Abstract
Abstract We present the ATLAS discovery and initial analysis of the first 18 days of the unusual transient event, ATLAS18qqn/AT2018cow. It is characterized by a high peak luminosity (∼1.7 × 1044 erg s−1), rapidly evolving light curves (>5 mag rise to peak in ∼3.5 days), and hot blackbody spectra, peaking at ∼27,000 K that are relatively featureless and unchanging over the first two weeks. The bolometric light curve cannot be powered by radioactive decay under realistic assumptions. The detection of high-energy emission may suggest a central engine as the powering source. Using a magnetar model, we estimated an ejected mass of 0.1–0.4 M , which lies between that of low-energy core-collapse events and the kilonova, AT2017gfo. The spectra cooled rapidly from 27,000 to 15,000 K in just over two weeks but remained smooth and featureless. Broad and shallow emission lines appear after about 20 days, and we tentatively identify them as He i although they would be redshifted from their rest wavelengths. We rule out that there are any features in the spectra due to intermediate mass elements up to and including the Fe group. The presence of r-process elements cannot be ruled out. If these lines are due to He, then we suggest a low-mass star with residual He as a potential progenitor. Alternatively, models of magnetars formed in neutron star mergers, or accretion onto a central compact object, give plausible matches to the data.
Highlights
The advent of wide-field transient surveys that scan the visible sky every few nights has led to the discovery of new classes of transients, such as superluminous supernovae (SLSNe; e.g., Quimby et al 2011), Type Iax SNe (e.g., Li et al 2003), and Ca-rich transients (e.g., Perets et al 2010)
These newly discovered rapidly evolving transients have a wide range of peak absolute magnitudes (−15 > M > −22 mag), rise times (∼1–10 days), and spectral properties that make them difficult to explain through a single progenitor scenario, but most are incompatible with a radioactively powered explosion
We discovered a new transient, ATLAS18qqn, in a 30 s exposure with the start time of 2018 June 16 10:35:38 UT Modified Julian Date (MJD) 58285.44141 at an AB magnitude of o = 14.74 ± 0.10
Summary
The advent of wide-field transient surveys that scan the visible sky every few nights has led to the discovery of new classes of transients, such as superluminous supernovae (SLSNe; e.g., Quimby et al 2011), Type Iax SNe (e.g., Li et al 2003), and Ca-rich transients (e.g., Perets et al 2010). The first confirmed kilonova (AT2017gfo) from a neutron star merger detected in gravitational waves (GW170817) is the fastest declining astrophysical transient (Abbott et al 2017) that approaches SN-like luminosities. These newly discovered rapidly evolving transients have a wide range of peak absolute magnitudes (−15 > M > −22 mag), rise times (∼1–10 days), and spectral properties that make them difficult to explain through a single progenitor scenario, but most are incompatible with a radioactively powered explosion. AT2018cow was detected in the X-ray, radio, and submillimeter (e.g., de Ugarte Postigo et al 2018; Rivera Sandoval et al 2018), but these observations are not the focus of this Letter
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