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IBVS 5427 (17 June 2003)

COMMISSIONS 27 AND 42 OF THE IAU
INFORMATION BULLETIN ON VARIABLE STARS
Number 5427

Konkoly Observatory
Budapest
17 June 2003
HU ISSN 0374 - 0676  (print)
HU ISSN 1587 - 2440  (on-line)

X-Ray Rotational Modulation in VXR45

Marino, A.1; Micela, G.2; Peres, G.1; Sciortino, S.2

(1) Dipartimento di Scienze Fisiche e Astronomiche, Sez. di Astronomia, Università di Palermo, Piazza del Parlamento 1, I-90134 Palermo, Italy;
e-mail: marino @ astropa . unipa . it, peres @ astropa . unipa . it

(2) INAF - Osservatorio Astronomico di Palermo G.S. Vaiana, Piazza del Parlamento 1, I-90134 Palermo, Italy;
e-mail: giusi @ astropa . unipa . it, sciorti @ astropa . unipa . it

SIMBAD object(s):    VXR45 = CD-52 2502 J2000: 8 42 14.8 -52 56 0.1

Type(s):    ZAMS

Keyword(s):    stars: variability

Abstract:   We present evidence of X-ray rotational modulation on VXR45, a young very fast rotator star member of IC2391 open cluster, observed with XMM-Newton observatory.


VXR45 is a young very fast rotator star (RA (J2000) = 8h42m 14s.8, Dec (J2000) = -52°56'01", V=10.70, B-V=0.81, photometric period =0.223 days (Patten & Simon, 1996)), member of IC2391 open cluster. The observation has been performed in November 20, 2001 with EPIC/PN on XMM-Newton satellite and covers, almost continuously, two photometric periods of VXR45. We extracted photon counts from a circular region centered on the VXR45 and selected in order to include   80% of the source photons

(Ghizzardi, 2001; Saxon, 2002), corresponding to a radius of 37". The total number of photon counts in the 0.3-7.8 keV band amounts to 7431; the local background has been determined from a source-free region near VXR45 for a total of 506 counts.

Fig. 1 shows the light-curve of VXR45 obtained in the 0.3-7.8 keV band (top) and that of the corresponding background (bottom). The X-ray rotational modulation, with a period very similar to the photometric one, is clearly evident. Note that other short-term variability not due to rotational modulation may also be present in the light-curve.

Fig. 2 shows the X-ray light-curve folded with the photometric rotational period. Phase-related variability is clearly evident; the amplitude of the variations in the X-ray light-curve is ~30% of the average count-rate.

We find, for the first time, unambiguous evidence of rotational modulation of X-ray stellar coronal emission in a very fast rotating star in the ``supersaturated'' regime.

The detection of X-ray rotational modulation implies the presence of not uniformly distributed active regions on the star. Furthermore we do not find evidence of spectral changes as function of the phase, consistently with the hypothesis that the modulation we observe is mainly due to a coverage effect, and that at all times the emission is largely due to the same mixture of emitting structures, probably composed by active regions hosting several flares.



X-ray light-curve of VXR45 (top) and background (bottom) as seen with EPIC/PN in the 0.3-7.8 keV band, time bins are 700 sec long. The photometric rotational period of ~ 19.3 ksec is well visible.



Folding of the X-ray data with the rotational period versus photometric period phase. Circles are points observed at t<0.223 days (the photometric period) and triangles those observed at t>0.223 days.

References:

Patten, B. M., & Simone, T., 1996, ApJS, 106, 489 (1996ApJS..106..489P)

Ghizzardi, S., 2001, ``EPIC-MCT-TN-011'', Tech. rep., EPIC Milano Calibration Team

Saxon, R. D., 2002, ``XMM-CCF-REL-116'', Tech. rep., XMM-SOC