1. Virgo cluster survey with MXT
Search for tidal disruption events
Roberto Soria (NAOC-Beijing)
Christian Motch (Observatoire de Strasbourg)

2. What can we learn from TDEs ?
Measure the mass distribution of dormant black holes.
In particular, provide insight on BHs mass distribution and density in low-mass galaxies (thus constraining massive BH seed formation at high redshift)
Quantify the frequency of intermediate mass black holes in globular clusters
Give information on the surrounding stellar population dynamics, etc..
Determine what fraction on nuclear BH growth comes from TDEs

3. Why is Virgo Cluster a good TDE target ?
Very close to the Northern B1 attitude law
Relatively nearby (at d = 16.5 Mpc, MXT can reach Lx ~ 1041 erg/s in 5 ks)
Compact and rich (~ 2,000 galaxies)
Well documented in terms of multi-wavelength data (Next Generation Virgo Survey, Chandra surveys of ellipticals (archival) and spirals (new, R. Soria PI))
Identified as a “perfect” SVOM target early in the project (e.g. in Mission Rationale and Requirement document)
CIRA 2013

4. Rate of tidal disruptions
In galaxies with a nuclear SMBH (including dwarfs)
(Wang & Merritt 2004)
Empirical rates (TDEs per galaxy)
- From ROSAT+XMM events ~ 2 x 10-4 /yr (Esquej et al 2008)
- From X-ray surveys up to 4.7 x 10-4 /yr (Auchettl et al. 2017)
In globular clusters with a nuclear IMBH
(e.g. Brockamp et al. 2011)
CIRA 2013

5. Density of tidal disruptions
Auchettl et al 2017
Milosavljevic et al. 2008
MBH > Msol
Stellar tidal radius reached within BH horizon
Is that really rate ? Or density ?
CIRA 2013

6. Example of TDE rates predicted for a sample of nearby galaxies
(Wang & Merritt 2004)
CIRA 2013

7. Rate of tidal disruptions in Virgo
Rate ~ 0.1 /(Mpc^2 yr) in the inner region of Virgo from Es, dEs only
Rate ~ 0.2 /yr including spirals
and dwarf spheroidals
In addition: at least 50,000 globular clusters in Virgo (NGVS)
 Another 0.1 /yr if they contain an IMBH
But theoretical rates can be up off by an order of magnitude!
 monitoring Virgo is a good way to constrain TDE rates
CIRA 2013

8. TDEs– temporal characteristics Typically last a few weeks or months but some rare events may remain bright during several years (Lin et al. 2017)
SDSS
NGC 3599
2MASS
Not biased because of 20 yr baseline
(Saxton et al. 2012)

9. Duration of TDE flares Super-Edd phase for a 106 Msun BH
L ∝ t−5/3
depends only weakly on adiabatic index
Note that was computed for an IMBH in a GC.
(Ramirez-Ruiz & Rosswog 2009)
CIRA 2013

10. Soft X-ray spectra during bright states, becoming harder when fainter
SDSS J
SDSS
NGC 3599
2MASS
Brem=0.3 keV
Bbod = 66 eV
Bbod = 95 eV
Disk_bbod = 59 eV
Γ=2, Bbod=114 eV
Γ=2, Bbod=86 eV
(Saxton et al. 2012)

11. Spectra of TDE flares Three alternative possibilities:
1. Direct thermal disk emission (or ``slim disk’’)
Early phase for a typical SMBH flare
Early phase for a typical IMBH flare
Decaying to a low/hard state (power-law index G ~ 1.7)
after ~ 1 to 10 years, when L <~ 0.03 Eddington
CIRA 2013

12. Spectra of TDE flares 2. Optically thick photosphere (UV emission)
Direct X-ray emission shrouded by thick inflow/outflow
Likely to occur during first few weeks of an IMBH flare, when
 early phase for an extremely super-Eddington IMBH flare
 evolving to a soft X-ray flare (kT ~ 200 eV)
when outflow becomes optically thin
3. Hard power-law (G ~ 1.4–1.7) when seen face-on into jet
Apparent (Doppler-boosted) X-ray luminosity L ~ 1E46 – 1E47 erg/s
CIRA 2013

13. 5-ks MXT spectra of a TDE from a 106 MSol BH
After ~ 1 year (L ~ 3x1043 erg/s)
After ~ 10 years (L ~ 3x1041 erg/s)
NH = 1021 cm-2
CIRA 2013

14. 5-ks MXT spectra of early-stage TDEs from a 106 MSol and a 104 MSol BH
SMBH (L ~ 3x1043 erg/s)
IMBH (L ~ 1042 erg/s)
NH = 1021 cm-2
CIRA 2013

15. Current plan for MXT Virgo survey: -- 10 ks observations
-- 3 x 3 degree field (~1 x 1 Mpc)
-- once a week for 5 years
-- centred on M87
Suggested plan for MXT Virgo survey:
-- 5 ks observations
-- 2 x (3 x 3) degree fields
-- once a week for 5 years
-- centred on M87 and M49
Increases chances to detect TDE flares
CIRA 2013

16. Virgo observed by NGVS and Chandra (stars) Planned MXT 10-ks field
CIRA 2013

17. Virgo observed
by NGVS
and Chandra
Proposed MXT
5-ks fields
CIRA 2013

18. ROSAT X-ray contours and NGVS galaxy types and luminosities
Proposed MXT
5-ks fields
adapted from
Durrell et al (2014)
ROSAT X-ray contours and NGVS galaxy types and luminosities
CIRA 2013

19. NGVS density map of GCs, and location of identified galaxies
Proposed MXT
5-ks fields
adapted from
Durrell et al (2014)
NGVS density map of GCs, and location of identified galaxies
CIRA 2013

20. Conclusions
MXT can significantly constrain TDE rates (especially at low Lx/mass) and is sensitive enough to detect possible transient IMBHs in globular clusters.
The expected number of TDEs detectable in the proposed survey is uncertain. It could be as low as 2 over 5 years but may be up to 6 or higher, taking into account current uncertainties on the rates.
Existing high quality multi-wavelength data (NGVS) will ease identifying the origin of the TDE (using e.g. follow-up Chandra /XMM imaging, SKA, etc).
MXT will provide useful comparisons with eROSITA’s survey. eRosita will obtain (before SVOM) 8 snapshots only and will miss peak Lx/LEdd from which BH mass can be estimated.
We suggest a revised observing plan including 5-ks observations of two 3x3 deg fields over 5 years.
MXT can monitor the densest Virgo regions and significantly constrain TDE rates (especially at low Lx). MXT is sensitive enough to detect possible transient IMBHs in globular clusters.
The expected number of TDEs detectable in the proposed survey is uncertain. It could be as low as 2 over 5 years but may be up to 6 or higher, taking into account current uncertainties on the rates. Simulations including visibility window, flare duration, galaxy distributions, would help.
The proximity of the Virgo Cluster to B1 attitude law and existing high quality multi-wavelength data (NGVS) will ease identifying the origin of the TDE (using e.g. follow-up Chandra /XMM imaging, SKA, etc).
MXT will provide useful comparisons with ROSAT’s all sky survey and with eROSITA’s survey. eRosita will obtain (before SVOM) 8 snapshots only and will miss peak Lx/LEdd from which BH mass can be estimated. Follow-up of eRosita TDEs with the MXT will allow be possible.
We suggest a revised observing plan including 5-ks observations of two 3x3 deg fields over 5 years.
Verifier les autres docs sur TDE virgos dans les doc white paper ?
Faire références aux études preliminaires (Qui a fait ca ?)
CIRA 2013