so the usingthefinalreferences(e.g.,Day1−Day327andDay time interval used for the measurement varies with the 2 − Day 327 for g-band). source redshifts (∆t = 0.55 days for SHOOT14or while Throughoutthepaper,wedonottakeintoaccountfull ∆t = 0.72 days for SHOOT14jr). The other three ob- K-correction for absolute magnitudes since only limited jects (SHOOT14gp, 14ha, and 14ef) are not detected in information about spectral energy distribution is avail- the subtracted images of Day 1 − Day 327. The 3 σ ableforoursamples. Instead,weonlycorrecttheeffectof lower limits of the rising rate measured in g-band are redshifts,i.e.,M =m−µ+2.5log(1+z),whereM andm |∆m/∆t| > 3.10, 1.21, and 1.17 mag day−1. These are are absolute and observed AB magnitudes (measured as alsohighenoughtomatchourcriterionforrapidlyrising f ), µ is the distance modulus. The last term originates ν transients. from the difference in the frequency bin in the restframe In the following sections, we compare the light curves andobserverframe,i.e.,L (ν )=[(4πd2)/(1+z)]f (ν ), ν e ν o of our samples with those of previously known SNe and where ν and ν are restframe and observer frame fre- e o transients. quency, and d is the luminosity distance (Hogg et al. 2002). 3.2. Comparison with SNe The absolute magnitudes of the five objects range from −16 to −19 mag in the restframe near-ultraviolet Figure 4 shows comparisonof rapidly rising transients (UV) wavelengths (2620Å− 3450Å, depending on the with normal SNe. Since the redshifts of our samples are redshifts). The photometric values of our samples are moderately high, z = 0.384−0.821, we compare our g- corrected for the extinction in our Galaxy but not for and r-band light curves with near-UV and u-band light the extinction in the host galaxy. Therefore, intrinsic curves of nearby SNe with good temporal coverage. We absolute magnitudes can be brighter than those shown use the Swift uvw1- and u-band data from Brown et al. in Figure 3. (2012) and Pritchard et al. (2014) with extinction cor- All of the five objects show blue g −r color on Day rection(both in ourGalaxy andhostgalaxies)using the 2, g − r ≃ −0.60,−0.21,−0.15, and −0.15 mag for extinctionlawbyBrown et al.(2010). Sincetheeffective SHOOT14gp, 14or, 14ha, and 14jr, respectively. For restframewavelengthsdonotalwaysmatchperfectly,we SHOOT14ef, the color is g −r < −0.39 mag. This in- always give effective restframe wavelengths in parenthe- dicates that, for blackbody case, the peak of the spectra sis. is located at wavelengths shorter than the wavelengths Figure 4 shows that the properties of our samples are corresponding to the observed r-band. Therefore the not consistent with those of Type Ia SNe at any phase, blackbodytemperaturesforourobjectsareT >13000, and those of core-collapse SNe at > a few days after BB ∼ ∼ 15000,13000,11000,and13000KforSHOOT14gp,14or, the explosion. The absolute magnitudes of our sam- 14ha, 14jr, and 14ef, respectively. Note that the intrin- ples are as luminous as the peak magnitude of Type Ia sic colors can be bluer due to the extinction in the host SN 2011fe (Brown et al. 2012) and Type IIP SN 2006bp 6 Tanaka, M., et al. -19.0 -19.0 g-band r-band -18.0 -18.0 SN IIP 06bp (3450A) SN IIP 06bp (2590A) -17.0 SN Ia 11fe (2600A) -17.0 e e d -16.0 d -16.0 SN Ia 11fe (3460A) u u gnit -15.0 gnit -15.0 S(3N4 6II0bA 0)8ax a a m 14gp (2920A) m ute -14.0 14or (2620A) ute -14.0 14gp (3810A) ol 14ha (3080A) ol 14or (3420A) SN Ib 07Y (3450A) Abs -13.0 1144jerf ((33405600AA)) S(2N6 0II0bA 0)8ax Abs -13.0 1144hjra ( 4(4500200AA))SN IIn 11ht (3450A) -12.0 -12.0 14ef (3990A) -11.0 SN IIn 11htSN Ib 07Y (2590A) -11.0 (2590A) -10.0 -10.0 -5 0 5 10 15 20 25 -5 0 5 10 15 20 25 Restframe days Restframe days Figure 4. Left: Comparisonbetweeng-bandlightcurvesofourobjectsandSwiftuvw1-bandlightcurvesofnearbynormalSNe: TypeIa SN2011fe (Brownetal.2012),TypeIIP SN2006bp, TypeIIbSN2011dh, TypeIInSN 2011ht, andTypeIbSN2007Y (Pritchard etal. 2014). Right: Comparison between r-band light curves of our objects and Swift u-band light curves. For Swift SN data, the estimated epoch of the explosion is taken to be t=0 day. The Swift data are corrected for the extinction both inour Galaxy and host galaxies as estimated byPritchardetal.(2014). Vegamagnitudes are converted toAB magnitudes usingthezeropoints presented byBreeveldetal. (2011). -23.0 on Days 1 and 2 could be interpreted as the very early SLSNe PS1-10ky (2440A) phase of SLSNe, which have never been caught. How- PS1-10ky (3180A) -22.0 ever,thedataonDays35and36areclearlyinconsistent with the declining part of SLSNe. -21.0 PS1-10awh (2500A) e PS1-10awh (3260A) d nitu -20.0 3.3. Comparison with very early phase of SNe g ma -19.0 SN 2010gx (2820A) WecompareoursampleswithearlierphasesofSNe(<∼ ute a few days after the explosion). First, we show compar- sol -18.0 ison with Type IIP SN 2010aq (Gezari et al. 2010) and Ab 14gp (2920A) PS1-13arp(Gezari et al.2015),withUVdetectionatthe -17.0 14or (2620A) 14ha (3080A) veryearlyphasewithGALEX. TheearlyemissionofSN -16.0 1144jerf ((33405600AA)) 2010aqisconsistentwithcoolingenvelopeemissionafter SN shock breakout(Gezari et al.2010). The emissionof -15.0 PS1-13arp is brighter and shorter, which may indicate -5 0 5 10 15 20 25 30 shock breakout emission from dense wind (Gezari et al. Restframe days 2015). Figure 5. Comparison of light curves with SLSNe The upper panel of Figure 6 shows a similarity of the (Pastorelloetal. 2010; Chomiuketal. 2011). Observed u- bandlightcurves areshownforSN2010gx,whileobservedg-and rising rate and brightness between our samples and SN r-band light curves are shown for PS1-10awh and PS1-10ky. For 2010aq and PS1-13arp. SN 2010aq and PS1-13arp also SLSNe,thepeakepochsareshiftedtot=13daysandmagnitudes showfastrise,|∆m/∆t|>0.989and>2.635magday−1, arecorrected foronlyGalacticextinction. respectively. They reach about −17 - −18 mag, which is also similar to our samples. Note that the effective (Pritchard et al.2014). However,the risingratesforour restframe wavelengths corresponding to the NUV filter samplesarefasterthantheveryearlyphaseofSN2011fe, ofGALEX (2130Åand1990ÅforSN2010aqandPS1- one of the best observed Type Ia SNe. We also com- 13arp, respectively) are shorter than those for our sam- pare our objects with Type IIb SN 2008ax,Type IIn SN ples (∼2600−3500 Å). 2011ht, and Type Ib SN 2007Y (Pritchard et al. 2014). For comparison, we also show non-filter magnitude of Theirrisingratesareslowerthanthoseofoursamplesat Type IIP SN 2006bp (Quimby et al. 2007), for which any epochs with available data, i.e., > a few days after very early phases were observed (see also Rubin et al. ∼ theexplosion. Inaddition,thebluecolorsofoursamples 2015for recent largersamples). It also shows a fast rise, (g−r ≤−0.2 mag) are not consistent with normal SNe |∆m/∆t|= 2.3 mag day−1. Again, although the differ- after a few days from the explosion. For nearby SNe af- ence in the restframe wavelengths should be cautioned, ter a few days from the explosion, the uvw1 magnitude these similaritiessuggestthatoursamples ofrapidlyris- is generally fainter than the u magnitude as shown in ing transients are the very early phase of SNe. Figure 4, i.e., the color is uvw1−u>0 mag. We also compare our samples with the very early part Our samples might correspond to the rising phase of of Type Ic SN 2006aj and Type Ib SN 2008D. They much brighter SNe, such as superluminous SNe (SLSNe, are among the best-studied stripped-envelope SNe. SN Quimby et al. 2011; Gal-Yam 2012). Figure 5 shows 2006aj is associated with low luminosity gamma-ray comparison of our samples with SLSN SN 2010gx, PS1- burst (GRB) 060218, and thus, good optical to NUV 10awh, and PS1-10ky with a good temporal coverage data are available from soon after the explosion (e.g., (Pastorello et al. 2010; Chomiuk et al. 2011). Our data Campana et al. 2006; Soderberg et al. 2006; Pian et al. Rapidly Rising Transients from Subaru/HSC Transient Survey 7 -20.0 -20.0 Type IIP (very early phase) Type Ic (very early phase) -19.0 -19.0 14gp (2920A) 14or (2620A) de -18.0 1144hjra ( 3(3405800AA)) de -18.0 1144gopr ((22692200AA)) SN Ic 2006aj (3350A) magnitu -17.0 14ef (3060A) PS1-13arp (1990A) SN 2010aq (2130A) magnitu -17.0 111444hjerfa ( (3(33400568000AAA))) SN Ic 2006aj (2520A) e e ut ut -16.0 ol ol bs -16.0 bs A A -15.0 SN Ib 2008D (3440A) -15.0 -14.0 SN 2006bp non-filter SN Ib 2008D (2580A) -14.0 -13.0 0 5 10 0 5 10 Restframe days Restframe days Figure 6. Upper: Comparison of light curves with the very early phase of Type IIP SNe: GALEX NUV data of Type IIP SN 2010aq (Gezari etal.2010)andPS1-13arp(Gezari etal.2015),andalsonon-filterdataofSN2006bp(Quimbyetal.2007). ThedataofSN2010aq and PS1-13arp are corrected for only Galactic extinction while those of SN 2006bp are corrected for both Galactic and host extinction. Lower: Comparisonwiththe very early phaseof TypeIcSN 2006aj (Campanaetal.2006;Šimonetal.2010,corrected foronlyGalactic extinction) and Type Ib SN 2008D (Modjazetal.2009, corrected forboth Galactic and host extinction). For the comparison with Type IbcSNe,theepochofourdataareshiftedsothatDay1corresponds tot=0day. -19.0 g-band -19.0 r-band Type II (2600A) Type II (3465A) 14gp (2920A) 14gp (3810A) Type IIb/Ib/Ic (2600A) Type IIb/Ib/Ic (3465A) 14or (2620A) 14or (3420A) -18.0 -18.0 14ha (3080A) 14ha (4020A) 14jr (3450A) 14jr (4500A) de -17.0 14ef (3060A) de -17.0 14ef (3990A) u u nit nit ag -16.0 ag -16.0 m m e e olut -15.0 PS1-13arp (1990A) olut -15.0 s s Ab SN 2010aq (2130A) Ab -14.0 -14.0 -13.0 -13.0 -12.0 -12.0 -10 -5 0 5 10 15 20 25 30 -10 -5 0 5 10 15 20 25 30 Restframe days Restframe days Figure 7. Left: Comparison between g-band light curves of our objects and Swift uvw1-band light curves of core-collapse SNe (Pritchard etal.2014;Modjazetal.2009)andTypeIIPSN2010aq(Gezari etal.2010)andPS1-13arp(Gezari etal.2015)withGALEX NUV data. Right: Comparison between r-band light curves of our objects and Swift u-band light curves of core-collapse SNe. The data from Pritchardetal.(2014)are corrected forestimated extinction both inour Galaxy and host galaxies. Vega magnitudes are converted toABmagnitudes. 2006; Mazzali et al. 2006; Sollerman et al. 2006; after the explosion (Soderberg et al. 2008; Modjaz et al. Modjaz et al. 2006; Mirabal et al. 2006; Šimon et al. 2009), and the rising rate in the first day is faster than 2010). SN 2008D is associated with X-ray transient that measured with 2-day interval. 080109(e.g., Soderberg et al. 2008; Mazzali et al. 2008; When we match our objects with core-collapse SNe Tanaka et al. 2009a,b; Modjaz et al. 2009). Emission within a few days after the explosion, our observations at the first 2 days of SN 2006aj and SN 2008D is on Day 35 and 36 correspond to the plateau phase of interpretedascooling envelopeemission(Waxman et al. Type IIP or the peak phase of Type Ibc SNe. As shown 2007; Soderberg et al. 2008; Modjaz et al. 2009; in Figure 7, the distribution of uvw1 brightness of core- Chevalier & Fransson 2008; Nakar 2015). collapse SNe at these epochs ranges from −12 to −17 The lower panel of Figure 6 shows that the rising rate mag. Since our limits in g-band correspond to −17.0 of SN 2006aj is as fast as our samples. The time to the mag, non-detection in g-band on Days 35 and 36 is not peakisonly∼0.5days,whichisasshortasthatinferred surprising. SHOOT14gp and 14or are marginally de- foroursamplesalthoughwecannotnotfirmlydetermine tected in r-band (right panel of Figure 7). Compared the peak dates only with 2-night data. SN 2008D lacks with Swift u-band data, their brightness is consistent the data at ∼ 1 day after the explosion. Nevertheless, with those of core-collapse SNe at the luminous end. the rising rate of SN 2008D in Swift u-band (measured 3.4. Comparison with rapidly rising transients from PS1 with2-dayinterval)is similarto SHOOT14jr. Note that if the early part of SN 2008D is interpreted as cooling Therapidrisingratesofoursamplesremindusofpop- envelope emission, the peak would be around ∼ 1 day ulationofrapidly evolvingand luminous transientsfrom PS1, which are compiled by Drout et al. (2014, see also 8 Tanaka, M., et al. PS1 luminous (Drout+14) PS1 luminous (Drout+14) -20.0 g-band PS1-11qr (g, 3600A) -20.0 r-band PS1-11qr (r, 4700A) PS1-12bv (g, 3400A) PS1-12bv (r, 4430A) PS1-13duy (g, 3760A) PS1-13duy (r, 4900A) e -19.0 e -19.0 d d u u nit nit g g a -18.0 a -18.0 m m e e ut ut sol -17.0 14gp (2920A) sol -17.0 14gp (3810A) b b A 14or (2620A) A 14or (3420A) 14ha (3080A) 14ha (4020A) -16.0 14jr (3450A) -16.0 14jr (4500A) 14ef (3060A) 14ef (3990A) -15.0 -15.0 -15 -10 -5 0 5 10 15 20 -15 -10 -5 0 5 10 15 20 Restframe days Restframe days PS1 faint (Drout+14) PS1 faint (Drout+14) -19.0 g-band -19.0 r-band PS1-10ah (g, 4440A) PS1-10ah (g, 4440A) PS1-10bjp (g, 4290A) PS1-10bjp (g, 4290A) e -18.0 e -18.0 d d u u nit nit g g a -17.0 a -17.0 m m ute 14gp (2920A) ute 14gp (3810A) bsol -16.0 1144ohra ((23602800AA)) bsol -16.0 1144ohra ((34402200AA)) A 14jr (3450A) A 14jr (4500A) 14ef (3060A) 14ef (3990A) -15.0 -15.0 -14.0 -14.0 -10 -5 0 5 10 15 20 25 -10 -5 0 5 10 15 20 25 Restframe days Restframe days Figure 8. ComparisonoflightcurveswithrapidlyevolvingandluminoustransientsfromPS1(Droutetal.2014). Thepeakepochofthe PS1samplesisselectedtobet=0day. Upper: ComparisonwiththePS1luminoussampleswiththepeakabsolutemagnitudesof<−19 mag. Leftandrightpanelsshowthelightcurvesing-andr-band(bothforHSCandPS1),respectively. Epochsofoursamplesareshifted sothatDay2datacorrespond tobet=−10days. Lower: ComparisonwiththePS1faintsampleswiththepeakabsolutemagnitudes of >−19mag. Leftandrightpanelsshowthelightcurvesing-andr-bandforHSCdata,respectively. ForthePS1sample,g-banddataare showninthebothpanels(asg-bandhasclosereffectivewavelengths). EpochsofoursamplesareshiftedsothatDay2datacorrespondto bet=−2days. ThemagnitudesofthePS1samplesarecorrected foronlyGalacticextinction. Poznanski et al. 2010; Kasliwal et al. 2010; Drout et al. SincethePS1faintsampleshavelowredshifts(z =0.074 2013). These transients show rapid luminosity evolution and0.113forPS1-10ahandPS1-10bjp,respectively),we both in rising and declining phases compared with nor- compare our g- and r-band data with PS1 g-band data. malSNewithatimeabovehalf-maximumoflessthan12 The peak magnitudes of the PS1 luminous samples days. Interestingly, they show a faster rising rate than are brighter than the magnitudes of our sample on Day a declining rate, which motivates the comparison with 2. Our samples could thus be interpreted to the rising our samples. In addition, they have blue g − r colors part of the PS1 samples. The dashed lines in the up- (g−r <−0.2 mag), similar to our samples. per left panel of Figure 8 shows the extrapolationof the Since the PS1 samples have a wide luminosity range, rising part by assuming the flux rises as f = (t−t )2 0 we divide the samples into two classes with the absolute (as often assumed for the early part of SNe, see e.g., magnitude brighter(hereafterPS1 luminous samples)or Nugent et al. 2011; Pastorello et al. 2013; Prieto et al. fainter(PS1faintsamples)than−19.0mag. Drout et al. 2013; Yamanaka et al. 2014), where t is the epoch with 0 (2014) interpret their rapid transients to be either (1) zero flux. Three of our samples (SHOOT14ha, 14jr, and the cooling envelope emission following shock breakout 14ef) show a nice agreement with the extrapolated ris- (especially for faint samples) or (2) shock breakoutfrom ing partif the epochs ofthese objects areshifted so that dense wind (for luminous samples). Day 2 corresponds to t ∼ −10 days. However, with this Figure 8 shows comparison of our samples with the assumption, the non detection of PS1-13duy before the PS1 samples (Drout et al. 2014) which are detected at peak in r-band is not consistent with our detection on the rising part in g-band. The peak dates of the PS1 Day 2. In addition, the brightness and upper limits at samples are taken to be t = 0 day. It should be cau- later epochs (Days 35 and 36)are much fainter than the tioned that the PS1 samples have a wider redshift range magnitudes of PS1-11qr for which the data at the de- than ours, and thus the rest wavelengths corresponding clining part is available. Therefore, our samples are not the observed filters have a wider variety. For the PS1 likely to be the same population as the PS1 luminous luminous samples, g- and r-band data for our samples samples. arecomparedwithPS1g-andr-banddata,respectively. Our samples show a better agreement with the PS1 Rapidly Rising Transients from Subaru/HSC Transient Survey 9 -1 |Δm/Δt| (mag day ) 10 1 0.1 -21.0 06aj t<1d PS1 rapid transients -20.0 (2520A) -19.0 e d -18.0 PS1-13arp u (1990A) t ni -17.0 11fe (2600A) g a 10aq m -16.0 (2130A) e 11ht ut -15.0 (2590A) ol s 08ax (2600A) b -14.0 14gp (2920A) A 14or (2620A) -13.0 14ha (3080A) 07Y (2590A) 14jr (3450A) -12.0 14ef (3060A) -11.0 0.1 1 10 -1 Rising timescale (day mag ) Figure 9. Summaryofabsolutemagnitudesandrisingtimescale(τrise≡1/|∆m/∆t|)oftransients. Oursamplesarecomparedwiththe followingobjects: SN2010aqandPS1-13arp(Gezari etal.2010,2015)withearlyUVdetectionwithGALEX,theearlypeakofSN2006aj (Campanaetal.2006;Šimonetal.2010,,Figure6),TypeIaSN2011fe(Brownetal.2012),core-collapseSNe(TypeIbSN2007Y, Type IIb SN 2008ax, and Type IIn SN 2011ht, Pritchardetal. 2014), and rapid transients from PS1 (Drout etal. 2014). For rapid transients from PS1, the rising timescale (rising rate) is measured with g-band data. The dashed lines show the absolute magnitude and rising timescaleofPS1-10ahandPS1-10bjpmeasuredwiththeinterpolated g-bandlightcurves. faintsamples(lowerpanelsofFigure8). Therisingrates 4. RISINGRATESOFTRANSIENTS ofthePS1samplesing-bandis|∆m/∆t|<1magday−1, Figure 9 shows a summary of rising rate and absolute whichdo notfulfill ourcriterion. However,PS1 dataare magnitudes of our samples and other transients shown taken with ∼ 3 days cadence, and thus, the rising rate in Figures 4, 6, and 8. The figure is shown as a function measuredwithashorterintervalcanbefaster. Infact,if of rising timescale τ ≡ 1/ |∆m/∆t|, time to have 1 the rising partis interpolatedwith f =(t−t0)2, the ris- mag rise. For our orbisjeects, SN 2010aq, PS1-13arp, and ingratecanbe asfastasthatmeasuredforoursamples. thePS1samples,therisingratesaremeasuredonlyatan Especially, three ofour samples(SHOOT14ha,14jr,and intervalontheriseastherearenotime-seriesdatabefore 14ef) show a good match if the epochs of these objects thepeak. Thetimeintervalis∆t>0.5days. Fornormal ∼ are shifted so that Day 2 corresponds to t ∼ −2 days. SNe, for which good time-series data are available, we Then, our data at later epochs are also consistent with measure the rising rate |∆m/∆t| as a function of time the PS1 samples at the declining phase. Since the esti- (connected with lines in Figure 9). In order to match mated epoch of zero flux for PS1-10ahand PS1-10bjp is the time interval with other objects, the time interval t ∼−4.2daysfromthepeak,theepochsofourobserva- 0 is kept to be ∆t > 0.5 days. For example, although tions correspondto ∼1.5−2.2 days after the explosion. ∼ fine time-series data are available for SN 2006aj before The agreement between the luminous 2 objects in our the peak, we measure the rising rate from t=0.082 and samples(SHOOT14gpand14or)andPS1faintsamplesis t=0.541 days from the burst (∆t = 0.45 days). For notasgoodasthatforthefaint3objects(SHOOT14ha, rest the PS1 faint samples (PS1-10ah and PS1-10bjp), the 14jr, and 14ef). Note that the direct comparison at the greendashedlinesshowthetherisingratemeasuredwith perfectly matched wavelengths is not possible (< 3000 ∆t =0.5daysusingthelightcurvesinterpolatedwith rest Å for SHOOT14gp and 14or while > 4000 Å for the f =(t−t )2. 0 PS1faintsamples). Nevertheless,SHOOT14gpand14or In this diagram, as also discussed in Section 3.2, it is show faster rises than the PS1 faint samples. The rising clear that Type Ia SN shows the fast rise only at the rates of SHOOT14gp and 14or are > 3.10 and 3.12+−10..1710 very early phase with faint magnitudes. Core-collapse mag day−1, respectively (Table 3). On the other hand, SNe after a few days from the explosion are located at the rising rate of the PS1 faint sample is |∆m/∆t|<1.3 theregionwithfaintermagnitudesandlongertimescales mag day−1 even at the fastest phase in the interpolated compared with our samples. light curves (see dashed lines in Figures 8 and 9). The Our samples share a region similar to SN 2010aq and nature of these objects are discussed in Section 5. PS1-13arp, SNe with early UV detection by GALEX (Gezari et al. 2010, 2015), as expected from the com- 10 Tanaka, M., et al. parison in the previous sections (Figure 6). The early forallthe objectsbyneglectingdifferentredshifts. Here, peak ofSN 2006ajalsohas a similar risingrate,but it is τ means the duration for which transients show a rapid brighter than our samples. risewithsufficientbrightnesssothattheyarerecognized The PS1 luminous samples (Drout et al. 2014) is lo- as rapidly rising transients in our survey. For the two cated at the regionwith brighter magnitudes and longer objectsdetectedbothonDays1and2(SHOOT14orand timescales. On the other hand, the PS1 faint sam- 14jr), the duration of the emission is about 1.2 days in ples are closer to the faint three objects in our samples the observedframe (0.67and0.86 daysin the restframe, (SHOOT14ha, 14jr, and 14ef). Especially, when the ris- respectively),andthus,τ isnotmuchshorterthan1day. ing rate is measured with the interpolated light curves A smaller τ is not excluded for the other three objects to have a similar ∆t with our samples,the brightness but they do not show clear intranightvariability for 1.6- rest and the rising timescale of the PS1 faint samples shows 3.1hrintheobservedframe(1.0-2.0hrintherestframe). fairly good agreement with SHOOT14ha, 14jr, and 14ef Comparisonwithpreviouslyknowntransients(Section3) (see also Figure 8). and also with models (see Section 5.2) suggest that it is unlikelythattherisingrateashighas|∆m/∆t|>1mag 5. DISCUSSION day−1 continuesfor>2daysinrestframewithsufficient The properties of our samples of rapidly rising tran- brightness. Thus, we adopt τ =1 day as a fiducial value sientsaresimilartothoseofveryearlycore-collapseSNe, for all objects. such as SN 2010aq, PS1-13arp, and SN 2006aj (Figure Atypical3σlimitingmagnitudefortheimagesusedfor 9). The faint three objects also show a similarity to the candidate selection is ≃ 26.0 mag. We use this value for faint population (with >−19 mag) of the rapidly rising thecalculationofthemaximumvolumeVmax. Infact,for transients from PS1 (Drout et al. 2014), which are also objectstoberecognizedasrapidlyrisingtransients,they interpreted as the very early phase of SNe. For both should be sufficiently brighter than the limiting magni- cases, the best match is obtained when our samples are tudeonDay2. Thus,theeffectivelimitingmagnitudefor assumed to be ∼1−2 days after the explosion. the rapidly rising transients tends to be shallower than By these facts, although we do not have photometric 26.0 mag. Since analysis with a shallower limiting mag- follow-upandspectroscopicidentificationofoursamples, nitude gives a smaller maximum volume and a higher we interpret that the rapidly rising transients presented event rate, our choice of deep limiting magnitude gives in this paper are the very early phase of core-collapse conservative estimates for the event rate. It is noted SNe. In the following sections, we discuss the nature of that the extinction in the host galaxy is not corrected therapidlyrisingtransientsbasedonthisinterpretation. and the true absolute magnitude of our samples should be brighter. However, if the extinction for the current 5.1. Constraints on the event rate samples represents an average degree of extinction, the estimate of V is not significantly affected (i.e., our Event rates of rapidly rising transients shown in this max estimate crudely includes the effect of extinction). paper are of interest. However, to estimate the event We estimate pseudo event rate for each object (R ). rates, we need detailed information about spectral en- i For example, the maximum redshift, in which our sur- ergydistribution,lightcurveshape,andluminosityfunc- vey would have detected SHOOT14gp, is z = 1.87 tion,whicharenotavailableforoursamples. Instead,we max with the limiting magnitude of 26.0 mag using absolute givecrude constraintsonhowhigheventrateisrequired magnitude of M = −18.67 mag and crude K-correction for short-timescale events to be detected with our short- (the term of 2.5log(1 + z)) as in Section 3. The co- period survey. moving volume within this redshift in 12 deg2 survey We estimate the event rates by using a method based area is V = 0.16 Gpc3. For this object to be de- on 1/V method (Schmidt 1968; Eales 1993), which max,i max tectedwithoursurvey,therequiredeventrateshouldbe is used for estimation of galaxy luminosity function. R ≃ 1/τ V ≃ 0.23×10−5(τ/1day)−1 yr−1 Mpc−3. The event rates of transients R can be written as R = i i max,i SimilaranalysisforSHOOT14or,14ha,14jrand14efgive P R = P 1 . Here, p is a detection efficiency i i i piτiVmax,i i zmax =1.28,0.70,0.82,and0.62,andtheeventratesare (pi <1),τiistherestframetimewindowforarapidlyris- Ri ≃ 0.47, 1.9, 1.3, 2.5 ×10−5(τ/1day)−1 yr−1 Mpc−3, ing transient to be detected with our survey, and Vmax,i respectively. is the maximum volume in which the transient is de- By summing up the pseudo rates, the lower limit of tectable with our survey. The summation is taken for the total event rate is R ≃ 6.4 × 10−5 (τ/1day)−1 all the detected objects. The difference from galaxy lu- yr−1 Mpc−3. It corresponds to about 9 % of core- minosity function is τ in the denominator to take into i collapse SN rate at z ∼ 1 (the core-collapse SN rate is accountthefactthattransienteventrateshouldbemea- (3−7)×10−4yr−1Mpc−3atz =0−1,Dahlen et al.2004; suredfor a giventime period. As the number ofsamples Botticella et al.2008;Li et al.2011;Dahlen et al.2012). is small, we do not take into account redshift evolution Note that the event rate is dominated by the less lumi- of the event rate. nous object with smaller maximum volumes. The event We do not correct detection efficiency since the selec- rateforthetwoluminousevents(SHOOT14gpand14or) tioncriteriaarecomplicated: we needspectroscopicred- isR=0.7×10−5 (τ/1day)−1 yr−1 Mpc−3 (∼1%ofthe shift to define the rapidly rising transients (Section 2.2). core-collapse SN rate at z ∼ 1), while the event rate Thus, we assume p = 1, so that the analysis gives a i for the three faint events (SHOOT14ha, 14jr, and 14ef) conservative lower limit for the event rate (see below for is R = 5.7×10−5 (τ/1day)−1 yr−1 Mpc−3 (∼ 8 % of possible impact of this assumption). the core-collapse SN rate). It is worthy to mention that Then, the free parameter in this analysis is only τ . i the event rate of the rapid transients from PS1 is esti- For simplicity,we assume this parameteris the same (τ)