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M101 ULX-1 Brightens

ATel #374; GSFC & LHEA
on 4 Jan 2005; 01:16 UT
Credential Certification: K.D.Kuntz (kuntz@milkyway.gsfc.nasa.gov)

Subjects: X-ray, Binary, Transient

Referred to by ATel #: 409

A series of four Chandra observations since 22 December 2004 show that M101 ULX-1 (CXO KM101 J140332.74+542102, the subject of Atels #222, 305, 306, and 311) has brightened significantly.

The count-rate of the first two observations is too low to allow spectral fitting. The spectrum from the 30 Dec 2004 observation provided about 620 counts, allowing a reasonable spectral fit. There is no significant flux above 1 keV, and the spectrum is well fit by an absorbed black-body spectrum (NH=3.5e21, kT=0.06+/-0.01). The peak of the emission is 0.5 keV. The spectrum from the 01 Jan 2005 observation provided about 1250 counts, again allowing spectral fitting. There is significant flux above 1 keV, and the peak of the emission is around 0.7 keV. It is also reasonably well fit by an absorbed black-body spectrum (NH=6.5e20, kT=0.166+/-0.02). There is clearly strong spectral evolution from 30 December to 1 January. The luminosities in the table below are the absorbed luminosities. For the last two observations, the luminosities are calculated from the model fits. For the first two, we have used the model of the 30 December observation scaled by the 0.3-2.5 keV count rate.

  
                         Absorbed   Exposure
Date Luminosity Time
(0.3-2.5 keV)
22 Dec 2004 01:17 UT 3.2e37 ergs/s 42 ks
24 Dec 2004 13:38 UT 4.5e37 ergs/s 41 ks
30 Dec 2004 02:10 UT 6.2e38 ergs/s 29 ks
01 Jan 2005 14:30 UT 1.5e39 ergs/s 21 ks

This object also brightened significantly in 05-11 July 2004, but had returned to its quiescent level by 23 July 2004. In that episode, the spectrum looked very similar to the 30 December 2004 spectrum, and although there are some signs that the spectrum did change over the course of the observations, it never acquired the hard tail seen in our 1 January 2005 observation. A hard tail has been observed in this object; the initial 26-27 March 2000 observation by Chandra, when the absorbed 0.5-2.0 keV luminosity was about 1.0e39 ergs/s(Pence et al,2001 ApJ 561, 189, Mukai et al, 2003, ApJ 582, 184). As on that occasion, the 0.5-2.0 keV flux is strongly and rapidly variable.

Optical spectra between 3500 and 5900 Angstroms of this object were obtained on 24 July 2004 with the Gemini telescope, one day after an XMM observation showed that the object had returned to its quiescent level. The spectrum showed some continuum, as well as strong HeII (4686) and HeI (5876), confirming the optical counterpart found by Kuntz et al (submitted). Further optical observations of this object, particularly when it is in outburst, are important for understanding the physical nature of this object.

The M101 Ms Collaboration