SN T Reections of Past Glory astro-ph/9407097 29 Jul 94 2 2 Brian P Schmidt and Rob ert P Kirshner HarvardSmithsonian Center for Astrophysics Garden St Cambridge MA 2 Bruno Leibundgut Europ ean Southern Observatory KarlSchwarzschildStrasse D Garching b ei M unchen Germany 3 Lisa A Wells and Alain C Porter 4 Kitt Peak National Observatory PO Box Tucson AZ Pilar RuizLapuente and Peter Challis HarvardSmithsonian Center for Astrophysics Garden St Cambridge MA Alexei V Filipp enko Department of Astronomy and Center for Particle Astrophysics University of California Berkeley California Submitted to The Astrophysical Journal Letters Abstract We have obtained photometry and sp ectra of SN T which extend more than days past maximum light by far the longest a SN Ia has b een followed Although SN T exhibited nearly normal photometric b ehavior in the rst days following maximum by days its decline had slowed and by days the sup ernova brightness was consistent with a constant apparent magnitude of m Sp ectra near maximum showed minor variations on the SN Ia theme which grew less B conspicuous during the exp onential decline At days the nebular sp ectrum was comp osed of Fe and Co lines common to SNe Ia However by days past maximum light these lines had shifted in wavelength and were sup erimp osed on a strong blue continuum The luminosity of SN T at 38 2 1 days is more than D Mp c ergs s with a rate of decline of less than mags days We show that this emission is likely to b e light that was emitted by SN T near maximum light which has reected from foreground dust much like the light echos observed around SN A 1 Based in part on observations made at the Multiple Mirror Telescope jointly op erated by the Smithsonian Institution and the University of Arizona 2 Guest Observer at Kitt Peak National Observatory op erated by AURA Inc under contract to the National Science Foundation 3 Deceased Octob er 4 Kitt Peak National Observatory National Optical Astronomy Observatories op erated by the Asso ciation of Universities for Research in Astronomy Inc AURA under co op erative agreement with the National Science Foundation Introduction SN T was discovered in NGC by a host of observers Waagen et al on April and reached a maximum of m mag days later Phillips et al Ford et al V So on after its discovery this ob ject was noted as an unusual member of the SN Ia o ck In a class known for exceptionally uniform sp ectral and photometric evolution SN T b efore maximum showed abnormally weak sp ectral features of intermediate mass elements Filipp enko et al RuizLapuente et al Phillips et al Jeery et al and an unusually broad maximum in its B and V light curves Phillips et al Ford et al Furthermore this black sheep of the SN Ia family app eared to b e more luminous at maximum than standard SNe Ia Filipp enko et al Phillips although other views on the absolute magnitude are tenable Phillips et al However in the weeks after maximum light SN T b egan to lose its unique sp ectroscopic and photometric features and so on app eared indistinguishable from other wellobserved SNe Ia Filipp enko et al RuizLapuente et al Phillips et al such as SN E Kirshner et al SN B Branch et al SN B Barb on et al Wells et al and SN N Leibundgut et al In this letter we present photometry and sp ectra of SN T which extend days past maximum the longest time for which a SN Ia has b een followed to date These observations show that ab out days after maximum SN T stopp ed the exp onential fading characteristic for SNe Ia Although SN T has once again strayed from its sibling SNe Ia we b elieve that this ob jects p eculiar late time b ehavior has nothing to with the eccentricities seen in its youth but instead is due to a light echo formed in intervening dust b etween the sup ernova and the Earth Observations Our optical light curve of SN T was determined from CCD images obtained using the Kitt Peak National Observatorys KPNO m telescop e and the Fred Lawrence Whipple Observatory FLWO m telescop e A sequence of standard stars Figure Plate near the SN was calibrated on two photometric nights with the FLWO m and one photometric night on the m by observing Landolt standard stars Transformation co ecients for each detector were derived using the metho d describ ed by Harris Fitzgerald Reed and used to determine the BVR I magnitudes of the c c sequence stars as given in Table The magnitude of the SN was determined by comparing the magnitude of the SN relative to the sequence stars using the p oint spread tting program DOPHOT Schechter Mateo Saha correcting for the dierence in the color b etween the standard stars and SN using the color terms determined on the photometric nights To estimate the error in each measurement we have used DAOPHOT Stetson to place several articial stars of known brightness onto regions of the galaxy similar to that where the SN lies The error estimate for each observation is then derived from the mean deviation b etween the articial stars known brightnesses and that output by DOPHOT The derived photometry and error estimates are presented in Table and Figure Our sp ectra Figure were obtained using the Multiple Mirror Telescope MMT red channel sp ec trograph Schmidt Weymann Foltz To obtain our last three sp ectra it was necessary to p osition h m s 0 00 the telescop e by osetting to SN Ts p osition B from 00 a nearby star Star Table The sp ectrograph has slit allowing us to subtract the background galaxy by tting it from the region immediately adjacent to the SN Although this is an eective metho d to remove the background contamination it is prone to errors if the background emission varies signif icantly over the scale of the spatial resolution After careful insp ection of the two dimensional sp ectra we b elieve that the features seen in these sp ectra are intrinsic to the SN and are not the result of con tamination from any underlying source The B V colors synthesized from the sp ectra using the lter functions of Bessell and zero p oints derived from the Vega sp ectrum of Hayes Latham Hayes agrees to magnitudes with our photometry providing further supp ort that the shap e of the sp ectrum is correct The last three sp ectra app ear identical within the noise and have b een combined to increase the signal to noise ratio The sp ectra are presented in Figure Discussion of Observations Few SNe Ia have b een well observed at late times however SN Ts photometric b ehavior is extraordinary Although the rate of decline b etween and days past maximum B d d mag mag is typical for SNe Ia Turatto et al Figure shows that by V days after maximum SN T had stopp ed the exp onential fading characteristic of SNe Ia Assuming they had the same luminosity at maximum SN T was intrinsically more than two magnitudes brighter than SN E Kirshner Oke and SN N Phillips et al at days If SN T was intrinsically brighter than SN E at maximum light as suggested by Filipp enko et al and Phillips then its excess luminosity at late times would b e even more pronounced Over our last three photometric observations b etween and days after maximum light SN Ts rate d of decline was consistent with zero mag The total luminosity of the SN on this B 2 38 04A 1 V late time plateau is substantial D Mp c ergs s b eing emitted in the optical B V RI alone At days past maximum SN Ts sp ectrum was similar to other SNe Ia at this ep o ch Ruiz Lapuente Filipp enko Correcting for the redshift of NGC lines primarily made up of FeII I multiplets are observed at A A A and A multiplets obs obs obs obs of FeII are contributing at A A A and A and lines obs obs obs obs from CoI I I are apparent at A and A The width of these lines indicate an obs obs 1 expansion velocity of the Fe p eak ejecta of v km s which is approximately faster than either SN E Kirshner Oke or SN N Phillips et al More than a year and a half later days past maximum the SNs sp ectrum was dierent SN T app ears to b e emitting a larger fraction of its ux blueward of A and the feature attributed to several Fe I I I lines at A at days is narrower and substantially blue shifted to obs A Emission features are also apparent at A and A It is not obs obs obs proven however that SN T is dierent from other SNe Ia sp ectra at this age SN E is the only other SN Ia with a sp ectrum at such an age Kirshner Oke and there is very little information in this observation Between and days past maximum no further evolution in SN Ts sp ectrum was detected Understanding SN Ts Late Time Luminosity There are several p ossible explanations for SN Ts large late time luminosity Before embarking on explanations for this observation a skeptical reader might wonder if we are seeing the SN at all We can b e condent that the ob ject we are seeing is indeed SN T and not some other source for several 00 00 reasons The observed ob ject is a p oint source FWHM lo cated within of the p osition of SN T The absolute magnitude of the ob ject M logD Mp c is equivalent to B O Ia stars Allen and could b e a luminous OB asso ciation however no ob ject is present at this p osition on a photographic plate taken a decade b efore the SN explosion Figure Plate Sandage Binggeli Tammann even though ob jects as faint as SN T are clearly visible Furthermore the ob jects color B V V R shows a large R deciency which is not consistent with the thermal sp ectrum of hot stars or the