arXiv subjects

Alexei V. Filippenko

Publications and source records attributed to Alexei V. Filippenko.

At least 307 records · Page 17Linked to original sources

arXiv2016

A 2.4% Determination of the Local Value of the Hubble Constant

We use the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) to reduce the uncertainty in the local value of the Hubble constant (H_0) from 3.3% to 2.4%. Improvements come from new, near-infrared observations of Cepheid variables in 11 new hosts of recent SNe~Ia, more than doubling the sample of SNe~Ia having a Cepheid-calibrated distance for a total of 19; these leverage the magnitude-z relation based on 300 SNe~Ia at z<0.15. All 19 hosts and the megamaser system NGC4258 were observed with WFC3, thus nullifying cross-instrument zeropoint errors. Other improvements include a 33% reduction in the systematic uncertainty in the maser distance to NGC4258, more Cepheids and a more robust distance to the LMC from late-type DEBs, HST observations of Cepheids in M31, and new HST-based trigonometric parallaxes for Milky Way (MW) Cepheids. We consider four geometric distance calibrations of Cepheids: (i) megamasers in NGC4258, (ii) 8 DEBs in the LMC, (iii) 15 MW Cepheids with parallaxes, and (iv) 2 DEBs in M31. H_0 from each is 72.25+/-2.51, 72.04+/-2.67, 76.18+/-2.37, and 74.50+/-3.27 km/sec/Mpc, respectively. Our best estimate of 73.24+/-1.74 km/sec/Mpc combines the anchors NGC4258, MW, and LMC, and includes systematic errors for a final uncertainty of 2.4%. This value is 3.4 sigma higher than 66.93+/-0.62 km/sec/Mpc predicted by LambdaCDM with 3 neutrinos with mass 0.06 eV and the Planck data, but reduces to 2.1 sigma relative to the prediction of 69.3+/-0.7 km/sec/Mpc with the combination of WMAP+ACT+SPT+BAO, suggesting systematic uncertainties in CMB measurements may play a role in the tension. If we take the conflict between Planck and H_0 at face value, one plausible explanation could involve an additional source of dark radiation in the early Universe in the range of Delta N_eff=0.4-1. We anticipate significant improvements in H_0 from upcoming parallax measurements.

astro-ph.CO↗

arXiv2016

Disappearance of the Progenitor of Supernova iPTF13bvn

Supernova (SN) iPTF13bvn in NGC 5806 was the first Type Ib SN to have been tentatively associated with a progenitor candidate in pre-explosion images. We performed deep ultraviolet (UV) and optical Hubble Space Telescope (HST) observations of the SN site 740 days after explosion. We detect an object in the optical bands that is fainter than the pre-explosion object. This dimming is likely not produced by dust absorption in the ejecta; thus, our finding confirms the connection of the progenitor candidate with the SN. The object in our data is likely dominated by the fading SN, which implies that the pre-SN flux is mostly due to the progenitor. We compare our revised pre-SN photometry with previously proposed progenitor models. Although binary progenitors are favored, models need to be refined. In particular, to comply with our deep UV detection limit, any companion star must be less luminous than a late-O star or substantially obscured by newly formed dust. A definitive progenitor characterization will require further observations to disentangle the contribution of a much fainter SN and its environment.

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arXiv2016

An Optical and Near-Infrared Study of the Type Ia/IIn Supernova PS15si

We present optical/near-infrared spectroscopy and photometry of the supernova (SN) PS15si. This object was originally identified as a Type IIn SN, but here we argue that it should be reclassified as a Type Ia SN with narrow hydrogen lines originating from interaction with circumstellar matter (CSM; i.e., SN Ia/IIn or SN Ia-CSM). Based on deep nondetections 27 days before discovery, we infer that this SN was discovered around or slightly before optical maximum, and we estimate the approximate time that it reached R-band maximum based on comparison with other SNe Ia/IIn. In terms of spectral morphology, we find that PS15si can be matched to a range of SN Ia spectral types, although SN 1991T-like SNe Ia provides the most self-consistent match. While this observation agrees with analysis of most other SNe Ia/IIn, we find that the implied CSM luminosity is too low to account for the overall luminosity of the SN at a time when the CSM should outshine the underlying SN by a few magnitudes. We infer that the similarity between PS15si and the hot, overluminous, high-ionisation spectrum of SN 1991T is a consequence of a spectrum that originates in ejecta layers that are heated by ultraviolet/X-ray radiation from CSM interaction. In addition, PS15si may have rebrightened over a short timescale in the B and V bands around 85 days after discovery, perhaps indicating that the SN ejecta are interacting with a local enhancement in CSM produced by clumps or a shell at large radii.

astro-ph.SR↗

arXiv2016

Light Curves of 213 Type Ia Supernovae from the ESSENCE Survey

The ESSENCE survey discovered 213 Type Ia supernovae at redshifts 0.1 < z < 0.81 between 2002 and 2008. We present their R and I-band photometry, measured from images obtained using the MOSAIC II camera at the CTIO 4 m Blanco telescope, along with rapid-response spectroscopy for each object. We use our spectroscopic follow-up observations to determine an accurate, quantitative classification and a precise redshift. Through an extensive calibration program we have improved the precision of the CTIO Blanco natural photometric system. We use several empirical metrics to measure our internal photometric consistency and our absolute calibration of the survey. We assess the effect of various potential sources of systematic bias on our measured fluxes, and we estimate that the dominant term in the systematic error budget from the photometric calibration on our absolute fluxes is ~1%.

astro-ph.CO↗

arXiv2016

Massive-Star Mergers and the Recent Transient in NGC4490: A More Massive Cousin of V838 Mon and V1309 Sco

The Galactic transient V1309 Sco was the result of a merger in a low-mass star system, while V838 Mon was thought to be a similar merger event from a more massive B-type progenitor. In this paper we study an optical/IR transient discovered in the nearby galaxy NGC4490, which appeared similar to these merger events (unobscured progenitor, irregular multi-peaked light curve, increasingly red color, similar optical spectrum, IR excess at late times), but which had a higher peak luminosity and longer duration in outburst. NGC4490-OT has less in common with the class of SN~2008S-like transients. A progenitor detected in pre-eruption HST images, combined with upper limits in the IR, requires a luminous and blue progenitor that has faded in late-time HST images. The same source was detected by Spitzer and ground-based data as a luminous IR transient, indicating a transition to a self-obscured state qualitatively similar to the evolution seen in other stellar mergers and in LBVs. The post-outburst dust-obscured source is too luminous and too warm at late times to be explained with an IR echo, suggesting that the object survived the event. The luminosity of the enshrouded IR source is similar to that of the progenitor. Compared to proposed merger events, the more massive progenitor of NGC4490-OT seems to extend a correlation between stellar mass and peak luminosity, and may suggest that both of these correlate with duration. We show that spectra of NGC4490-OT and V838 Mon also resemble light-echo spectra of eta Car, prompting us to speculate that eta Car may be an extreme extension of this phenomenon.

astro-ph.SR↗

arXiv2016

Optical Observations of the Type Ia Supernova 2011fe in M101 for Nearly 500 Days

We present well-sampled optical observations of the bright Type Ia supernova (SN~Ia) SN 2011fe in M101. Our data, starting from $\sim16$ days before maximum light and extending to $\sim463$ days after maximum, provide an unprecedented time series of spectra and photometry for a normal SN~Ia. Fitting the early-time rising light curve, we find that the luminosity evolution of SN 2011fe follows a $t^n$ law, with the index $n$ being close to 2.0 in the $VRI$ bands but slightly larger in the $U$ and $B$ bands. Combining the published ultraviolet (UV) and near-infrared (NIR) photometry, we derive the contribution of UV/NIR emission relative to the optical. SN 2011fe is found to have stronger UV emission and reaches its UV peak a few days earlier than other SNe~Ia with similar $Δm_{15}(B)$, suggestive of less trapping of high-energy photons in the ejecta. Moreover, the $U$-band light curve shows a notably faster decline at late phases ($t\approx 100$--300 days), which also suggests that the ejecta may be relatively transparent to UV photons. These results favor the notion that SN 2011fe might have a progenitor system with relatively lower metallicity. On the other hand, the early-phase spectra exhibit prominent high-velocity features (HVFs) of O~I $λ$7773 and the Ca~II~NIR triplet, but only barely detectable in Si~II~6355. This difference can be caused either by an ionization/temperature effect or an abundance enhancement scenario for the formation of HVFs; it suggests that the photospheric temperature of SN 2011fe is intrinsically low, perhaps owing to incomplete burning during the explosion of the white dwarf.

astro-ph.HE↗

arXiv2016

An Ultraviolet Spectrum of the Tidal Disruption Flare ASASSN-14li

We present a Hubble Space Telescope STIS spectrum of ASASSN-14li, the first rest-frame UV spectrum of a tidal disruption flare (TDF). The underlying continuum is well fit by a blackbody with $T_{\mathrm{UV}} = 3.5 \times 10^{4}$ K, an order of magnitude smaller than the temperature inferred from X-ray spectra (and significantly more precise than previous efforts based on optical and near-UV photometry). Super-imposed on this blue continuum, we detect three classes of features: narrow absorption from the Milky Way (probably a high-velocity cloud), and narrow absorption and broad (FWHM $\approx 2000$-8000 km s$^{-1}$) emission lines at/near the systemic host velocity. The absorption lines are blueshifted with respect to the emission lines by $Δv = -(250$-400) km s$^{-1}$. Due both to this velocity offset and the lack of common low-ionization features (Mg II, Fe II), we argue these arise from the same absorbing material responsible for the low-velocity outflow discovered at X-ray wavelengths. The broad nuclear emission lines display a remarkable abundance pattern: N III], N IV], He II are quite prominent, while the common quasar emission lines of C III] and Mg II are weak or entirely absent. Detailed modeling of this spectrum will help elucidate fundamental questions regarding the nature of the emission process(es) at work in TDFs, while future UV spectroscopy of ASASSN-14li would help to confirm (or refute) the previously proposed connection between TDFs and "N-rich" quasars.

astro-ph.HE↗

arXiv2016

Late-time Spectroscopy of Type Iax Supernovae

We examine the late-time (t > 200 days after peak brightness) spectra of Type Iax supernovae (SNe Iax), a low-luminosity, low-energy class of thermonuclear stellar explosions observationally similar to, but distinct from, Type Ia supernovae. We present new spectra of SN 2014dt, resulting in the most complete published late-time spectral sequence of a SN Iax. At late times, SNe Iax have generally similar spectra, all with a similar continuum shape and strong forbidden-line emission. However, there is also significant diversity where some late-time SN Iax spectra display narrow P-Cygni features and a continuum indicative of a photosphere in addition to strong narrow forbidden lines, while others have no obvious P-Cygni features, strong broad forbidden lines, and weak narrow forbidden lines. Finally, some SNe Iax have spectra intermediate to these two varieties with weak P-Cygni features and broad/narrow forbidden lines of similar strength. We find that SNe Iax with strong broad forbidden lines also tend to be more luminous and have higher-velocity ejecta at peak brightness. We estimate blackbody and kinematic radii of the late-time photosphere, finding the latter an order of magnitude larger than the former. We propose a two-component model that solves this discrepancy and explains the diversity of the late-time spectra of SNe Iax. In this model, the broad forbidden lines originate from the SN ejecta, while the photosphere, P-Cygni lines, and narrow forbidden lines originate from a wind launched from the remnant of the progenitor white dwarf and is driven by the radioactive decay of newly synthesized material left in the remnant. The relative strength of the two components accounts for the diversity of late-time SN Iax spectra. This model also solves the puzzle of a long-lived photosphere and slow late-time decline of SNe Iax. (Abridged)

astro-ph.HE↗

arXiv2016

A Multiwavelength Study of the Relativistic Tidal Disruption Candidate Sw J2058+05 at Late Times

We report a multiwavelength (X-ray, ultraviolet/optical/infrared, radio) analysis of the relativistic tidal disruption event candidate Sw J2058+05 from 3 months to 3 yr post-discovery in order to study its properties and compare its behavior with that of Sw J1644+57. Our main results are as follows. (1) The long-term X-ray light curve of Sw J2058+05 shows a remarkably similar trend to that of Sw J1644+57. After a prolonged power-law decay, the X-ray flux drops off rapidly by a factor of $\gtrsim 160$ within a span of $Δ$$t$/$t$ $\le$ 0.95. Associating this sudden decline with the transition from super-Eddington to sub-Eddington accretion, we estimate the black hole mass to be in the range of $10^{4-6}$ M$_{\odot}$. (2) We detect rapid ($\lesssim 500$ s) X-ray variability before the dropoff, suggesting that, even at late times, the X-rays originate from close to the black hole (ruling out a forward-shock origin). (3) We confirm using HST and VLBA astrometry that the location of the source coincides with the galaxy's center to within $\lesssim 400$ pc (in projection). (4) We modeled Sw J2058+05's ultraviolet/optical/infrared spectral energy distribution with a single-temperature blackbody and find that while the radius remains more or less constant at a value of $63.4 \pm 4.5$ AU ($\sim 10^{15}$ cm) at all times during the outburst, the blackbody temperature drops significantly from $\sim$ 30,000 K at early times to a value of $\sim$ 15,000 K at late times (before the X-ray dropoff). Our results strengthen Sw J2058+05's interpretation as a tidal disruption event similar to Sw J1644+57.

astro-ph.HE↗

arXiv2016

Parallax of Galactic Cepheids from Spatially Scanning the Wide Field Camera 3 on the Hubble Space Telescope: The Case of SS Canis Majoris

We present a high-precision measurement of the parallax for the 12-day Cepheid SS Canis Majoris, obtained via spatial scanning with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). Spatial scanning enables astrometric measurements with a precision of 20-40 muas, an order of magnitude better than pointed observations. SS CMa is the second Cepheid targeted for parallax measurement with HST, and is the first of a sample of eighteen long-period >~ 10 days) Cepheids selected in order to improve the calibration of their period-luminosity relation and eventually permit a determination of the Hubble constant H_0 to better than 2%. The parallax of SS CMa is found to be 348 +/- 38 muas, corresponding to a distance of 2.9 +/- 0.3 kpc. We also present a refinement of the static geometric distortion of WFC3 obtained using spatial scanning observations of calibration fields, with a typical magnitude <~0.01 pixels on scales of 100 pixels.

astro-ph.SR↗

arXiv2015

Type II supernova energetics and comparison of light curves to shock-cooling models

During the first few days after explosion, Type II supernovae (SNe) are dominated by relatively simple physics. Theoretical predictions regarding early-time SN light curves in the ultraviolet (UV) and optical bands are thus quite robust. We present, for the first time, a sample of $57$ $R$-band Type II SN light curves that are well monitored during their rise, having $>5$ detections during the first 10 days after discovery, and a well-constrained time of explosion to within $1-3$ days. We show that the energy per unit mass ($E/M$) can be deduced to roughly a factor of five by comparing early-time optical data to the model of Rabinak & Waxman (2011), while the progenitor radius cannot be determined based on $R$-band data alone. We find that Type II SN explosion energies span a range of $E/M=(0.2-20)\times 10^{51} \; \rm{erg/(10 M}_\odot$), and have a mean energy per unit mass of $\left\langle E/M \right\rangle = 0.85\times 10^{51} \; \rm{erg/(10 M}_\odot$), corrected for Malmquist bias. Assuming a small spread in progenitor masses, this indicates a large intrinsic diversity in explosion energy. Moreover, $E/M$ is positively correlated with the amount of $^{56}\rm{Ni}$ produced in the explosion, as predicted by some recent models of core-collapse SNe. We further present several empirical correlations. The peak magnitude is correlated with the decline rate ($Δm_{15}$), the decline rate is weakly correlated with the rise time, and the rise time is not significantly correlated with the peak magnitude. Faster declining SNe are more luminous and have longer rise times. This limits the possible power sources for such events.

astro-ph.HE↗

arXiv2015

Constraining the Progenitor Companion of the Nearby Type Ia SN 2011fe with a Nebular Spectrum at +981 Days

We present an optical nebular spectrum of the nearby Type Ia supernova 2011fe, obtained 981 days after explosion. SN 2011fe exhibits little evolution since the +593 day optical spectrum, but there are several curious aspects in this new extremely late-time regime. We suggest that the persistence of the $\sim5800$~Å feature is due to Na I D, and that a new emission feature at $\sim7300$~Å may be [Ca II]. Also, we discuss whether the new emission feature at $\sim6400$~Å might be [Fe I] or the high-velocity hydrogen predicted by Mazzali et al. The nebular feature at 5200~Å exhibits a linear velocity evolution of $\sim350$ $\rm km\ s^{-1}$ per 100 days from at least +220 to +980 days, but the line's shape also changes in this time, suggesting that line blending contributes to the evolution. At $\sim 1000$ days after explosion, flux from the SN has declined to a point where contribution from a luminous secondary could be detected. In this work we make the first observational tests for a post-impact remnant star and constrain its temperature and luminosity to $T \gtrsim 10^4$ $\rm K$ and $L \lesssim 10^4$ $\rm L_{\odot}$. Additionally, we do not see any evidence for narrow H$α$ emission in our spectrum. We conclude that observations continue to strongly exclude many single-degenerate scenarios for SN 2011fe.

astro-ph.SR↗

arXiv2015

The Persistent Eruption of UGC~2773-OT: Finally, a Decade-Long Extragalactic Eta Carinae Analog

While SN impostors resemble the Great Eruption of eta Car in the sense that their spectra show narrow H lines and they have typical peak absolute magnitudes of -13 to -14 mag, most extragalactic events observed so far are quite different from eta Car in duration. Their bright phases typically last for 100~d or less, rather than persisting for several years. The transient object UGC2773-OT had a similar peak absolute magnitude to other SN impostors, but with a gradual 5-yr prediscovery rise. In the 6 yr since discovery, it has faded very slowly (0.26 mag/yr). Overall, we suggest that its decade-long eruption is so far the best known analog of eta Car's 19th century eruption. We discuss extensive spectroscopy of the ongoing eruption. The spectra show interesting changes in velocity and line shape that we discuss in detail, including an asymmetric Halpha emission line that we show is consistent with the ejection of a bipolar nebula that could be very much like the Homunculus of eta Car. Moreover, changes in the line width, line profile, blue excess emission resembling that of Type IIn supernovae, and the intensity of Halpha suggest the presence of strong circumstellar interaction in the eruption at late times. This supports the hypothesis that the extended plateau of eta Car's eruption may have been powered by shock interaction as well. One interesting difference compared to eta Car, however, is that UGC2773-OT so far does not exhibit the repeated brief spikes in luminosity that have been associated with binary periastron events.

astro-ph.SR↗

arXiv2015

Stripped-envelope supernova rates and host-galaxy properties

The progenitors of stripped-envelope supernovae (SNe Ibc) remain to be conclsuively identified, but correlations between SN rates and host-galaxy properties can constrain progenitor models. Here, we present one result from a re-analysis of the rates from the Lick Observatory Supernova Search. Galaxies with stellar masses less than $\sim 10^{10}~{\rm M_\odot}$ are less efficient at producing SNe Ibc than more massive galaxies. Any progenitor scenario must seek to explain this new observation.

astro-ph.HE↗

arXiv2015

What Powers the 3000-Day Light Curve of SN 2006gy?

SN 2006gy was the most luminous SN ever observed at the time of its discovery and the first of the newly defined class of superluminous supernovae (SLSNe). The extraordinary energetics of SN 2006gy and all SLSNe (>10^51 erg) require either atypically large explosion energies (e.g., pair-instability explosion) or the efficient conversion of kinetic into radiative energy (e.g., shock interaction). The mass-loss characteristics can therefore offer important clues regarding the progenitor system. For the case of SN 2006gy, both a scattered and thermal light echo from circumstellar material (CSM) have been reported at later epochs (day ~800), ruling out the likelihood of a pair-instability event and leading to constraints on the characteristics of the CSM. Owing to the proximity of the SN to the bright host-galaxy nucleus, continued monitoring of the light echo has not been trivial, requiring the high resolution offered by the Hubble Space Telescope (HST) or ground-based adaptive optics (AO). Here we report detections of SN 2006gy using HST and Keck AO at ~3000 days post-explosion and consider the emission mechanism for the very late-time light curve. While the optical light curve and optical spectral energy distribution are consistent with a continued scattered-light echo, a thermal echo is insufficient to power the K'-band emission by day 3000. Instead, we present evidence for late-time infrared emission from dust that is radiatively heated by CSM interaction within an extremely dense dust shell, and we consider the implications on the CSM characteristics and progenitor system.

astro-ph.HE↗

arXiv2015

Dissecting the Power Sources of Low-Luminosity Emission-Line Galaxy Nuclei via Comparison of HST-STIS and Ground-Based Spectra

Using a sample of ~100 nearby line-emitting galaxy nuclei, we have built the currently definitive atlas of spectroscopic measurements of H_alpha and neighboring emission lines at subarcsecond scales. We employ these data in a quantitative comparison of the nebular emission in Hubble Space Telescope (HST) and ground-based apertures, which offer an order-of-magnitude difference in contrast, and provide new statistical constraints on the degree to which Transition Objects and low-ionization nuclear emission-line regions (LINERs) are powered by an accreting black hole at <10 pc. We show that while the small-aperture observations clearly resolve the nebular emission, the aperture dependence in the line ratios is generally weak, and this can be explained by gradients in the density of the line-emitting gas: the higher densities in the more nuclear regions potentially flatten the excitation gradients, suppressing the forbidden emission. The Transition Objects show a threefold increase in the incidence of broad H_alpha emission in the high-resolution data, as well as the strongest density gradients, supporting the composite model for these systems as accreting sources surrounded by star-forming activity. The narrow-line LINERs appear to be the weaker counterparts of the Type 1 LINERs, where the low accretion rates cause the disappearance of the broad-line component. The enhanced sensitivity of the HST observations reveals a 30% increase in the incidence of accretion-powered systems at z~0. A comparison of the strength of the broad-line emission detected at different epochs implies potential broad-line variability on a decade-long timescale, with at least a factor of three in amplitude.

astro-ph.GA↗

arXiv2015

Spectropolarimetry of SN 2011dh in M51: geometric insights on a Type IIb supernova progenitor and explosion

We present seven epochs of spectropolarimetry of the Type IIb supernova (SN) 2011dh in M51, spanning 86 days of its evolution. The first epoch was obtained 9 days after the explosion, when the photosphere was still in the depleted hydrogen layer of the stripped-envelope progenitor. Continuum polarization is securely detected at the level of P~0.5% through day 14 and appears to diminish by day 30, which is different from the prevailing trends suggested by studies of other core-collapse SNe. Time-variable modulations in P and position angle are detected across P-Cygni line features. H-alpha and HeI polarization peak after 30 days and exhibit position angles roughly aligned with the earlier continuum, while OI and CaII appear to be geometrically distinct. We discuss several possibilities to explain the evolution of the continuum and line polarization, including the potential effects of a tidally deformed progenitor star, aspherical radioactive heating by fast-rising plumes of Ni-56 from the core, oblique shock breakout, or scattering by circumstellar material. While these possibilities are plausible and guided by theoretical expectations, they are not unique solutions to the data. The construction of more detailed hydrodynamic and radiative-transfer models that incorporate complex aspherical geometries will be required to further elucidate the nature of the polarized radiation from SN 2011dh and other Type IIb supernovae.

astro-ph.SR↗

arXiv2015

Search for precursor eruptions among Type IIb supernovae

The progenitor stars of several Type IIb supernovae (SNe) show indications for extended hydrogen envelopes. These envelopes might be the outcome of luminous energetic pre-explosion events, so-called precursor eruptions. We use the Palomar Transient Factory (PTF) pre-explosion observations of a sample of 27 nearby Type IIb SNe to look for such precursors during the final years prior to the SN explosion. No precursors are found when combining the observations in 15-day bins, and we calculate the absolute-magnitude-dependent upper limit on the precursor rate. At the 90% confidence level, Type IIb SNe have on average $<0.86$ precursors as bright as absolute $R$-band magnitude $-14$ in the final 3.5 years before the explosion and $<0.56$ events over the final year. In contrast, precursors among SNe IIn have a $\gtrsim 5$ times higher rate. The kinetic energy required to unbind a low-mass stellar envelope is comparable to the radiated energy of a few-weeks-long precursor which would be detectable for the closest SNe in our sample. Therefore, mass ejections, if they are common in such SNe, are radiatively inefficient or have durations longer than months. Indeed, when using 60-day bins a faint precursor candidate is detected prior to SN 2012cs ($\sim2$% false-alarm probability). We also report the detection of the progenitor of SN 2011dh which does not show detectable variability over the final two years before the explosion. The suggested progenitor of SN 2012P is still present, and hence is likely a compact star cluster, or an unrelated object.

astro-ph.HE↗