A status report of the microlensing search by the pixel method in the direction of M31, on the 2 meter telescope at Pic du Midi is given. Pixels are stable to a level better than 0.5%. Pixel variations as small as 0.02 magnitude can clearly be detected

Ansari, R.

Auriere, M.

Baillon, P.

Bouquet, A.

Coupinot, G.

Coutures, C.

Ghesquiere, C.

Giraud-Heraud, Y.

Gondolo, P.

Hecquet, J.

Kaplan, J.

Le Du, Y.

Melchior, A.L.

Moniez, M.

Picat, J.P.

Soucail, G.

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arXiv:astro-ph/9602015v1  2 Feb 1996AGAPE, a microlensing search in the direction of M31:status reportR. Ansaria, M. Aurie`reb, P. Baillonc, A. Bouquetd,f , G. Coupinotb,C. Couturese, C. Ghesquie`ref , Y. Giraud-He´raudf , P. Gondolod,g,J. Hecquetb,J. Kapland,f , Y. Le Duf , A.L. Melchiorf , M. Monieza, J.P. Picatb,and G. Soucailba LAL, Universite´ Paris Sud, Orsay, France,b Observatoire Midi-Pyre´ne´es Bagne`res de Bigorre et Toulouse, France,c CERN, Gene`ve, Switzerland, d LPTHE, Universite´s Paris 6 et 7, France,e DAPNIA, CEN Saclay, France, f LPC Colle`ge de France, Paris, France.g Theoretical Physics, University of Oxford, United KingdomPresented at the second Workshop : “The dark side of the Universe : experimental effortsand theoretical framework” Roma 13 -14 November 1995, by J. Kaplan.LPC 96-04/confThe M31 galaxy in Andromeda is the nearest large galaxy after the Small and Large Magel-lanic Clouds. It is a giant galaxy, roughly 2 times as large as our Milky Way, and has its ownhalo. As pointed by some of us [1, 2] and independantly by A. Crotts [3], M31 provides arich field of stars to search for MACHO’s in galactic halos by gravitational microlensing [4].M31 is a target complementary to the Magellanic Clouds used by the current experiments[5, 6]. It is complementary in that it allows to probe the halo of our galaxy in a directionvery different from that of the LMC. Moreover, the fact that M31 has its own halo and istilted with respect to the line of sight provides a very interesting signature [3] : assuming anapproximately spherical halo for M31, the far side of the disk lies behind a larger amountof M31 dark matter, therefore more microlensing events are expected on the far side of thedisk. Such an asymmetry could not be faked by variable stars.In other words, M31 seems very appropriate to detect brown dwarfs through microlensing.However, as very few stars of M31 are resolved, we had to develop a new approach to lookfor microlensing by monitoring the pixels of a CCD, rather than individual stars [1, 2]. TheAGAPE collaboration has set out to implement this idea.Monitoring pixelsIn the case of a crowded field such as M31, the light flux Fpixel on a pixel comes fromthe many stars in and around it, plus the sky background. The light flux of an individualstar, Fstar, is spread among all pixels of the seeing spot and only a fraction of this light,Fpixel = {seeing fraction} × Fstar, reaches the central pixel. If the star luminosity isamplified by a factor A, the pixel flux increases by :∆Fpixel = (A− 1) {seeing fraction} Fstar . (1)The amplification of the star luminosity allows an event to be detected if the flux on thebrightest pixel rises sufficiently high above its rms fluctuation σpixel :∆Fpixel > Q σpixel . (2)Typically, in our simulations, we requireQ to be larger than 3 during 3 consecutive exposuresand larger than 5 for at least one of them.1Pixel flux distribution before calibrationPhotometric calibrationΦ(ADU)dN/dΦ0100020003000400050000 500 1000 1500 2000 2500 3000 3500 4000aPixel flux distribution after calibrationPhotometric calibrationΦ(ADU)dN/dΦ0100020003000400050000 500 1000 1500 2000 2500 3000 3500 4000bFigure 1: The matching of pixel histograms before (a) and after (b) photometric alignmentAll our simulations [1, 2, 7] indicate that the statistics should be significant if the relativefluctuation of the current pixels (i.e. where no particular activity occurs) can be made suf-ficiently small. This crucial point is the first step of our data analysis and is discussed indetail below.Status of the analysisWe took data during 60 nights of observation on the 2 meter “Bernard Lyot” telescope atObservatoire du Pic du Midi in the French Pyre´ne´es, from September 29 to November 24 in1994, and 93 nights, from July 28 to Dec 31 in 1995. Of these 153 nights, only 61 came outwith good weather. The field regularly covered was 8′ × 8′,with 4 exposures on a 800× 800part of a thin Tektronix CCD camera with pixels 0.3 ′′ wide. Two other 4′ × 4′ fields wereoccasionally covered.Images have been taken with both red (Gunn) and blue (Johnson) filters, but less regularlyin blue. We have not yet started the analysis of the blue frames.Geometrical alignment. A definite pixel never points exactly in the same direction of thesky on two different exposures. Successive images have therefore to be realigned geometri-cally by software, in order that the light curve of a pixel really represents the light curveof a definite region of the sky. To this aim, we match the positions of bright stars betweenthe current image and a reference image, using an adaptation to our case of the programPEIDA devised by the EROS collaboration [8]. The precision of the geometrical realignmentthus obtained is better than 0.3 pixel (0.1 ′′). Such a precision is fully satisfactory, as weconstruct our light curves on super-pixels of size 5 pixel× 5 pixel or larger.Photometric alignment. The sky background light and the atmospheric absorption differfrom one picture to the other. Before any photometric follow-up, it is necessary to correctfor these differences. We do that by matching the dispersion and the mean value of the his-togram of pixel intensities between each frame and a reference one. This method works wellin this case because the local luminosity gradient in M31 largely supersedes all other sourcesof dispersion. Figure 1 shows how well pixel histograms, that look very different beforetreatment, coincide up to small structures after a renormalization by only two parameters :an overall multiplicative factor for the atmospheric absorption and an additive constant forthe sky background. To check the quality of the photometric alignment obtained, we eval-2AGAPE - A field - seeing<1.8″- 1994-1995Relative fluctuation of the 5x5 superpixels (1.5˝)200.0020.0040.0060.0080.010.0120.0140.016100200300400500600700300 400 500 600 700 800 900Figure 2: The relative fluctuation on field Auated the relative intensity fluctuation of each super-pixel among all exposures. Figure 2shows a map of this relative dispersion for 5 × 5 super-pixels for one of our fields. We seethat this dispersion does not exceed 0.5% on most of the field, which is around twice thephoton noise. This can be looked at in a different way (figure 3): after constructing a lightχ2 distribution with σ=1.7σγAGAPE 1994-1995    -    1.2˝<seeing<1.4˝χ2dN/dχ2020004000600080001000012000140000 10 20 30 40 50 60 70 80 90 100χ2 distribution with σ=1.7σγAGAPE 1994-1995    -    seeing<1,8˝χ2dN/dχ201000200030004000500060007000800090000 20 40 60 80 100 120 140 160 180 200Figure 3: The relative fluctuation on field A3curve for each super-pixel, one can compute along each light curve the χ2 of the intensitycompared with its average. On figure 3, we display for two different seeing intervals the dis-tribution of this χ2. The error σ entering the χ2 is chosen in such a way that the maximumof the distribution coincides with that of the ideal poissonnian χ2 distribution. The truedistibutions show non poissonnian tails. Clearly there are non gaussian contributions to thefluctuations and a comparison between figure 3a and 3b shows that they are largely due toseeing variations. Further work is in progress to cope with seeing variations.Present results.Let us examplify the kind of results we get by the light curve of a 7× 7 super-pixel shownB field - superpixel 7x7 (342,335)390004000041000420000 50 100 150 200 250 300 350 400 450daysφ (ADU)Figure 4: A typical light curveon figure (4). This light curve first shows the degree of stability (≃ 0.5%) we achieve ona stable pixel. It appears consistent with error bars taken as twice the photon noise asexplained earlier. A second point apparent on this light curve is that a magnitude variation∆m ≃ 0.02 (800/40000 ADU) of the super-pixel is clearly detectable.Figure 5 shows another light curve. On graph 5a all frames are retained, whereas onlyframes with seeing between 1.2 ′′ and 1.8 ′′ are kept in graph 5b. This illustrates the insta-bilities introduced by seeing variations. However, when extreme seeing have been excluded(graph 5b), a magnitude change ∆m ≃ 0.015 is clearly detected. There are two luminosityvariations, therefore it is not a microlensing event.Plots 5c and 5d display the intensity in a square of side 30 elementary pixels (10 ′′). Thehills in the landscape are structures of M31 and appear in the same way on both plots.However, a tiny hill at the center of plot 5c has grown to a high peak on plot 5d. The clearpoint-spread-function shape of the growing peak tells us that we are really looking at thevariation of the luminosity of a star. This is confirmed by the progressive rise and fall ofthis peak on exposures before and after the maximum shown on plot 5d. Clearly, such afaint variable object would not have been detected by monitoring resolved stars.We have a catalog of a few hundred such variations, most of which are multiple and there-fore are not microlensing events. We are know working i) to treat the seeing variationsii) to interprete the variations we see in terms of known types of variable stars, iii) to tryand isolate events compatible with microlensing and in any case to evaluate our sensitivitythreshold for the detection of microlensing events.We thank professor A. Gould for useful discussions and suggestions.References4D field - superpixel 7x7 (693,182)40000420000 50 100 150 200 250 300 350 400 450a : all points daysφ (ADU)4100042000430000 50 100 150 200 250 300 350 400 450b : seeing selected points daysφ (ADU)680 690700166176.33186.67197800850900c : 17 :  24/11/94 - s=1.3˝680 690700166176.33186.67197800850900d : 33 :  14/10/95 - s=1.3˝Figure 5: Another typical light curve[1] Baillon P. Bouquet A. Giraud-He´raud Y. & Kaplan J. 1992. “Search for dark matteras brown dwarves by looking at Andromeda (M31)”, Proceedings of the first PalaiseauWorkshop, Fleury P. Vacanti G. editors, Edition Frontie`res 1992; p151.[2] Baillon P. Bouquet A. Giraud-He´raud Y. & Kaplan J. 1993. A&A 277; 1.[3] Crotts A. P. S. 1992. ApJ. 399; L43.[4] Paczyn´ski B. 1986. ApJ. 304; 1.[5] Alcock et al., 1993. Nature 365; 621, and these proceedings.[6] Aubourg E. et al. 1993. Nature 365; 623.[7] Ansari et al. 1995. 17th Texas symposium on relativistic astrophysics and cosmology,Annals of the New York academy of sciences 759; 608.[8] “Programme de reconstruction photome´trique PEIDA Version V”, 1995. EROS techni-cal note.5

AGAPE: a microlensing search in the direction of M31

AGAPE: a microlensing search in the direction of M31

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