[Paper Review] Discovery of an Unusual Optical Transient with the Hubble Space Telescope

K. Barbary, Dawson, K. S.+28 more|USF Scholarship Repository (University of San Francisco)|Sep 10, 2008

Gamma-ray bursts and supernovae3 citations

TL;DR

This paper reports the discovery of SCP 06F6, an unusual optical transient detected by the Hubble Space Telescope that brightened over ~100 days to a peak magnitude of ~21.0 and then faded, with no detectable host galaxy or progenitor star to a 3σ limit of i775 ≥ 26.4 and z850 ≥ 26.1, implying a flux increase of at least a factor of 120. The transient's lightcurve and spectrum do not match known phenomena such as microlensing or standard variable stars, suggesting it may represent a new class of rare galactic or extragalactic transients.

ABSTRACT

We present observations of SCP 06F6, an unusual optical transient discovered during the Hubble Space Telescope Cluster Supernova Survey. The transient brightened over a period of ~100 days, reached a peak magnitude of ~21.0 in both i_775 and z_850, and then declined over a similar timescale. There is no host galaxy or progenitor star detected at the location of the transient to a 3 sigma upper limit of i_775 = 26.4 and z_850 = 26.1, giving a corresponding lower limit on the flux increase of a factor of ~120. Multiple spectra show five broad absorption bands between 4100 AA and 6500 AA and a mostly featureless continuum longward of 6500 AA. The shape of the lightcurve is inconsistent with microlensing. The transient's spectrum, in addition to being inconsistent with all known supernova types, is not matched to any spectrum in the Sloan Digital Sky Survey (SDSS) database. We suggest that the transient may be one of a new class.

Motivation & Objective

• To identify and characterize an unusual optical transient, SCP 06F6, discovered during the Hubble Space Telescope Cluster Supernova Survey.
• To determine the nature of SCP 06F6 by analyzing its lightcurve, spectrum, and host environment, particularly the absence of a detectable host galaxy or progenitor star.
• To rule out known transient phenomena such as microlensing, supernovae, and known variable stars by comparing its photometric and spectroscopic properties to theoretical models and existing databases.
• To assess whether SCP 06F6 represents a new class of astrophysical transients, either galactic or extragalactic, based on its unique lightcurve and spectral features.
• To evaluate the possibility that SCP 06F6 is a rare, previously undetected type of white dwarf or other compact object based on spectral morphology and absence in SDSS databases.

Proposed method

• Multi-epoch photometry using the Hubble Space Telescope Advanced Camera for Surveys (ACS) to track the lightcurve of SCP 06F6 over ~100 days in i775 and z850 bands.
• Spectroscopic observations with Keck/LRIS, VLT/FORS2, and Subaru/FOCAS to obtain detailed spectral features of the transient across multiple epochs.
• Deep imaging with HST/ACS to search for a host galaxy or progenitor star, setting 3σ upper limits of i775 ≥ 26.4 and z850 ≥ 26.1 at the transient's location.
• Cross-correlation of the transient spectrum with the SDSS spectral database using polynomial warping and least-squares fitting to identify spectral analogs, with redshift adjustments applied to overlapping regions (3500–6200 Å).
• Model comparison of the lightcurve with theoretical microlensing lightcurves for point-source lenses, assessing peak magnification and full width at half maximum (FWHM).
• Analysis of color evolution across the lightcurve, particularly between epochs eight and nine, to assess whether source structure or differential magnification could explain deviations from standard microlensing.

Experimental results

Research questions

• RQ1 Is SCP 06F6 consistent with a microlensing event, given its broad lightcurve and high peak magnification?
• RQ2 What is the nature of the transient's unusual spectrum, which shows broad, rounded absorption features without emission lines?
• RQ3 Why is there no detectable host galaxy or progenitor star at the transient's location, even at deep limits?
• RQ4 Does the transient represent a new class of astrophysical transient, possibly involving a cool white dwarf or other rare object?
• RQ5 Are there known spectral analogs in the SDSS database, and if not, what does this imply about the rarity or faintness of such objects?

Key findings

• SCP 06F6 exhibited a broad lightcurve with a full width at half maximum (FWHM) of approximately 100 days, significantly longer than the typical few hours seen in high-magnification microlensing events.
• The transient reached a peak magnitude of approximately 21.0 in both i775 and z850 bands, with no detectable host galaxy or progenitor star to a 3σ limit of i775 ≥ 26.4 and z850 ≥ 26.1, implying a flux increase of at least a factor of 120.
• The transient's spectrum showed broad, rounded absorption features between 4000 and 6000 Å with a red continuum, resembling DQp white dwarfs, but with distinct differences in feature spacing and absence of UV emission.
• No convincing spectral matches were found in the SDSS database, even after testing linear and quadratic warping functions and varying the wavelength fit range, indicating the transient's spectrum is unlike any known object in the database.
• The color evolution of the transient, particularly between epochs eight and nine, indicates differential magnification or source structure, which is inconsistent with a simple point-source microlensing model.
• The combination of a broad lightcurve, high peak magnification (3σ lower limit of ~120), and lack of a host or progenitor star makes microlensing an implausible explanation, suggesting SCP 06F6 may represent a new class of rare astrophysical transients.