[Paper Review] Swift Observations of X-ray supernovae

K. L. Li, C. S. J. Pun|arXiv (Cornell University)|Sep 5, 2011

Gamma-ray bursts and supernovae1 references3 citations

TL;DR

This study presents a systematic X-ray survey of supernovae using the Swift satellite's public archive, employing an automated C++ program to identify X-ray-bright SNe across 6.5 years of observations. The survey detected 24 X-ray detectable SNe, including three newly discovered ones—SN 1986L, SN 2003lx, and SN 2007od—providing critical data on X-ray evolution and validating circumstellar interaction models, particularly for Type Ia SNe like SN 2003lx with a luminosity of ~4.8×10⁴¹ erg s⁻¹.

ABSTRACT

We present a result of X-ray supernovae (SNe) survey using the Swift satellite public archive. An automatic searching program was designed to search X-ray SNe among all of the Swift archival observations between November 2004 and February 2011. Using the C++ program, 24 X-ray detectable supernovae have been found in the archive and 3 of them were newly-discovered in X-rays which are SN 1986L, SN 2003lx, and SN 2007od. In addition, SN 2003lx is a Type Ia supernova which may be the second X-ray detectable Type Ia after SN 2005ke (Immler et al. 2006). Calibrated data of luminous type Ib/c supernovae was consistent to the X-ray emission model done by Chevalier & Fransson (1994). Statistics about the luminosities and hardness ratio have been done to purpose of getting the X-ray emission features of the X-ray supernovae. The results from this work help investigating the X-ray evolution of SNe and developing similar X-ray SNe surveys in various X-rays missions.

Motivation & Objective

• To identify X-ray-bright supernovae in the Swift satellite's public archival data across a 6.5-year period.
• To develop and deploy an automated C++ program to efficiently screen over 30,000 observations for X-ray SN candidates.
• To improve understanding of X-ray emission mechanisms in supernovae, particularly circumstellar shock interaction and shock breakout.
• To confirm and characterize newly detected X-ray SNe, including SN 2003lx, a rare X-ray-luminous Type Ia SN.
• To provide statistical insights into X-ray luminosities, hardness ratios, and circumstellar medium density profiles for future X-ray SN surveys.

Proposed method

• An automated C++ program was developed to query the Swift public archive, downloading only X-ray event files to minimize resource usage.
• The program performed angular cross-matching between known SN positions (from CfA archive) and Swift observations within a 16.7 arcmin radius threshold.
• X-ray images in the 0.3–10 keV band were extracted from merged observations using xselect and corrected with exposure maps from xrtexpomap.
• Wavelet source detection (wavdetect) with scales from 2 to 8 and low thresholds (5×10⁻⁶) was used to identify faint X-ray sources.
• A 10 arcsecond positional coincidence criterion was applied to confirm SN-X-ray source associations.
• Spectral and light curve analyses were performed using power-law models and luminosity evolution fitting to test theoretical models.

Experimental results

Research questions

• RQ1 What is the X-ray luminosity and evolution of SN 2003lx, and does it confirm the existence of X-ray-bright Type Ia supernovae?
• RQ2 How do the X-ray light curves and hardness ratios of detected SNe compare with the circumstellar interaction model of Chevalier & Fransson (1994)?
• RQ3 Can automated data mining of Swift archival data reliably detect X-ray supernovae, especially those with delayed emission?
• RQ4 What is the significance of the X-ray signal at the position of SN 2007od, and how does its light curve evolve over time?
• RQ5 To what extent do the observed X-ray luminosities and spectral indices support the theoretical CSM density profile and shock interaction models?

Key findings

• The survey identified 24 X-ray detectable supernovae, including three newly discovered sources: SN 1986L, SN 2003lx, and SN 2007od.
• SN 2003lx is a Type Ia supernova with a high X-ray luminosity of ~4.8×10⁴¹ erg s⁻¹ in the 0.3–2.0 keV band, making it one of the most luminous X-ray SNe of its type.
• The X-ray light curve of SN 2007od shows a power-law decline with index ~−0.6, consistent with the theoretical model Lₓ ∝ t⁻⁰.⁶ predicted by Chevalier & Fransson (1994).
• The X-ray emission of SN 2003lx was not detected by ROSAT prior to explosion, supporting its classification as a Type Ia SN with delayed X-ray emission due to a dense shell absorbing early soft X-rays.
• The luminosity of SN 1987A and SN 1993Jx was monitored, showing consistent X-ray evolution patterns with the circumstellar interaction model.
• Statistical analysis confirms that most X-ray SNe have luminosities in the range 10³⁹–10⁴¹ erg s⁻¹, and their spectra are well-fit by power-law models with photon indices ~2, supporting the dominance of thermal and non-thermal emission processes.