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

TLDR: 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.

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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).