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A new soft X-ray transient from the XMM-Newton slew survey

ATEL # 1537; R. Saxton(ESAC), P. Esquej(MPE), P. Evans(LU), J. Osborne(LU), K. Page(LU), A. Read(LU), M. Sanchez(ESAC)
on 20 May 2008; 9:16 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Richard Saxton (

Subjects: Optical, X-ray, Request for Observations, Transients

We report the discovery of a new soft X-ray transient, XMMSL1 J072822.2+745406, found in an XMM-Newton slew of April 26th. The source position is RA: 07 28 22.2 DEC: 74 54 06 with an 8", 1 sigma, error circle.

The source count rate was 1.1 c/s in the EPIC-pn detector and the spectrum is soft with no events above 2 keV. The unabsorbed flux is 2.3E-12 ergs/s/cm^2 (0.2-2 keV; for a black-body spectral model of temperature 130eV and galactic absorption of NH=3.75E20). This flux is 24x greater than an upper-limit calculated from the ROSAT All Sky Survey using the same spectral parameters.

One catalogued optical source, EO1295-0467919, lies within the error circle with a magnitude m_b=20.1, m_r=18.3. This source is flagged as extended in the APM catalogue.

An optical spectrum was taken by the WHT on May 13 with the ISIS spectrometer using the R158R and R158B low-resolution gratings. The Ca II H and K, H_beta, H_delta, H_gamma absorption lines can be identified and have a redshift consistent with zero. i.e. the object is located within our galaxy.

A Swift XRT observation was performed on May 15 for 2.4ks and confirmed the source at a position: RA: 07 28 21.95, DEC: +74 54 02.4 (error 2.0", 90% confidence), still consistent with EO1295-0467919. The extrapolated 0.2-2 keV unabsorbed flux is 1.2E-12 ergs/s/cm^2 implying that the source has faded by a factor ~1.8 since the slew observation. The Swift XRT spectrum is soft and can be fit by a power-law of slope 3.7+/-0.5 or a black-body of temperature 130+/-20 eV.

The source type remains unknown. If it is a star then the spectral type will be F or G and given the magnitudes, the distance would be a few kpc and the Luminosity high for a stellar coronal event.

We welcome further observations in particular to investigate the reported extended optical emission.

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R. E. Rutledge , Editor
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