A hot and fast ultra-stripped supernova that likely formed a compact neutron star binary

TL;DR: The discovery of iPTF 14gqr is interpreted as evidence for ultra-stripped supernovae that form neutron stars in compact binary systems.

Abstract: Compact neutron star binary systems are produced from binary massive stars through stellar evolution involving up to two supernova explosions. The final stages in the formation of these systems have not been directly observed. We report the discovery of iPTF 14gqr (SN 2014ft), a type Ic supernova with a fast-evolving light curve indicating an extremely low ejecta mass (≈0.2 solar masses) and low kinetic energy (≈2 × 1050 ergs). Early photometry and spectroscopy reveal evidence of shock cooling of an extended helium-rich envelope, likely ejected in an intense pre-explosion mass-loss episode of the progenitor. Taken together, we interpret iPTF 14gqr as evidence for ultra-stripped supernovae that form neutron stars in compact binary systems.

Summary

Introduction

• Taken together, the authors interpret iPTF 14gqr as evidence for ultra-stripped supernovae that form neutron stars in compact binary systems.
• If massive enough, the highly stripped core eventually collapses to produce a faint and fast evolving SN explosion which ejects a small amount of material (7, 8).

Discovery and follow-up of iPTF 14gqr

• The authors obtained rapid ultraviolet (UV), optical and near-infrared (NIR) follow-up observations of the source, including a sequence of four spectra within 24 hours from the first detection (12).
• The authors early spectra also exhibit blackbody continua with temperatures consistent with those inferred from the photometry, superimposed with intermediate width emission lines of He II, C III and C IV.
• Additional constraints based on light travel time arguments also suggest that the envelope was located at r ≤ 6 × 1015 cm from the progenitor (12).

An ultra-stripped progenitor

• The low ejecta mass and explosion energy, as well as the presence of an extended He-rich envelope, indicate an unusual progenitor channel for iPTF 14gqr.
• The temporal coincidence of the ejection with the final SN suggests that the envelope was likely associated with an intense pre-SN mass loss episode of the progenitor (12).
• The timescale of the ejection is similar to that expected for silicon flashes (∼ 2 weeks before explosion) in the terminal evolution of low mass metal cores (29), that have been suggested to lead to elevated mass loss episodes prior to the explosion.
• The presence of the extended He-rich envelope in iPTF 14gqr along with the lack of He in the low mass of ejecta suggest that the progenitor was highly stripped by a compact companion, such that only a thin He layer was retained on its surface.
• While wide binaries containing a NS and another compact object may be formed in noninteracting systems of binary massive stars, ultra-stripped SNe have been suggested to precede the formation of almost all compact NS binary systems (8).

Author contributions

• KD and MMK initiated the study, conducted analysis and wrote the manuscript.
• DAP, GED and YC conducted Keck and Palomar observations and contributed to data reduction and manuscript preparation.
• AH conducted the VLA observations and data reduction.
• TJM and PAM prepared the ultrastripped SN models presented in the paper.
• EOO, CF, AGY, RL, PEN and ALP contributed to manuscript preparation.

Data and materials availability

• All photometric data used in this paper are provided in the supplementary material (Table S1 and Table S2), while all spectra are available via the WISeREP repository at https: //wiserep.weizmann.ac.il/.
• The codes used for the ultra-stripped SN modeling are presented in (50), while the synthetic spectra presented in this paper are available at https: //goo.gl/9gkc9M.

Observations

• The authors nominally adopt the average MJD 56943.75 ± 0.43 as the explosion date, and calculate all phases with reference to this epoch.
• The actual explosion could have taken place before the last non-detection depending on the behavior of the early emission.
• Hence, the authors allow the explosion time to vary as a free parameter in their modeling, and discuss the last non-detection individually in the context of the physical models.

Optical light curves

• The authors obtained g band photometry of iPTF 14gqr with the P48 CFH12K camera, along with additional follow-up photometry in the Bgri bands with the automated 60-inch telescope at Palomar Observatory (P60; (56)).
• Point spread function (PSF) photometry was performed on the P48 images using the Palomar Transient Factory Image Differencing and Extraction pipeline (11), while the P60 images were reduced using an automated pipeline (57).
• Since P60 obtained contemporaneous observations with LCO with much higher signal to noise ratio, the authors chose not to include the LCO data in their analysis.
• The authors triggered Swift follow-up of the source in the V , B, UVW1 and UVW2 bands with the Swift Ultra-Violet/Optical Telescope (UVOT; (61)) and X-ray follow-up with the Swift X-ray telescope (XRT; (62)).
• The corresponding flux limits are summarized in Table S2.

Optical spectroscopy

• All spectra were reduced using standard tasks in IRAF and IDL, including wavelength calibration using arc lamps and flux calibration using standard stars.
• The spectroscopic observations are summarized in Table S3.
• The authors were unable to obtain a high signal-to-noise ratio (SNR) spectrum of the transient at epochs beyond≈ 30 days from light curve peak.
• The authors also obtained a spectrum of the apparent host galaxy nucleus with APO DIS on 2014 October 14 which was found to exhibit narrow emission lines of Hα, Hβ, [SII], [NII], [OII] and [OIII].
• Additionally, the authors obtained one spectrum of the transient location ∼ 800 days after the explosion as a part of a spectroscopic mask observation and did not detect any nebular emission features at the source location.

NIR imaging

• The data were processed using a custom reduction pipeline including flat-fielding and sky subtraction as well as special filtering steps to remove artifacts associated with the replacement detector in use at the time.
• The source is well-detected in all three filters in the final stacks.

Radio observations

• Each observation was performed using C-band (centered at 6.1 GHz) and K-band (centered at 22 GHz) in the C configuration.
• The authors analyzed the data with standard AIPS routines, using 3C 48 as the flux calibrator and NVSS J234029+264157 as the phase calibrator.
• The authors observations resulted in null detections in both bands at each epoch.
• The observational limits are 11.6 microJanskys (µJy) and 11.7µJy at C-band and K-band [measured as the 1σ root-mean-squared (RMS) noise of the reduced image], respectively, on 2014 October 15.

Late-time imaging

• The data were reduced and processed with standard image reduction procedures in lpipe (73).
• This constrains the presence of any stellar association at the location of the transient toMR >−11.4 mag andMg >−11.1 mag.
• Late-time images of the host galaxy region are shown in Figure S14.

Host environment spectroscopy

• IPTF 14gqr was discovered in the outskirts of an extended spiral galaxy showing clear signs of tidal interactions with nearby companions.
• The authors selected a total of 32 extended sources classified as galaxies (including the apparent spiral host) in the Sloan Digital Sky Survey (SDSS) within 5.4′ of the transient location (out of a total of 254 objects) to place the slits on the spectroscopic mask, along with one slit at the location of the transient.
• The spectra were reduced with standard routines in IRAF.
• Amongst the galaxies whose redshifts could be determined, the faintest source had SDSS magnitude of r ≈ 22.11 mag, while the same for galaxies within 100 kpc of the transient was r ≈ 21.60 mag.

Basic properties

• The authors find the peak magnitudes, time of maximum and corresponding rise time (between assumed explosion time and peak of light curve) in each filter by fitting a low order polynomial to the photometry near peak.
• The absence of photometric data points beyond ∼ 10 days after peak does not allow us to estimate the more commonly used quantity ∆m15, the drop in magnitude in 15 days after light curve peak.
• The uncertainties on these parameters were estimated by Monte Carlo sampling of 1000 realizations of the photometric data points using their associated uncertainties.
• The rise times of the light curves are shorter in the bluer bands as typically observed in Type Ib/c SNe (74).
• The rapid first peak of the light curve is perhaps the most distinguishing feature of iPTF 14gqr when compared to this sample of transients, and hence the authors compare this first peak to that of other known SNe exhibiting double peaked light curves in Figure S5.

Optical / UV SEDs

• In particular, the authors have two epochs with photometric data from all optical / UV bands, and the resulting blackbody SEDs are shown in Figure 3.
• The first epoch was within the first peak of the light curve (at ≈ 14 hours after explosion), where the UV / optical photometry is consistent with a blackbody of temperature > 30,000 K.
• This spectrum is also well described by a blackbody consistent with the photometric fit within the uncertainties.
• The UV photometric points are found to be significantly fainter than the optical blackbody fit at this epoch (with T ∼ 10000 K), which is indicative of significant line blanketing at UV wavelengths (as expected from Fe group elements in the ejecta).
• The NIR photometric magnitudes obtained near this epoch (≈ 1 day earlier) are also consistent with the optical blackbody fit.

Bolometric light curve

• The authors construct a bolometric light curve of iPTF 14gqr using three methods.
• The authors first fit a Planck blackbody function to the observed photometry at all epochs where they have detections in 3 or more filters to obtain a best-fitting blackbody and corresponding temperature, radius and luminosity.
• The corresponding radius and temperature evolution is also shown in Figure 4.
• Since the authors do not have simultaneous spectroscopy with all epochs of multi-band photometry, they choose to scale the fluxes obtained from a trapezoidal integration by an average factor of 1.48, while conservatively adding an uncertainty of 10% to account for the possible errors on this fraction.
• Hence, the authors use the pseudo-bolometric luminosities for modeling the properties of the second light curve peak.

First peak

• The spectroscopic sequence for iPTF 14gqr is shown in Figure 3.
• The He II λ4686 line is a common prominent feature of the flash ionized spectra of these events, and the C III λ4650 and C IV λ5801 lines were also observed in iPTF 13ast.
• The spectral evolution of the C high ionization lines in the early spectra is very similar to that seen in the WC sub-type evolution of galactic Wolf-Rayet stars (82, 83).
• In particular, the C III λ5696 / C IV λ5801 ratio increases in the later and cooler sub-types (WC7 - WC9) of this class, consistent with the increasing ratio observed in this source with decreasing photospheric temperature.

Photospheric phase

• The authors compare the photospheric phase spectra of iPTF 14gqr to those of other fast and normal Type Ic SNe in Figure S7.
• The comparison clearly shows that the photospheric spectra of iPTF 14gqr remain relatively blue and featureless compared to those of the normal Type Ic SNe (SN 1994I and SN 2004aw) at similar phases.
• SNe generally also exhibit the nearby Fe II lines of λ5018 and λ4924, which are blended into a single blue-shifted feature with respect to the λ5169 line .
• The three features are blended into a single broad absorption component in the case of the high velocity Type Ic-BL SNe, and can potentially cause errors in a velocity measurement if this effect is not taken into account (86).

Early nebular phase

• The authors final spectrum (with good SNR) was obtained ≈ 34 days after explosion, and show that the source was transitioning very early into the nebular phase.
• The authors compare the only nebular spectrum of iPTF 14gqr to the nebular spectra of other Type I SNe which exhibited an early nebular transition at similar phases in Figure S8.
• The best spectral match to iPTF 14gqr in this sample are to that of the Ca-rich gap transients PTF 10iuv and SN 2012hn.

Modeling

• Arnett Model for the main peak Type I SNe which do not show signs of interaction (such as iPTF 14gqr) are predominantly powered by energy released in the radioactive decay chain of 56Ni to 56Co to 56Fe.
• The best-fitting explosion time is earlier than their last non-detection (0.43 days before assumed explosion), although the predicted flux from the Arnett model would be below their detection threshold.
• The authors show the degeneracies between the various model parameters in Figure S9, where the degeneracy between t0 and τM is particularly prominent, since τM controls the width of the light curve.
• As in the case of the Arnett model, the authors expect the simple assumptions of this model to produce values which are only approximately correct.

Interaction with a companion

• Such interaction signatures have been previously observed in some Type Ia SNe ( (97–99); see (16) for a review of SN classification) where comparison of the data to theoretical models (100) allows the inference of the orbital separation of the binary system.
• The authors see no evidence for the presence of broad lines, as expected from the reverse shock produced in the ejecta-companion interaction.
• The authors consider the photometric properties of the first peak in a companion interaction scenario.
• As shown in (100), the early luminosity evolution depends on the viewing angle of the observer, where the excess flux is most prominent along the direction of the companion and relatively weak along directions perpendicular to or oriented away from the companion.
• This is readily apparent when comparing the luminosity and temperature evolution of the model with the observations – the analytical model predicts a luminosity evolution scaling with time as t−1/2, which is almost the same as the color temperature evolution, which scales as t−37/72.

Analysis of early spectra

• The early spectra of iPTF 14gqr exhibit prominent emission features of highly ionized He and C that are broader (FWHM ∼ 3000 km s−1) than that typically observed in the flash ionized spectra of other core-collapse events.
• The inferred radius of the optically thin material is larger than the envelope producing the early shock cooling emission as seen in the light curve, and suggests that the highly ionized lines likely arise from a lower density extension of the same envelope.
• The authors thus constrain the first spectrum to have been taken between ≈ +14 h (for the assumed explosion time) and ≈ +36 h after explosion (for an explosion at 0.92 days before the assumed time).
• Comparing to their observations, the authors note that the blue-shifted peak of the λ4686 feature and C III λ5696 lines in the +13.9 h and +25.2 h spectra can potentially be explained by LTT effects.

Radio analysis

• Radio emission in SNe arises from synchrotron radiation produced by shock accelerated electrons in the circumstellar medium.
• The authors show a comparison of the observed radio light curves of other Type Ib/c SNe to the radio limits on iPTF 14gqr in Figure S11.
• Since their observations were obtained at relatively early times after the explosion (within ∼ 10 days of explosion), the authors assume that the shock velocity is constant, instead of assuming a density structure for the outer envelope and a corresponding velocity evolution.
• The 22 GHz radio upper limits barely intersect the optically thick locus of the models, while the 6 GHz upper limits are well above the optically thick locus.
• Hence, these upper limits do not constrain the cicrumstellar environment.

Ultra-stripped SN modeling

• The authors compare the bolometric light curve and peak photospheric spectra of iPTF 14gqr to models of ultra-stripped SNe (as described in (7,28)) in Figure 5.
• Hence, the authors compare these models to the bolometric luminosity for the first peak, and to the pseudo-bolometric luminosity for the second peak.
• This assumption is valid because most of the lines are formed just above the photosphere.
• As shown earlier, the low γ-ray opacity of the ejecta also suggest that the lack of He excitation lines is likely due to a genuine low abundance of He in the ejecta, even in the limit of very low 56Ni mixing.
• The authors modeling of the peak photospheric spectra and light curve of iPTF 14gqr in the context of ultra-stripped SNe suggest that the 56Ni needs to be centrally located in order to explain the lack of line blanketing at < 4000 Å near peak optical light.

Host identification

• IPTF 14gqr was discovered at a large projected offset of ≈ 24′′ from the nearest apparent host galaxy, a two-tailed spiral galaxy showing signs of tidal interaction with at least three companion galaxies.
• The authors nominally adopt this galaxy as the host of iPTF 14gqr, but also discuss several other possible scenarios below.
• It was not possible to rule out the presence of a faint dwarf galaxy or globular cluster underneath the transient.
• The authors thus undertook deep late-time imaging of the host region and did not find any stellar association down to absolute magnitude limits ofMr > −11.4 mag and Mg > −11.1 mag.
• They are not stringent enough to rule out the entire population of dwarf galaxies and stellar clusters given the observed luminosity function of these systems in the local group (118–120).

Host properties

• The authors estimate the metallicity of the host galaxy using the pyMCZ code (123) to calculate the host oxygen metallicity (12 + log(O/H)).
• The authors fit the broadband photometry measurements of the galaxy to estimate parameters for the host stellar population.
• Thus, taken at face value, the host galaxy of iPTF 14gqr is indeed similar to the typical host galaxies of Type Ic SNe.

Explosion site properties

• The 29 kpc central offset of the location of iPTF 14gqr corresponds to a host normalized offset of≈.
• 8 Reff (where Reff is the half-light radius of the galaxy), placing this source at the extreme high end of the distribution of host offsets found for all SNe ( (135); (77)).
• IPTF 14gqr has the second largest host offset in terms of physical distance when compared to the PTF sample of core-collapse SNe, with SN 2010jp (PTF 10aaxi; (136)) as the only other corecollapse event with a larger offset (≈ 33 kpc).
• This would be in contrast to the general trend where Type Ib/c SNe are found to be more likely to be associated with H II regions than Type II events, in accordance with the shorter lifetimes of their more massive progenitors (139,141).

Tidally interacting environment

• As shown in Figure S2, the galaxies marked Obj2 and Obj3 were found to be at a redshift consistent with that of the nominal host Obj1.
• The small systemic velocities arise in particular due to the requirement of wide binary systems that can survive the common envelope (CE) ejection after the High Mass X-ray Binary (HMXB) phase (see (40) for a review).
• There are several possible explanations of this discrepancy.
• The authors late-time images of the host galaxy clearly show that the tidal tails of the host galaxy extend to larger projected distances compared to the offset of iPTF 14gqr, and hence a faint tidal tail at the location of iPTF 14gqr is not unexpected.
• There are also known examples of tidal dwarf galaxies forming in tidal tails that can host star formation several 100 Myrs after the formation of the parent tail (144, 148).

The nature of the companion star

• Stripping of the outer H and He envelopes in stripped envelope SNe can arise either due to mass loss via strong winds or due to stripping by a binary companion (2).
• The authors thus first consider the possibility of explaining the highly stripped progenitor of iPTF 14gqr from single star evolution.
• Several theoretical calculations for a wide range of stellar mass loss rates and metallicity show that the minimum progenitor mass from single star evolution of massive stars is > 5 M (152– 154), and hence the authors expect ejecta masses of>.
• This is again an order of magnitude lower than these predictions.
• This allows only the most compact companions to explain the stripping.

The remnant of the explosion

• When considering the required presence of a close compact companion to explain the low ejecta mass, a possible explanation of the expanding envelope could be due to a common envelope ejection preceding the terminal core-collapse.
• Multiple theoretical studies on the evolution of compact.
• He stars indeed suggest that they undergo rapid expansion at the onset of the He shell burning phase and as they approach core-collapse, reaching radii of the order of 102 − 103 R (162).
• The peak luminosity of iPTF 14gqr is much larger than those of the Ca-rich gap transients, which occupy the luminosity ‘gap’ between novae and SNe (peak absolute magnitude −15.5 ≥ Mpeak ≥ −16.5).

Figures

Figure 4: Bolometric light curve and Arnett modeling of iPTF 14gqr. A. Bolometric light curve of iPTF 14gqr. Filled black points indicate blackbody luminosities obtained from fitting multi-color photometry while the magenta points correspond to pseudo-bolometric luminosities (12). The empty black circles indicate g-band luminosities obtained by multiplying the g-band flux Fλ with the wavelength λ of the filter. The inverted triangles denote estimated predetection 5σ upper limits on the respective luminosities (12). The inset shows the bolometric light curves zoomed into the region of the first peak. B. The radius and temperature evolution of the fitted blackbody functions. C. Best-fitting Arnett model of the pseduo-bolometric light curve of the main (second) peak of iPTF 14gqr. The 56Ni mass MNi and diffusion timescale τM corresponding to the model are indicated in the legend (12).

Figure 3: Spectroscopic evolution of iPTF 14gqr. A. Observed spectra before (gray) and after (black) binning. The epochs of the spectra along with the scaling and vertical shifts used are indicated next to each spectrum. B. Zoom-in of the early spectra, indicated by the black dashed box in (A), showing rapid evolution of the λ4686 feature within 24 hours of discovery. The x-axis indicates the velocity shift from the He II λ4686 line. The orange and cyan lines mark the locations of the λ4686 line and the C III λ4650 line respectively. For the +13.9 h and +25.2 h spectra, additional magenta lines show the profiles of the C IV λ5801 and the C III λ5696 features respectively, at the same epochs. C. Scaled optical / UV SEDs of the photometry and spectra obtained within the first light curve peak (see Figure 2) in magenta, along with photometry near the second peak in orange. The circles indicate observed photometric fluxes, while the triangle is a 5σ upper limit. The dashed black lines indicate the best fitting blackbody SEDs including all optical / UV data points for the first peak and including only the optical data points for the second peak (12).

Figure 2: Multi-color photometric observations of iPTF 14gqr. A. Multi-color light curves of iPTF 14gqr from our photometric follow-up observations (magnitudes are corrected for galactic extinction, and offset vertically as indicated in the legend). Inverted triangles denote 5σ upper limits while other symbols denote detections. Hollow inverted triangles are upper limits from P48/P60 imaging and the filled inverted triangles are upper limits from Swift observations (filled green triangles are V band limits from Swift). Epochs when spectra were obtained are marked in both panels by vertical black dashed lines. B. Zoom-in of the early evolution of the light curve. The black solid line shows the assumed explosion epoch. The colored solid lines show the best-fitting shock cooling model for extended progenitors (25). Only photometric data before the cyan dot-dashed vertical line were used in the fitting (12).

Figure 6: Stellar evolutionary sequence leading from a binary system of massive stars (starting from the top left) to a NS-NS system, adapted from (9). NS-BH systems are expected to arise from binaries where the first formed compact object is a BH. NS-WD systems follow a similar evolutionary sequence starting from the HMXB stage (where the NS is replaced by the WD), but require additional mass transfer in the earlier stages (52). The material composition of the stars is indicated by their colors – red indicates H-rich material, cyan / blue indicate He-rich material, grey indicates CO-rich material and green indicates degenerate matter (in NS). The specific phase of the evolution is indicated by the text next to the systems, with black text indicating phases that have been observed previously, while red text indicates phases that have not been previously observed, and bold red text phases we observed in this work.

Figure 1: Discovery field and host galaxy of iPTF 14gqr. A. An optical image of the field from the Sloan Digital Sky Survey (SDSS); r and g filter images have been used for red and cyan colors respectively). B. Composite RGB image (r, g and B filter images have been used for red, green and blue colors respectively) of the iPTF 14gqr field from images taken near the second peak (19 October 2014) with the Palomar 60-inch telescope (P60), showing a blue transient inside the white dashed circle at the discovery location. C. Late-time composite R+G image (R and G filter images have been used for red and cyan colors respectively) of the host galaxy taken with the Low Resolution Imaging Spectrograph on the Keck-I telescope.

Figure 5: Comparison of iPTF 14gqr to theoretical models of ultra-stripped SNe. A. The bolometric light curve of iPTF 14gqr shown with a composite light curve consisting of ultrastripped Type Ic SN models (28) and early shock cooling emission (25). The blue dashed line corresponds to the 56Ni powered peak in the ultra-stripped SN models for Mej = 0.2 M , MNi = 0.05 M and EK = 2 × 1050 ergs, the magenta line corresponds to the early shock cooling emission and the orange line is the total luminosity from summing the two components. We use the blackbody (BB) luminosities to represent the early emission, while we use the pseudo-bolometric (pB) luminosities for the second peak (12). B. Comparison of the peak photospheric spectra of iPTF 14gqr (the epoch is indicated by the cyan dashed line in (A)) to that of the model in (A). The overall continuum shape, as well as absorption features of O I, Ca II, Fe II and Mg II are reproduced (12).